First Results from the SRG/eROSITA All-Sky Survey: From Stars to Cosmology

15 Sept 2024, 09:00 → 20 Sept 2024, 18:00 Europe/Berlin

HS 1 Hörsaal/lecture hall 1 (Garching)

Esra Bulbul (MPE), Johannes Buchner (MPE)

First Results from the SRG/eROSITA All-Sky Survey: From Stars to Cosmology

International conference

In little more than half a century, X-ray astronomy has established itself as a fundamental domain of observational astrophysics. X-rays probe the hot and energetic components of the Universe, encompassing, among others, the million-degree coronae of stars, the remnants of supernovae, the ultra-dense matter in neutron stars, the immediate surroundings of black holes, the plasma filling galaxy clusters – the most massive objects in the Universe.

The new X-ray telescope eROSITA launched successfully on July 13, 2019, on board the Russian/German X-ray mission Spektr-RG (SRG). After commissioning and a successful performance verification program, eROSITA has started mapping the entire sky at unprecedented depths. Before being placed into safe mode on February 26, 2022, eROSITA completed 4.4 all-sky surveys. As in any other astrophysics domain, the eROSITA all-sky surveys unlock large swathes of discovery space, provide large statistical samples to study various classes of objects, and explore sufficiently large volumes to serve as cosmological tools for the study of the Universe as a whole. Serendipitous scientific highlights include an X-ray flash from a nova, quasi-periodic eruptions, tidal disruption events, and the eROSITA bubbles. The high sensitivity, large field of view, and high survey efficiency of eROSITA are revolutionizing X-ray astronomy: The first all-sky survey includes in the Western Galactic Hemisphere, among others, more than 12.000 clusters of galaxies, 700.000 Active Galactic Nuclei, and 180.000 stars or compact stellar objects. This exceeds the total number of previously discovered celestial X-ray objects since the dawn of X-ray astronomy.

This conference follows the public data release of the first all-sky survey, including source catalogs of point-like and extended sources, images, spectra, light curves, and X-ray photon events. We invite presentations of the first scientific results driven by the international community. We plan to cover the following topics:

• Galaxy Clusters and Cosmology
  •  Physical properties of distant and nearby galaxy clusters, scaling relations
  • Clusters as tracers of the large-scale structure of the Universe
  • Tests of cosmological models with cluster statistics
  • Theory and simulations of structure formation
• Active galactic nuclei and galaxy evolution
  • Evolution and properties of the AGN population
  • AGN as tracers of large-scale structure
  • Extreme sub-populations (high redshift, high luminosity)
• Galactic compact objects, stars, and planets
  • Heliosphere
  • Active stars: physics and population studies
  • Cataclysmic variables and X-ray binaries
  • Isolated neutron stars
  • ULX and X-ray sources in nearby galaxies
• The Transient X-ray sky
  • Tidal Disruption Events
  • Quasi-periodic eruptions
  • Gamma-ray bursts and afterglows
  • Galactic X-ray transients
  • Gravitational Waves and other multi-messenger counterparts
• Diffuse X-ray emission
  • Hot plasmas in the Milky Way, LMC, and SMC
  • Circum-Galactic Medium
  • Supernova Remnants
  • Cosmic X-ray background and its fluctuations
  • High-energy physical processes in the solar neighborhood
• Synergies with multi-wavelength surveys and multi-messenger probes

Invited speakers:

• James Aird (Edinburgh Univ.)
• Riccardo Arcodia (MIT)
• Franz Bauer (Pontificia Universidad Católica de Chile)
• Matteo Costanzi (INAF)
• Jeremy Drake (CfA)
• Gus Evrard (Univ Michigan)
• Peter Jonker (SRON)
• Juna Kollmeier (Carnegie Theoretical Astrophysics Center)
• Chandreyee Maitra (MPE)
• Francois Mernier (NASA Goddard Space Flight Center)
• James Miller-Jones (Curtin Univ)
• Annalisa Pillepich (MPIA)
• Gabriele Ponti (INAF)
• Katja Poppenhäger (AIP)
• Richard Saxton (ESA)
• Eleonora Troja (Univ. Roma)
• Natalie Webb (IRAP)
• Irina Zhuravleva (Univ. Chicago)

 

We hope that the attendees will enjoy the famous Oktoberfest of Munich, which is right after the conference.

CHECKOUT

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Sunday, 15 September

eROSITA conference: Registration

eROSITA conference: Reception in the foyer of the conference building

Monday, 16 September

X-ray View of the Milky Way: SNe and ISM

1 eROSITA's view of the local interstellar medium

The solar neighbourhood is a volume devoid of neutral material. Instead, it is displaced by a soft X-ray-emitting plasma. This region is known as the local hot bubble (LHB). A holistic view of this emission requires a large covering area, first delivered by the ROSAT All Sky Survey about three decades ago. The properties of the LHB were measured but were unfortunately clouded by the discovery of the solar wind charge exchange (SWCX) emission at similar energies emanating from solar wind ions interacting with Earth's exosphere and neutral matter within the heliosphere.

eROSITA's halo orbit around L2 avoided the contamination from the SWCX from the exosphere, and the favourable timing of eRASS1 at solar minimum provided a valuable data set for studying any diffuse emission outside of the Solar System, particularly emissions spanning large areas in the sky, like the LHB. In this talk, I will describe our half-sky spectral analysis of the diffuse soft X-ray background (SXRB), emphasising the isolation and characterisation of the LHB emission. We found a clear North-South temperature dichotomy in the LHB. Its cause is unclear but could be linked to the most recent off-centre supernova explosions that inflated the LHB. The shape of the LHB is much more extended at high Galactic latitudes, presumably due to the higher mid-plane pressure preventing expansion on the disk. We also observed the presence of interstellar tunnels filled with hot plasma, potentially connecting to the nearest superbubbles.

Speaker: Michael Yeung (Max Planck Institute for Extraterrestrial Physics)

2 Revisiting Dust Scattering Halos from X-ray Surveys with eROSITA

In X-ray observations, scattering of photons on dust along the line of sight can lead to the formation of an extended X-ray halo around any source. We have used data from eROSITA to study these dust scattering halos around 29 point sources in eRASS 1.

For bright sources, which are needed to observe halos at the high scattering angles made accessible by eROSITA, pile-up limits our abilities to constrain the source and halo flux by fitting the surface brightness distribution, as has been done in previous studies with, e.g., ROSAT. Instead, we fit spectra extracted from annuli around the sources using the xscat model, excluding the inner, piled-up source regions. This allows us to simultaneously fit the unscattered source spectrum and the dust distribution.

We find that the hydrogen column density in absorption provides an upper limit for the scattering column densities, likely due to absorption within some source systems as well as scattering close to the sources, which is not resolved by eROSITA's PSF.

Speaker: Christian Kirsch (Remeis-Observatory & ECAP, FAU Erlangen-Nürnberg)

3 The eROSITA view of the hot phase of the interstellar medium in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is a large and nearby satellite galaxy of the Milky Way, which underwent multiple recent star formation episodes. Due to its low inclination and foreground absorption, the properties of the hot phase of its interstellar medium (ISM) can be ideally probed via its diffuse soft X-ray emission. This, in combination with deep available exposure due to the location of the LMC close to the SRG/eROSITA survey pole, makes the LMC an ideal target for studying the interplay of the hot and cold ISM phases as well as the ISM heating and enrichment through massive stars, using eROSITA and multiwavelength data.

In this contribution, we will present the results of an X-ray morphological and spectral analysis campaign of the ISM in the LMC, based on the full available eROSITA All-Sky Survey, complemented with radio, infrared, and optical data.
We will discuss the anticorrelation of the hot and cold ISM phases, caused by both X-ray absorption and macroscopic inhomogeneities in their distribution. Further, we will show to what extent the distribution of the hot ISM phase in the LMC can be linked to massive stars. Finally, we will present evidence for star-formation-fueled outflows in the east of the LMC, traced by filaments in X-rays and optical line emission, enhanced elemental abundances, and possibly X-ray synchrotron emission from accelerated cosmic rays.

Speaker: Martin Mayer (Remeis Observatory Bamberg (ECAP, FAU))

10:55 Coffee break

AGN Surveys and History of Acceration, Clustering

4 X-ray beacons at late cosmic dawn

From extrapolations of contemporaneous X-ray active galactic nuclei (AGN) luminosity functions, the eROSITA all-Sky Survey (eRASS) is expected to contain ~100 X-ray ultra-luminous quasars, that emitted their light when the universe was less than a billion years old, at z>5.6. In the luminosity regime probed by eROSITA at these early times, the powerful X-ray output of these quasars is driven by rapid accretion onto black holes, often boosted by non-thermal jet contributions. eRASS reionization-era quasars are thus powerful probes of the evolution of black hole growth and active galactic nuclei (AGN) jet demographics. Theses objects should further shed light on the later stages of reionization and are prime subjects for feedback studies in young host galaxies, before building up a complex merger history and before the peak of star-formation. However, this tip-of-the-iceberg population is hidden away in a haystack of cool Galactic stellar objects, which share their red colours in the deepest optical/IR imaging surveys, but have a sky density that is many sorders of magnitude higher.
I will present the sample of all luminous quasars at z>5.5 with spectroscopic redshifts in the eRASS hemisphere. It was assembled by cross-matching all single-epoch and the stacked eRASS eRASS:4-5 to the latest compilation of ~400 quasars known at these cosmic time.
In addition, I have designed a quasar selection pipeline combining eRASS X-ray data with optical/IR imaging data from the Dark Energy Survey DR2, the Vista Hemisphere Survey DR5 and CatWISE2020. The core selection method relies on optical/IR SED template fitting and X-ray aperture photometry. The method led to discovery of two of the most X-ray luminous quasars at late cosmic dawn in eRASS: a blazar at z=5.6 and an X-ray variable broad-absorption line quasar at z=5.7.
High-redshift eRASS quasars appear in general X-ray over-luminous with respect to their disk UV emission. These findings hint at the existence of an aox-LUV outlier population that could not be sampled in the smaller footprints of previous X-ray surveys. I will discuss implications for the AGN population as a whole and the dominant emission mechanisms in these sources.

Speaker: Juliem Wolf (MPIA)

5 An X-Ray Census of Active Galactic Nuclei in the Virgo and Fornax Clusters of Galaxies with SRG/eROSITA

We present a uniform and sensitive X-ray census of active galactic nuclei (AGNs) in the two nearest galaxy clusters, Virgo and Fornax, utilizing the newly released X-ray source catalogs from the first all-sky scan of Spectrum-Roentgen-Gamma/eROSITA. A total of 50 and 10 X-ray sources are found positionally coincident with the nuclei of member galaxies in Virgo and Fornax, respectively, down to a 0.2–2.3keV luminosity of ∼10^39 erg/s and reaching out to a projected distance well beyond the virial radius of both clusters. The majority of the nuclear X-ray sources are newly identified. There is weak evidence that the nuclear X-ray sources are preferentially found in late-type hosts. Several hosts are dwarf galaxies with a stellar mass below ∼10^9 M_sun. We find that contamination by nonnuclear X-ray emission can be neglected in most cases, indicating the dominance of a genuine AGN. In the meantime, no nuclear X-ray source exhibits a luminosity higher than a few times 10^41 erg/s, which might be owing to a steep intrinsic luminosity function. The X-ray AGN occupation rate is only ∼3% in both clusters, apparently much lower than that in field galaxies inferred from previous X-ray studies. Both aspects suggest that the cluster environment effectively suppresses AGN activity. The findings of this census have important implications for the interplay between galaxies and their central massive black holes in cluster environments.

Speaker: Meicun Hou (Kavli Institute for Astronomy and Astrophysics, Peking University)

6 Simulation-based inference of the AGN population based on the radiation-regulated unification model in the X-rays

X-ray surveys of AGN provide direct constraints on the properties of individual AGN, such as their accretion, reflection, and obscuration. Previous AGN population synthesis models have not addressed such constraints self-consistently. Here we use a simulation-based inference (SBI) approach to constrain the geometrical and physical properties of the AGN population. We perform numerical simulations with our ray-tracing code, RefleX, which allows the self-consistent modelling of the X-ray emission of AGN with flexible circumnuclear and source geometries. We create our synthetic population by sampling the black hole mass function and Eddington ratio distribution function of local AGN and constructing the radiation-regulated unification model. Using the RefleX-simulated emission of the AGN population, we attempt to simultaneously reproduce several observed properties of Swift/BAT detected AGN, such as the N$\mathrm{_H}$ distribution, the fraction of obscured AGN as a function of Eddington ratio, and their logN – logS. With this approach, we can test the consistency of the radiation-regulated model in the local Universe with the most comprehensive set of X-ray observables, while constraining the size and density of the dusty torus.

Speaker: Dimitra Gerolymatou (University of Geneva)

13:00 Lunch Break

AGN and host Galaxies

7 The AGN content in eROSITA DR1

Galaxy evolution can only be understood if their AGN phases are accounted for. For that, a complete and pure census of AGN is needed. Hunting for AGN in X-ray is the most obvious way to go, given the low emission from galaxies at this frequency. In the last 20 years, XMM and Chandra have provided us mostly with pencil-beam surveys, thus sampling the faint and high-redshift regime. Finally, with eROSITA, we can also sample the rare (local and z>5.5) and faint Universe. In my talk, I will review the multi-wavelength properties (including redshifts) of the first eROSITA AGN sample, also in comparison with AGN selected from other surveys.

Speaker: Mara Salvato (MPE)

8 Hard X-ray selected sources in the first eROSITA all-sky survey

I will present the hard ($2.3-5~$keV) X-ray selected sample of sources detected with eROSITA during the first all-sky survey, comprising 5466 X-ray sources. From this catalog, I have produced a large uniform sample of hard-X-ray selected AGN, and characterised them with supporting multi-wavelength astrometry, photometry and spectroscopy. For the 2863 sources within the sky coverage of the DESI imaging Legacy Survey Data Release 10 (LS10; $>15000$ deg$^2$), counterparts are identified and classified. I also perform comparisons with the Swift-BAT sample and HEAO-1 AGN sample to attempt to better understand the effectiveness and sensitivity of eROSITA in the hard band. A total of 2547 sources within the LS10 area are found to have good-quality counterparts, and 111 of these are detected only in the hard band. Comparing with other hard X-ray selected surveys, the eROSITA hard sample covers a larger redshift range and probes dimmer sources, providing a complementary and expanded sample as compared to Swift-BAT. Examining the column density distribution of missed and detected eROSITA sources present in the follow-up catalog of Swift BAT 70 month sources, it is demonstrated that eROSITA can detect obscured sources with column densities $>10^{23}~$cm$^{-2}$ corresponding to $\sim14\%$ of the full sample, but that the completeness drops rapidly thereafter. A sample of hard-only sources, many of which are likely to be obscured AGN with column densities $\sim10^{23}~$cm$^{-2}$ is also presented and discussed. X-ray spectral fitting reveals that these sources have extremely faint soft X-ray emission and their optical images suggest that they are found in more edge-on galaxies with lower b/a. The resulting X-ray catalog is demonstrated to be a powerful tool for understanding AGN, in particular heavily obscured AGN found in the hard-only sample. I will discuss these results in the context of future analyses of hard X-ray selected AGN, and also in the context of future eROSITA all-sky surveys.

Speaker: Sophia Waddell (Max Planck Institute for Extraterrestrial Physics)

9 Hyper-luminous AGN candidates in eFEDS viewed with SCUBA-2 on JCMT and KOOLS-IFU on Seimei Telescope

We report the follow-up observations for hyperluminous quasar candidates discovered in the eFEDS field using SCUBA-2 on JCMT and KOOLS-IFU on the Seimei Telescope. Galaxies whose infrared (IR) luminosity exceeds 10$^{13}$ $L_{\odot}$ have been termed hyper-luminous IR galaxies (HyLIRGs). The IR luminosity can arise from active galactic nucleus (AGN) and star formation (SF) activity. According to the galaxy and supermassive black hole (SMBH) growth scenarios predicted by numerical simulation of galaxy mergers, HyLIRGs correspond to the most crucial phases in which the growth rates of galaxies and SMBH peak. Therefore, HyLIRGs are expected to serve as a vital laboratory for probing the growth phase of galaxy-SMBH co-evolution. The advent of the eROSITA enables us to investigate hyperluminous AGNs systematically. This work conducts follow-up observations for HyLIRG candidates discovered in the eFEDS field (Toba et al. 2022). To estimate accurate IR luminosity and address the dust properties, we observed six HyLIRG candidates by SCUBA-2 on JCMT. For objects without spectroscopic redshifts, we measured them using KOOLS-IFU on the Seimei Telescope in Japan. As a result, we discovered a hyperluminous quasar at $z_{\rm spec}$ = 1.622 with a super-Eddington ratio ($\lambda_{\rm Edd} > 3$) (Toba et al. 2024, submitted). We also report AGN host properties of those HyLIRGs based on the SED fitting from X-ray to Radio and characterize those objects in the context of galaxy-SMBH co-evolution.

Speaker: Yoshiki Toba (NAOJ)

10 The LOFAR-eFEDS survey: Global energetics and feedback as probed by AGN incidences

We use the complete, spectroscopic GAMA09 survey to measure the fraction of galaxies hosting radio and X-ray AGN, defined using LOFAR and eROSITA data, as functions of mass-scaled power indicators. We recover the previously found mass-invariant triggering and fueling mechanisms in the incidence of X-ray AGN as a function of λEdd. However, the story is more perplexing in the case of radio AGN, especially when considering different radio morphologies, as their incidence as a function of λJet shows a residual mass and jet power dependence. Interestingly, these effects cannot be explained by more powerful radio AGN residing in more dense environments (or more massive dark matter haloes). We demonstrate that this statistical incidence approach is a powerful way to probe the fundamental accretion physics, in particular the disk-jet connection, in varying accretion modes. Lastly, we compute the average λJet as a function of stellar mass and radio morphology to understand which types of radio AGN exert the most impactful feedback on their host galaxies. This knowledge serves as a useful calibrator for AGN feedback simulations.

Speaker: Zsofi Igo (MPE)

11 AGN in dwarf galaxies: the eRASS1 revolution

Supermassive black holes (SMBHs) reside in the heart of virtually every massive galaxy. However, much less is known about whether dwarf galaxies commonly host SMBHs.
Exploiting the recently published eROSITA X-ray catalogue, we built a sample of actively accreting SMBHs in local dwarf galaxies. This uniformly selected sample of 74 AGN is, to date, one of the largest compilations of X-ray-selected SMBHs hosted by dwarf galaxies.
I will present the sample-building procedure we adopted and the properties of the sources we selected. I will also discuss how, by comparing our sample with semianalytical models, it is possible to extract precious information about the growth history of SMBHs which could help constrain different seeding mechanisms of the most mysterious building blocks of our Universe.

Speaker: Andrea Sacchi (Center for Astrophysics | Harvard & Smithsonian)

16:00 Coffee break

Time domain and multi-messenger astronomy

12 X-ray observations in time domain and multi-messenger astronomy

In this talk I will review recent and future (possible)
progress on time domain and multi-messenger astronomy from an X-ray
perspective.

Speaker: Prof. Peter Jonker (Radboud University & SRON)

Poster Viewing: Happy Hour

Tuesday, 17 September

AGN Physics, Variability, TDE, QPE

13 Tidal disruption events and other extreme nuclear transients uncovered with eROSITA

The SRG/eROSITA all-sky survey provides unique capabilities to explore the extragalactic transient and variable X-ray sky. While previous studies showed a dominance of AGN variability, a rare subset of sources in such a survey is expected to arise from more exotic phenomena such as tidal disruption events (TDEs), quasi-periodic eruptions (QPEs) or other short-lived non-AGN events associated with supermassive black hole accretion. In this talk, I will highlight the extraordinary properties of candidate TDEs discovered by eROSITA in eRASS1-5 and show the diversity of their X-ray and multi-wavelength behaviour. I will cover repeating partial TDE with timescales from months to decades, TDEs with unusual host galaxies, eROSITA’s constraints on the onset of accretion disk formation and the time scales for corona formation and destruction, and show that the X-ray evolution of TDEs rarely follows the canonical fall-back driven expectation.
Furthermore, I will describe the methodology used to compile the largest systematically selected sample (>300 sources) of extragalactic X-ray transients without prior signs of AGN activity (eRO-ExTra) from which a golden sample of ~30 TDEs was selected. The fundamental properties, including the X-ray and multi-wavelength light curves, spectral characteristics, and host galaxy properties, will be summarised, and the TDE luminosity function and occurrence rate will be discussed.

Speaker: Iuliia Grotova

14 The radio properties of eROSITA selected tidal disruption event candidates

When a star passes too close to a supermassive black hole (SMBH) it can be destroyed, temporarily increasing the accretion rate onto the SMBH. Such tidal disruption events (TDEs) produce bright flares across the electromagnetic spectrum that provide a unique window into the central region of a galaxy, including the previously dormant black hole. Radio observations of TDEs are essential for probing synchrotron emission from electrons that are accelerated in the shocks formed from outflows. However, <30 TDEs have published radio detections so the origin of these outflows is still under debate, with scenarios including accretion disk winds, weak radio jets launched by accretion onto the SMBH, or collisions between debris streams. Some events have even displayed unexplained late-time radio flares years after the disruption. A comprehensive analysis of the radio properties of X-ray bright TDEs, and therefore any link between radio outflows and strong accretion, was not possible due to the lack of dedicated radio observations of X-ray selected events. However, the eROSITA all-sky surveys allowed the search for and identification of many X-ray bright TDE candidates. In this talk I will present an overview of our systematic radio follow-up of the eROSITA TDE “golden sample” with the Australian Compact Array Telescope (ATCA). I will discuss the prevalence of radio emission from bright X-ray TDEs, any link between X-ray and radio bright emitters, and the properties of the outflows launched.

Speaker: Adelle Goodwin (ICRAR - Curtin University)

15 Systematic Collapse of the Accretion Disc Across the Black Hole Population

Active galactic nuclei (AGN) are powered by the accretion of material onto a supermassive black hole (SMBH) but the structure of this flow is not well understood. Standard accretion disc models match only to zeroth order in predicting the substantial energy dissipated in optically-thick material producing a strong blue/UV continuum. More detailed comparisons to the observed spectral shapes fail along with the ability to produce their variable nature. Based on stellar mass black holes in our own galaxy, the accretion disc should transition into an X-ray hot, radiatively inefficient flow below a (mass scaled) luminosity of 0.02Ledd. However, this is difficult to disentangle from increased dust obscuration and/or host galaxy contamination drowning out the AGN emission for the majority of the AGN population out to z~1. Here we use the new eROSITA eFEDS Survey to identify unobscured AGN from their X-ray emission, matched to excellent optical imaging from Subaru’s Hyper Suprime-Cam to subtract out the host galaxy contamination. The resulting spectra clearly show the shape of the distorted disc emission in bright AGN, and demonstrate that there is an intrinsic drop in this disc continuum below 0.02Ledd, revealing fundamental aspects of accretion physics in AGN.

Speaker: Scott Hagen (Durham University)

16 A systematic study of the most extreme variability in AGN detected with eROSITA

eROSITA has so far completed four all-sky X-ray surveys, detecting around one million AGN in each survey. We conduct a large-scale and systematic search among all individual surveys, to identify the most extreme X-ray variability events in extra-galactic objects. Among those are significant ignition and shut-down events associated with large changes in accretion rate or line-of-sight absorption. The most significant events are followed-up in a multi-wavelength campaign, which includes optical photometry & spectroscopy and observations in UV, and X-rays.
In this talk, I will cover the results of our search covering the first four eROSITA all-sky surveys. I will introduce our sample of extremely variable sources by detailing our selection methods. In total our sample consists of ~2,000 vetted sources, of which approximately 10% have additional multi-wavelength follow-up data. I will provide an overview of some of the most interesting sources detected (extreme ignition and shutdown events in AGN), which are currently published in several papers. I will also discuss our results in the context of the link between extreme X-ray and optical variability, specifically 'changing-look' behaviour in AGN, and the time-scales involved in large-scale accretion changes around SMBHs. Finally, I will introduce the first statistical study of the largest X-ray selected changing-look AGN sample to date. From this sample conclusions can be drawn on the occurrence rate of changing-look events, as well as their preferred AGN parameter space.

Speaker: Mirko Krumpe (AIP)

17 An X-ray view of 'Scary Barbie'/AT2021lwx: the most extreme Narrow Line Seyfert I flare

AT2021lwx/ZTF20abrbeie, also known as Scary Barbie, is an ultra-luminous optically discovered transient at z $\sim$ 1 with no previously cataloged host. It is the most energetic non-jetted transient ever observed, exhibiting a single optical brightening by a factor of over 100 to an optical luminosity of $few \times 10^{46}$ erg s$^{-1}$, surpassing any known optical transient. Here, we present SRG/eROSITA, XMM-Newton, and Chandra X-ray observations of the source throughout its evolution.
We demonstrate that the X-ray spectral shape and light curve, along with multi-wavelength data, indicate that the source is an Active Galactic Nucleus (AGN) rather than a tidal disruption event (TDE, as suggested by the discovery authors). Specifically, it has all the characteristics of the Narrow Line Seyfert 1 class of AGN. Through comprehensive spectral energy distribution (SED) fitting from infrared to X-rays, we show that the bolometric luminosity of the source has varied from less than $10^{44}$ erg s$^{-1}$ to a peak of approximately a few $\times 10^{47}$ erg s$^{-1}$. The peak bolometric luminosity is observationally and theoretically inconsistent with a TDE, but it makes the source, for a brief period, one of the brightest AGNs in the z $\leq$ 1 universe. This work demonstrates the fundamental role of X-ray data in understanding astronomical transients' nature.

Speaker: Muryel Guolo (Johns Hopkins University)

18 X-ray quasi-periodic eruptions: where do we stand?

X-ray Quasi-Periodic Eruptions (QPEs) are high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central black holes in galactic nuclei of low-mass galaxies. So far, only a handful of such events has been found, although with rising interest in the broader community given their observational and theoretical connection with tidal disruption events and, possibly, low-frequency gravitational wave sources. As a matter of fact, some of the latest models suggest that these eruptions are triggered by extreme mass ratio inspirals, in which the secondary body interacts with the accretion flow around the primary. This accretion flow is suggested to be short-lived and fed by a previous TDE. I will outline the observational properties of QPE sources and the latest insights from theoretical models.

Speaker: Riccardo Arcodia (MIT Kavli Institute)

19 Hubble space telescope ultraviolet observations of a quasi-periodic X-ray eruption source

eROSITA's time-domain capabilities have enabled the discovery and characterization of four new quasi-periodic X-ray eruption (QPE) sources. A wealth of X-ray follow-up observations exists, as does a large number of theoretical models that can broadly explain the X-ray properties of QPEs. While characterizing these sources in more detail with X-ray observations is valuable, multi-wavelength observations can provide complementary information with potentially more constraining power to discriminate between (or rule out) existing theories and models, and to drive future modeling efforts.

I will present new ultraviolet observations of a QPE source taken with the Hubble space telescope. These observations represent the highest spatial resolution time-resolved data available for a QPE, are at least 2 orders of magnitude more sensitive than existing UV data, and are likely the most stringent constraints on UV variability for the foreseeable future. I will discuss our results and their implications in the context of theoretical models for QPEs.

Speaker: Thomas Wevers (Space Telescope Science Institute)

20 Lensed QSOs in the eROSITA all-sky surveys

The flux magnification by strong gravitational lensing allows detailed observations of QSOs at high redshifts which would otherwise only be possible for a few extremly luminous objects. Recently there has also been renewed interest in the study of lensed QSOs due to the possibility of independent and accurate measurements of the Hubble constant using time-delay cosmography in lensed QSO systems. We have been using X-ray sources from the eRASS1 catalogue in combination with Gaia measurements to identify new lensed QSO candidates. We have so far spectroscopically confirmed 4 new QSO lenses and 14 physical pairs of AGN. For the brightest of the new systems, eRASS1 J050129.5-073309, we were able to measure a time delay of ~100 days between the light curves of the 2 lensed components. Due to its brightness (among the 10 X-ray brightest QSO lenses in the eRASS1 survey) and its potential use for time-delay cosmography we have initiated follow-up observations of this object with Chandra, XMM-Newton and HST.

Speaker: Georg Lamer (Leibniz-Institut für Astrophysik)

21 Optical spectroscopy of eROSITA AGN

Multiwavelength information is crucial for a complete understanding of the Universe. In the era of big data, large-number statistics is the ideal tool to characterize the demography of galaxy populations and understand the complex phenomena involved in galaxy evolution. Within SPIDERS (Spectroscopic Identification of ERosita Sources), we selected eROSITA X-ray sources to be observed in the optical domain by SDSS-V. This survey allows us to compare physical properties obtained from optical spectra and X-rays, and I will present the results of the detailed analysis for ~10 000 Active Galactic Nuclei (AGN) at the eFEDS field. This is one of the largest uniformly selected X-ray AGN samples with systematic optical spectroscopic data. We will study the relation between the black hole growth and the host galaxy by comparing properties such as black hole mass, accretion rate, luminosity, outflow rates, column density, etc. For a subsample of AGN with redshift lower than 1, we will present results of accurate AGN-host spectral decomposition that reveal properties from the host galaxy and their connection with AGN. Some insights about AGN spectra will also come from the challenges of performing the fit of this X-ray-selected sample.

Speaker: Catarina Aydar (Max Planck Institute for Extraterrestrial Physics)

22 Photo-z for eROSITA AGN via Deep Learning

A complete census of SMBH increases our understanding of the role of AGN evolution over cosmic time. As AGN detection is less affected by obscuration effects in the X-ray window, eROSITA offers increased likelihood and purity in detecting these objects. That being said, a substantial fraction of spectroscopic redshifts for AGN identified by eROSITA will be available only in 2-3 years from now at best. In the meantime, we must rely on photometric redshifts (photo-z), where for wide-area surveys, the quality of current estimates for AGN using broad-band photometry is poor. The limited number of photometric bands is insufficient to disentangle complex convolved AGN/host-galaxy contributions, resulting in a high fraction of outliers, as relevant parameters established by the source detection and flux estimate algorithms, are usually fine-tuned for galaxies and not AGN.
More recent efforts to compute photo-z for AGN utilizing the radial light distribution with aperture photometry (provided by the Legacy Survey) via ML, have shown promising improvements, as its shape changes with redshift given a fixed resolution. For this reason, we further extend our novel single-survey Deep Learning algorithm by raising the spatial light distribution resolution through images, alleviating previous empirical approaches by decreasing the fraction of outliers. In my talk, I will show how our works "CircleZ" and "PICZL" outperform previous results while uncovering various sources of contaminants.

Speaker: William Roster (Max Planck for extraterrestrial Physics (MPE))

Poster Viewing: Coffee break

12:45 Lunch Break

X-ray emission around galaxies from CGM to WHIM

23 The hot CGM of the Milky Way as seen by eROSITA

The first all-sky maps of high ionization lines observed in X-rays by eROSITA provide an excellent probe for the study of the hot phase (T ∼ 10^6 K) of the Milky Way (MW) circumgalactic medium (CGM). In this work we analyse the OVII and OVIII line detected in the eROSITA data. We fit sky maps made in narrow energy bins around the lines, with physical emission models embedded in a 3D geometry to constrain the density distribution of the hot gas around our Galaxy, with a focus on mid and high (absolute) Galactic latitudes. By filtering out the eROSITA bubbles and other foreground sources, we find that an oblate geometry of the hot gas (T ≡ 0.15 keV), flattened around the Galactic disk with scale height zh ∼ 1 − 3 kpc, best describes the observed eROSITA maps, with most of the observed emission resulting to be produced within a few kpc from the Sun. We find that the soft-X background emission attributed to the CGM in general does not probe the medium at distances ≫ kpc from the Sun. The additional presence of a large scale hot spherical halo, while providing a minor contribute to the X-ray emission, accounts for the high OVII absorption column densities detected with XMM, as well as most of the baryon budget of the MW CGM. In addition, we exploit the ratio between the OVIII and OVII intensities to constrain the temperature distribution of the same medium down to <5% of the average temperature <T>~0.2 keV. The eROSITA data carry the largest amount of information and detail of OVIII CGM intensities to date, allowing to highly reduce the statistical uncertainties of the inferred physical parameters.

Speaker: Nicola Locatelli (INAF - OAB)

24 eROSITA All Sky Survey: X-ray emission around galaxies

The properties of the hot circumgalactic medium (CGM) help constrain the galaxy evolution models and the bi-modality of the galaxy population, but its observation is challenging. By applying the stacking technique to the X-ray data from four eROSITA all-sky surveys (eRASS:4) and a volume-limited central galaxy sample containing 85,222 galaxies with 10$<\log(M_*)<$11.5, we measure the X-ray surface brightness profiles of the hot CGM and estimate the baryon budget of it. We provide the scaling relationships between the X-ray luminosity of hot CGM and the stellar or halo mass of the galaxies. We discuss if the X-ray emission around star-forming and quiescent galaxies is different and the implication on the galaxy evolution models.

Speaker: Yi Zhang (MPE)

25 Exploring the Properties of Hot Gas in Galaxy Groups from the CGM to the IGrM

Low-mass galaxy groups are the most common environments for galaxies in the Universe, and they provide a crucial link between cosmology and galaxy evolution. However, their hot gas and baryon content are poorly constrained by current X-ray observations due to their low surface brightness. In this talk, I will demonstrate the efficiency of X-ray spectral stacking as a method to overcome these limitations for galaxy groups coming from the major optical spectroscopic surveys, such as the SDSS, GAMA and DESI, and then observed by eROSITA All Sky Survey. I will show how this approach allows us to probe the temperature-mass relation and examine the shape of temperature and entropy profiles down to Milky Way-sized halos, where estimations from individual data are still not possible. The stacking results from eRASS1 will be compared with predictions from eROSITA mock observations based on hydro-dynamical simulations like Magneticum and IllustrisTNG. This approach not only probes the properties of intra-group medium gas in clusters on scales comparable to our own Local Group, but also expands and validates existing X-ray scaling relations to the very low-mass end. Furthermore, it traces the influence of AGN feedback on baryon distribution in groups, providing insights into their internal dynamics.

Speaker: Victoria Toptun (European Southern Observatory (ESO))

26 SRG/eROSITA’s insights on the hot phase of the Circumgalactic medium in Milky-Way-sized halos: modelling and interpretation

The CGM's hot phase probes the modern puzzles of galaxy formation and evolution as galaxies' feedback processes imprint their signatures in its volume-filling hot gas distribution. This talk presents state-of-art observed hot gas profiles of the CGM in Milky-Way-sized halos, obtained with the depth of eRASS:4 and the large area optical coverage of the LSS DR10 galaxy survey. Together with other observational results detecting hot CGM radial profiles with eROSITA (Zhang et al. 2024a,b), we present a new empirical model for interpreting the soft X-ray observations of the CGM's hot phase ($>10^6$ K). The model considers emission in the X-ray wavelength range from the hot gas surrounding galaxies, the population of X-ray binaries in galaxies and their AGN. Furthermore, I will present the importance of accounting for all observed projection effects, such as (1) large-scale structure along the line of sight, (2) contamination by mis-centring on satellite galaxies, and (3) contribution of point sources within the two-halo terms. This paves the way for enhancing the interpretability of emission studies of the hot CGM with current and future X-ray observations. Given the observational benchmarks with the modelling technique presented here, we also show how future state-of-the-art hydro-dynamical simulations could use our results to calibrate gas properties in the Milky Way mass halo regime.

Speaker: Soumya Shreeram (Max Planck Institute for Extraterrestrial Physics)

Poster Viewing

X-ray emission around galaxies from CGM to WHIM

27 eRASS: Stacked WHIM Emission from Long Cosmic Filaments

The warm/hot intergalactic medium (WHIM) within cosmic filaments is one of the least well-characterized baryon repositories in the local Universe. The extremely weak signals in either X-rays or the Sunyaev-Zeldovich effect challenge its robust detection. We utilize SRG/eROSITA All-Sky Survey data to examine X-ray emission from >20 Mpc long cosmic filaments. We detect the total X-ray emission spatially coincident with cosmic filaments traced by galaxy distribution with a high significance. With a further modeling of undetected active nuclei and circumgalactic medium X-ray emission, we disentangle the WHIM emission from the emission that is associated with galaxies. We also stack a broadband 100-eV resolution spectrum of cosmic filaments and probe the physical properties of the detected WHIM.

Speaker: Xiaoyuan Zhang (MPE)

Exoplanets and Solar System

28 Heliospheric X-ray emission in the SRG/eROSITA all-sky surveys

The discovery of cometary X-ray emission has revealed the efficiency of charge exchange for the generation of soft X-rays. As this process is characterized by very high cross sections, even tenuous traces of gas can become a source of soft diffuse X-ray emission when exposed to highly ionized solar wind. Such gas is not only present in Earth's exosphere but is also found throughout the solar system, in the form of interstellar particles streaming through it.

Thus, the X-ray glow of geocoronal and heliospheric gas may be present in any observation, affecting all studies of the local hot bubble, the interstellar medium, the galactic corona, the circum-galactic medium, and the cosmic X-ray background. After this fundamental problem had been realized, many attempts have been made to identify and isolate the geocoronal and heliospheric components, but they all face severe limitations, leading to uncertainties which propagate into many studies of the diffuse X-ray emission from beyond. Now, however, a major step forward has been achieved with SRG/eROSITA.

Thanks to its privileged location well outside Earth's exosphere, we see with eROSITA for the first time ever a sky that is free of geocoronal X-ray emission. Furthermore, by having mapped the full sky repeatedly and almost continuously with high grasp for more than two years, eROSITA data have opened up a new window for heliospheric studies. We will show how we utilize this unique treasure for reliably isolating the heliospheric X-ray emission and for investigating its temporal, spatial, and spectral properties in unprecedented detail.

Speaker: Konrad Dennerl

eROSITA conference: Social Dinner

Wednesday, 18 September

X-ray View of the Milky Way: Compact Objects

29 The search for X-ray dim isolated neutron stars in the SRG/eROSITA All-Sky Survey

X-ray dim isolated neutron stars (XDINSs) are famous for their amazingly clean thermal X-ray emission. The small known population that comprises only a handful sources, however, demands new identifications before their population properties, evolution, and links to other isolated neutron star (INS) families can be constrained. With the potential to constitute a significant fraction of all Galactic INSs, new members of the elusive XDINS class are ought to be hiding among the myriad of X-ray sources that are detected in the SRG/eROSITA All-Sky Survey (eRASS). In this talk, we will present the strategy to identify new XDINSs in the eRASS, discuss the selected candidates that will form the foundation of a representative sample of these objects and report the results of recent observations with NICER, XMM-Newton and the ESO-VLT that already confirm the thermal INS nature for several candidates.

Speaker: Jan Kurpas (Leibniz-Institut für Astrophysik Potsdam)

30 Whispering in the dark: X-ray faint black holes around OB stars

Despite the potential of GAIA DR3 to reveal a large population of black holes (BHs), only a few BHs have been discovered to date in orbit with luminous stars without an X-ray counterpart. It has recently been shown that black holes in orbit with main sequence companions seldom form accretion disks, from where observable X-ray flux is conventionally thought to be produced. Yet, even without accretion disks, dissipative processes in the hot, dilute and magnetized plasma around the BH can lead to radiation. For instance, particles accelerated through magnetic reconnection can produce non-thermal emission through synchrotron. We study the X-ray luminosity from this large unidentified population of black holes using detailed binary evolution models computed with MESA, having initial donor masses from 10-90 Msun and orbital periods from 1-3162 d. A significant fraction (0.1% to 50%) of the gravitational potential energy can be converted into non-thermal radiation for realistic particle acceleration efficiency. A population synthesis analysis predicts at least 28 BH+OB star binaries in the Large Magellanic Cloud (LMC) to produce X-ray luminosity above 10**31 erg/s, observable through focused Chandra observations. We identify a population of observed SB1 systems in the LMC comprising O stars with unseen companions above 1.8 Msun that aligns well with our predictions of the orbital period and luminosity distribution of faint X-ray emitting BH+OB star binaries. The peak in the luminosity distribution of OB companions to these faint X-ray-emitting BHs lies around log(L/LSun) ∼ 4.5-5. Finally, the X-ray luminosity from hot accretion flows around the faint BH can be ∼one order of magnitude above the typical X-ray luminosity expected from embedded shocks in the stellar wind of the OB star companion.

Speaker: Koushik Sen (Nicolaus Copernicus University Torun)

31 The population of high-mass X-ray binaries in the LMC detected during the first eROSITA all-sky survey

The Magellanic Clouds are our closest star-forming galaxies with low Galactic foreground absorption. This makes them a unique laboratory to study the population of high-energy sources. The SMC hosts a large population of Be/X-ray binaries associated with high star formation activity 25-40 Myr ago. It has been proposed that the HMXB population in the LMC is associated with more recent star formation. However, due to the large angular extent and resulting insufficient coverage of the LMC, this association with SFR is not well established yet.

An essential asset for studying the HMXB population in the entire LMC was the launch of eROSITA. eROSITA scans the sky in great circles crossing at the ecliptic poles. Due to the vicinity of the south-ecliptic pole, sources in the LMC are monitored for up to several weeks during each all-sky survey, leading to a deep total exposure and the possibility of studying long-term temporal behaviour. This allowed us to discover several new HMXBs, verify candidate HMXBs and construct a complete, flux-limited catalogue. During my presentation, I will first focus on HMXB population properties in the LMC. Then I will discuss individual systems we discovered with eROSITA, such as a Be-WD and an SFXT candidate.

Speaker: David Markus Kaltenbrunner (Max Planck Institute for extraterrestrial Physics)

32 First Study of the SNR Population in the LMC with eROSITA

The supernovae (SNe) explosions expel the stellar interior of the star in the surrounding which enrich the interstellar medium (ISM) with metals. The interaction between the stellar interior and the ISM produce the supernova remnants (SNRs).. Their emission is visible in different wavelength from radio to X-ray. The SNRs can be studied to infer information about the explosion it self and on the property of the surrounding ISM. The SNR are one of the main responsible for injecting energy into the ISM and therefore influence the evolution of the entire galaxy. A complete sample of SNR inside a galaxy is important to understand the chemical enrichment and the energy budget inside such a galaxy.
The best laboratory for the study the SNRs population in a galaxy is the Large Magellanic Cloud (LMC). The LMC is the nearest star-forming galaxy with low absorption along the line of sight.
The eROSITA telescopes are the best instrument available to make such a survey thanks to the large field of view and the high sensitivity in the softer part of the X-ray emission. We present the initial results from eROSITA data collected to inspect the SNR population in the LMC in the X-ray band. The complete coverage of the LMC and its surronding provided by eROSITA we investigate the very recent SNR candidate detected in the radio band using ASKAP interferometry, among the other SNR candidates proposed in radio and optical. Furthermore we present the detection of new SNR candidates never observed by other X-ray telescope before. Of particular interest is the increasing population of SNR detected outside of the main body of the galaxy which have been recently followed up by deep observation of XMM-Newton satellite.

Speaker: Federico Zangrandi (Dr. K. Remeis (FAU))

Poster Viewing: Coffee break

Cluster Surveys

33 Update on the South Pole Telescope Cluster Samples

The South Pole Telescope (SPT) is a 10-meter millimeter-wavelength telescope located at the geographic South Pole, one of the world's premier sites for millimeter-wave observations. The SPT has been used to conduct several generations of wide-field high resolution cosmic microwave background (CMB) surveys including the 2500-square-degree SPT-SZ survey, the SPTpol 500d and ECS surveys, and now the 10,000 square-degree SPT-3G survey.

One of the primary objectives of these surveys has been the construction of mass-limited samples of galaxy clusters identified via the thermal Sunyaev- Zel'dovich (SZ) effect, through which massive clusters imprint subtle temperature distortions on the CMB. The abundance of such clusters is a powerful cosmological probe as it depends sensitively upon both the expansion history of the universe and the growth of density fluctuations. In this talk I will discuss these datasets, including progress on the SPT-3G cluster sample. I will also highlight areas of synergy with eROSITA.

Speaker: Lindsey Bleem (Argonne National Laboratory)

34 The connectivity-mass relation of eROSITA clusters and groups

The cosmic web is a network of structures composed of dark matter, galaxies, and gas. In this network, galaxy clusters and groups occupy the position of nodes, connected among them by filaments. Connectivity is a measurement of the number of filaments connected to a node of the cosmic web, be it a cluster or a group. Theoretical works predict a relationship between the connectivity of a node and its mass, with more connected structures also being more massive. Connectivity, thus, has a strong importance to study the accretion of matter on clusters, structure growth, and cosmology. I will describe our measurement of the connectivity-mass relation obtained with the eRASS:1 cluster sample extracted from the eROSITA data and the filament samples detected with DisPerSE in spectroscopic surveys (GAMA) and with photometric redshifts (DESI Legacy Surveys). We characterize the relation, in an attempt to identify a change of slope to identify the transition regime from clusters to groups and to provide a fit to the relation to be used for further studies and comparison with numerical simulations.

Speaker: Nicola Malavasi (Max Planck Institute for Extraterrestrial Physics)

35 Characterizing galaxy clusters in the eRASS1 point source catalog

The X-ray-emission of high-redshift galaxy clusters and compact nearby groups hosting bright, active galactic nuclei (AGN) can be misclassified as point source emission by the source detection algorithms due to the sizeable point-spread function of eROSITA.
In an analogous way to the treatment of the extended source catalog, we search for clusters in the eROSITA point source catalog. For this, we scan all positions of X-ray point sources with a high detection likelihood in the optical and near-infrared DESI Legacy Imaging Surveys data using the eROMaPPer algorithm. This procedure finds red-sequence galaxy overdensities at $\sim 16\,000$ locations after removing the lower richness ($\lambda \leq 16$) candidates and likely duplicates.
To characterize these sources, we additionally consider the optical and near-infrared counterparts to the X-ray point sources under the assumption that they are genuine point sources (e.g., AGN) identified via the NWAY algorithm, and compare the photometric cluster redshifts to the photometric redshifts of these single-source counterparts.
Of the $\sim14\,000$ point source cluster candidates with counterparts having sufficient photometry, we note that $\sim 11\,500$ are not yet listed in the literature.
We suspect that a high amount of genuine clusters in the higher redshift regime that would normally lie below the detection limit of eROSITA is found here because the X-ray emission of a coincident genuine point source along the line of sight boosts it.
In this talk/poster, I will present the properties of the clusters misclassified as point sources in the eROSITA X-ray catalog, and conclude by showcasing some of the most interesting candidates.

Speaker: Fabian Balzer (Max Planck Institute For Extraterrestrial Physics)

36 Optical Identification and Properties of Galaxy Clusters and Groups in the Western Galactic Hemisphere

The first SRG/eROSITA All-Sky Survey (eRASS1) provides the largest intracluster medium-selected galaxy cluster and group catalog covering the western Galactic hemisphere. Compared to samples selected purely on X-ray extent, the sample purity can be enhanced by identifying cluster candidates using optical and near-infrared data from the DESI Legacy Imaging Surveys. Using the red-sequence-based cluster finder eROMaPPer, we measured individual photometric properties (redshift $z_\lambda$, richness $\lambda$, optical center, and BCG position) for 12 000 eRASS1 clusters over a sky area of 13 116 deg$^2$, augmented by 247 cases identified by matching the candidates with known clusters from the literature. The median redshift of the identified eRASS1 sample is $z=0.31$, with 10% of the clusters at $z>0.72$. The photometric redshifts have an accuracy of $\delta z/(1+z)\lesssim0.005$ for $0.050.05$. For these and further quality assessments of the eRASS1 identified catalog, we applied our identification method to a collection of galaxy cluster catalogs in the literature, as well as blindly on the full Legacy Surveys covering 24 069 deg$^2$. Using a combination of these cluster samples, we investigated the velocity dispersion-richness relation, finding that it scales with richness as $\log(\lambda_{\rm norm})=2.401\times\log(\sigma)-5.074$ with an intrinsic scatter of $\delta_{\rm in}=0.10\pm0.01$ dex. The primary product of our work is the identified eRASS1 cluster catalog with high purity and a well-defined X-ray selection process, opening the path for precise cosmological analyses presented in companion papers.

Speaker: Matthias Kluge (MPE)

37 eROSITA cross-calibration with Chandra and XMM-Newton using galaxy cluster gas temperatures

Galaxy cluster gas temperatures ($T$) are crucial for numerous cosmological and astrophysical applications. Potential $T$ biases can propagate to several such cluster applications. Thus, it is important to accurately cross-calibrate X-ray instruments to account for systematic biases. We present the first cross-calibration between eROSITA-Chandra, and between eROSITA-XMM-Newton, using a large sample of galaxy cluster $T$. To do so, we use the eRASS1 data to spectroscopically measure X-ray $T$ for 186 independent cluster regions with both eROSITA and Chandra for three energy bands; 0.7-7 keV (full), 0.5-4 keV (soft), and 1.5-7 keV (hard). We do the same with eROSITA and XMM-Newton for 71 different cluster regions and all three bands. We find that eROSITA measures systematically lower $T$ than the other two instruments, with hotter clusters deviating more than cooler ones. For the full band, eROSITA returns 20$\%$ and 14$\%$ lower $T$ than Chandra and XMM-Newton respectively, when the two latter instruments measure $k_{\text{B}}T\approx 3$ keV each. The discrepancy increases to 38\% and 32\% when Chandra and XMM-Newton measure $k_{\text{B}}T\approx 10$ keV respectively. Moreover, a broken power law fit demonstrates a break at the eROSITA-Chandra scaling relation at $k_{\text{B}}T\approx 1.7-2.7$ keV. The soft band shows a marginally lower discrepancy than the full band. TIn the hard band, the cross-calibration of eROSITA and the other instruments exhibits substantial differences. We tested several possible systematic biases to identify the reason behind the $T$ discrepancies but none could significantly alleviate the tension. Most importantly, we simulated several clusters with different 3D profiles for gas density and $T$ and fitted their projected spectra with all three instruments. We found no expected $T$ discrepancies due to the presence of multitemperature gas and the different effective areas of the telescopes. For now, it is most likely that the systematically lower eROSITA $T$ can be attributed to systematic effective area calibration uncertainties.

Speaker: Konstantinos Migkas (Leiden University)

12:40 Lunch break

Cosmology

38 eROSITA cosmology

Galaxy groups and clusters trace the distribution of the most prominent peaks in the matter density field at late time. Therefore, they provide valuable insights into the growth of structure in our Universe, the nature of dark matter, and, in general, the cosmological parameters that describe the content of our Universe and govern its formation and evolution. The primary science goal of eROSITA, on board the SRG Mission, launched in 2019, is to perform a precision cosmology experiment through the evolution of cluster mass function. I will present the cosmological constraints from the 5259 clusters of galaxies securely detected and optically confirmed in the area of 13791 deg^2 of the Western Galactic Hemisphere covered by Legacy Survey DR10-South. The overlap of 4968 deg^2 containing 2348 clusters between eROSITA survey and DES, KiDS, and HSC are used to perform mass calibration using the weak gravitational lensing effect. In particular, we have tested four different cosmological models: the standard LCDM, wCDM where we fit for dark energy equation of state, nuCDM where we fit for the right-handed neutrino summed masses, and the nuwCDM where we fit for both dark energy equation of state and summed neutrino masses. I will present our recent work from the galaxy clusters catalog, to the optical confirmation and redshift measurments, to the cosmological results and focus particularly on the key aspects of this analysis that allow us to perform a precision cosmological experiment using cluster number counts.

Speaker: Vittorio Ghirardini (Max Planck Institute for extraterrestrial physics)

39 From the largest structures to the tiniest particles: Constraining ultra-light axions with eRASS:1 galaxy cluster number counts

Axion-like particles are viable dark matter candidates that would fit naturally in the concordance $\Lambda$CDM cosmological model. As bosonic particles, they form Bose-Einstein condensates with scales determined by their thermal de Broglie wavelength. Due to their extremely small masses, ultra-light axions with $10^{-22}$ eV masses can form condensates on scales comparable to dark matter halos. If their mass is even smaller, around $10^{-33}$ eV, they exhibit a slow roll behavior and effectively behave like dark energy. Both the dark matter and dark energy regimes of axionlike particles can be constrained by using clusters of galaxies as tracers for the highest peaks in the late-time matter density field. With its 5259 securely detected and optically confirmed galaxy clusters, eROSITA on board the SRG Mission, which was launched in 2019, offers the possibility of precision cosmology by using the halo mass function. We use the observed cluster abundance to constrain a cosmology including an ultra-light axion species with a certain mass and dark matter abundance. Selection effects are fully accounted for, while the mass calibration is performed with the weak gravitational lensing data from the DES, KiDS, and HSC surveys. I will present the constraints on ultra-light axion mass and abundance as well as the cosmological parameters obtained using the first All-Sky Survey of eROSITA in the Western Galactic Hemisphere.

Speaker: Silas Zelmer (Max Planck Institute for Extraterrestrial Physics)

40 New measurement of the growth of structures with eROSITA cluster abundance

Several tensions are currently challenging the standard cosmological model. Among them, the Hubble constant measured at early times is lower than the one inferred from late-time probes. Additionally, Planck indicates cosmological perturbations growing faster than what is inferred from large-scale structure probes. Consequently, it is fundamental to investigate potential deviations from its prediction and potential alternative theories.
In this paradigm, the evolution of the cluster mass function traces the growth
of the linear density perturbations and can be used as a probe for potential new physics. We present new constraints on deviations from general relativity by measuring the growth rate of structures and by investigating the Hu-Sawicki parametrization of f(R) gravity with the first SRG/eROSITA All-Sky Survey (eRASS1) cluster catalog in the Western Galactic Hemisphere in combination with the overlapping Dark Energy Survey Year-3, KiloDegree Survey, and Hyper Supreme Camera data for weak lensing mass calibration. We find a strict upper limit on the parameter log |fR0| < -4.12 jointly with a sum of the masses of the neutrinos smaller than 0.44 e.V. at a 95% confidence level. It is the first time that constraints are obtained from clusters only.
We then parameterized the growth factor with the cosmic linear growth index gamma, which fits LambdaCDM predictions well when gamma = 0.55. We present new constraints from cluster abundance by exploiting the statistical constraining power of eRASS1, the largest intra-cluster medium selected cluster sample to date.
We find a higher growth index than the general relativity prediction, in agreement with other large-scale structure probes. Those constraints suggest that eRASS1 clusters are better fitted with a reduction of the amplitude of the power spectrum.
We finally perform a direct measurement of the growth of structures by dividing the sample into five redshift bins containing the same number of clusters and measuring the cosmological parameters in each bin. In agreement with the cosmic linear growth index parameterization, we find that structures evolve slower than the predictions of the Planck satellite. Our results align with other cluster surveys but are tense with weak lensing shear predictions.
Deeper surveys with eROSITA will disentangle the origin of these findings and if the higher value of the cosmic linear growth index originates from a change in fundamental physics.

Speaker: Emmanuel Artis (Max Planck Institut for Extraterrestrial Physics)

41 Investigating the sigma8 cosmological tension with galaxy cluster count

Recent measurements of cosmological parameters highlight possible tensions between values determined by CMB measurements and those determined by late epoch observations which could potentially challenge the current theoretical model, in particular for the clumpiness parameter σ8 which is related to the normalization of the primordial matter power spectrum. In this project, we investigate this tension by constructing a forward cosmological modeling pipeline to determine this parameter from galaxy cluster counts using simulation based inference. The implemented pipeline generates samples of galaxy clusters from the halo mass function and includes all the relevant observational effects, which allows us to apply the exact selection function of a survey. We go beyond examining the redshift and flux distributions of clusters by incorporating as well their temperature distribution, which allows to some extent to break the degeneracy between Ωm and σ8. We then train a convolutional neural network to learn the mapping between model parameters and observed data. Finally, we apply our framework to the measured distributions of redshifts, fluxes, and temperatures in the XXL survey to obtain the parameters that best reproduce the observed sample. Our simulation based inference framework can be readily applied to the new large-scale galaxy clusters surveys such as eROSITA, Euclid and SPT-3G.

Speaker: Ms Manon Regamey (Unige)

42 Cosmological constraints from the Planck cluster catalogue with new multi-wavelength mass calibration

To compare the observations of galaxy clusters with theoretical predictions and thus constrain the cosmological parameters of the underlying model, precise knowledge of cluster masses and redshifts is required. We provide a new $Y_{\text{SZ}} - M_{500}^{Y_X}$ scaling relation using a sample of clusters from the Planck Early Sunyaev-Zeldovich (ESZ) catalogue that was observed in X-rays by Chandra, and compare it to the results of the Planck collaboration obtained from XMM-Newton observations of a subsample of the ESZ. We calibrate a mass bias for the new scaling relation as well as that from the Planck collaboration with a subset of the Planck cosmological cluster sample using published weak-lensing data from the Canadian Cluster Cosmology Project (CCCP) and Multi Epoch Nearby Cluster Survey (MENeaCS). We propose a novel method to account for selection effects . With these mass biases, we obtain $Y_{\text{SZ}} - M_{500}$ scaling relations that we apply to the full Planck cosmological cluster sample, to obtain new constraints on the cosmological parameters. We also provide constraints with a redshift evolution of the scaling relation fitted from the data instead of fixing it to the self-similar value. We find a redshift evolution significantly deviating from the self-similar value, leading to a 1.5$\sigma$ shift of $S_8$. We compare our results to those from recent analyses based on various cosmological probes.
We also plan to have some results using DES data to calibrate the mass bias, and to compare those with the results obtained with CCCP and MENeaCS.

Speaker: Gaspard Aymerich (IAS, Université Paris-Saclay)

Poster Viewing

Physics of Galaxy Clusters and Groups

43 SRG/eROSITA: View of the Virgo Cluster

eROSITA allows us to resolve the entire Virgo Cluster and its outskirts on scales between 1 kpc and 3 Mpc, covering a total area on the sky of about 25 by 25 degrees. We present an exploration of the SRG/eROSITA data of the Virgo Cluster from five all-sky surveys. We utilize image manipulation techniques and surface brightness profiles to search for extended emission, surface brightness edges, and features in the outskirts. We employ a method of comparing mean and median profiles to measure gas clumping out to and beyond the virial radius. Surface brightness analysis of the cluster and individual sectors of the cluster reveal the full extent of previously identified cold fronts to the north and south. The emissivity bias due to gas clumping, which we quantify over three orders of magnitude in radial range, is found to be mild, consistent with previous findings. We find uniform clumping measurements in all directions, with no enhancements along candidate filaments.

Speaker: Hannah McCall (University of Chicago)

44 The cosmic web in X-rays and the radio

Cosmological simulations predict the presence of warm, thermal gas in the cosmic filaments that connect galaxy clusters. This gas is thought to constitute an important part of the missing baryons in the Universe. In addition to the thermal gas, radio observations show that cosmic filaments between close pairs of galaxy clusters are filled with a population of relativistic particles and magnetic fields. We present our search for thermal and non-thermal diffuse emission from inter-cluster regions using a combination of eROSITA and radio telescopes. Our search involves stacked data as well as deep observations. We discuss our results in the context of the models for the formation of magnetic fields in cosmic filaments, the implications for particle acceleration and the comparison to cosmological simulations.

Speaker: Marcus Brüggen (University of Hamburg)

45 The SRG/eROSITA All-Sky Survey: View of the Fornax galaxy cluster

The Fornax cluster is one of the most nearby X-ray bright galaxy groups. Its proximity allows us to study its properties at high spatial resolution. We exploit the essentially unlimited field-of-view of five eROSITA all-sky surveys to study the Fornax cluster from kpc to Mpc scale, tracing the intracluster medium out to beyond the virial radius. We interpret the observations in combination with the distribution of globular clusters, dwarf galaxies, and HI-tail galaxies.

Speaker: Prof. Thomas Reiprich (Argelander-Institut für Astronomie, Universität Bonn)

Thursday, 19 September

Cluster Surveys

46 eROSITA X-ray properties of galaxy clusters selected with different methods

Galaxy clusters are the largest gravitationally bound systems in the universe. Most of the baryonic matter in clusters is found not in the stars and galaxies, but in the surrounding X-ray emitting intra-cluster medium (ICM). Using observations of the ICM we can determine key properties of these clusters such as their mass, temperature, luminosity, and density profiles. However, statistical studies of ICM properties may be biased when clusters are selected based on ICM properties (i.e. via X-rays or the Sunyaev-Zel'dovich effect). For example, recent work has found that using an optically selected sample of clusters returned a shallower and more scattered $L-M$ scaling relation than what is recovered with traditional selection techniques. We will present a study of the X-ray properties of galaxy clusters selected with different methods to explore the impact of these biases. The breadth of the all-sky eRASS1 data provides an ideal test bed for this investigation as it provides automatic X-ray coverage of large samples of clusters detected by different techniques.

Speaker: Joseph Hall (University of Bristol)

47 Dark Energy Survey optical cluster calibration using the eROSITA All-Sky Survey

In this talk I will present the cross-match of Dark Energy Survey (DES) redMaPPer clusters with the eROSITA All-Sky Survey (eRASS), which offers an unprecedented ~4000 deg$^2$ area of overlap between the DES and eRASS survey footprints. Previous efforts of cross-matching DES RM clusters with various X-ray archives, e.g., the XMM Cluster Sample, resulted in a few hundred clusters detected in non-contiguous survey overlaps. With the public release of the first eRASS data, we detail the construction of the cross-matched sample delivering ~1700 RM clusters with a corresponding eRASS confirmed cluster detection. Using this sample, we will investigate initial forms of the mis-centering distribution and scaling relations of this sample, with particular emphasis on lower mass galaxy group scale systems. Finally, we outline how these results can impact our assumptions in preparation for the cluster samples constructed from the Legacy Survey of Space and Time.

Speaker: Paul Giles (University of Sussex)

48 X-ray properties of optically selected galaxy clusters: a joint study of eROSITA and Subaru telescopes

The SRG/eROSITA all-sky survey delivers an unprecedented X-ray scan of the whole sky, which offers a great opportunity for large-scale structure studies. At the same time, the Subaru telescope is amongst the deepest and most powerful optical instruments, which provides a large selection of optically selected galaxy clusters via the detection of the overdensities of the red sequence galaxy population. We present a study focusing on the X-ray properties of optically selected galaxy clusters, leveraging data from the 140 deg2 field eROSITA Final Equatorial Depth Survey (eFEDS) and the Subaru telescope (Nguyen-Dang et al in prep.; Ota et al. 2023). Our analysis encompasses a sample of nearly 1000 optically selected galaxy clusters, offering insights into their X-ray characteristics. Notably, only about 20% of these optically selected clusters are detected in X-rays, motivating the application of stacking analysis to investigate the properties of faint X-ray clusters. Our findings reveal significant differences between the X-ray properties of optically selected clusters and those of X-ray-selected samples. Additionally, we explore the dynamical state of these clusters and establish relationships between their luminosity, richness, and weak-lensing mass. Our results contribute to understanding cluster selection effects and inform cosmological models concerning the formation and evolution of galaxy clusters.

Speaker: Nhan Nguyen-Dang (University of Tuebingen)

49 The morphology of clusters in the first eROSITA sky survey

Clusters of galaxies are usually not described by simple symmetric beta models. Their morphologies are important for a number of reasons. Many physical processes talking place inside or between clusters impacts their morphology. For example, merging clusters create disturbed morphologies and cool core clusters containing AGN feedback have steeply-peaked surface brightness profiles. It is therefore important to have a good understanding of cluster morphology distributions to study their evolution and physical processes. In addition, cluster morphology impacts how they are selected in surveys such as the eROSITA all-sky survey, as it becomes easier or harder to distinguish them from point sources or background fluctuations. We describe the results of our analysis of the morphology of over 12 thousand clusters from the first eROSITA all-sky survey (eRASS1). We develop new forward-modelled parameters for characterising how disturbed clusters are, including a "slosh" parameter. We also apply existing measurements of disturbance, including concentration and inner density slopes, to our sample of clusters, taking account of the instrument PSF and background when possible. The result of this is a catalogue of parameters for each detected cluster in eRASS1. We model how these parameters impact the selection of clusters in the survey. We investigate the underlying scaling relation of several of these parameters, taking into account the effects of selection. We develop a new combined disturbance parameter. We compare our findings to previous analyses of the morphology of clusters.

Speaker: Jeremy Sanders (MPE)

Poster Viewing: Coffee break

AGN feedback

50 Incidences and energetics of AGN winds in a distant universe

In the context of an evolutionary model, the outflow phase of an active galactic nucleus (AGN) occurs at the peak of its activity, once the central supermassive black hole (SMBH) is massive enough to generate sufficient power to counterbalance the potential well of the host galaxy. This outflow feedback phase plays a vital role in galaxy evolution. I will present results from our two recent studies. In the first study, we develop an approach to select powerful AGNs in the feedback phase using optical/IR colours, and optical and X-ray spectral properties from the eROSITA Final Equatorial-Depth Survey (eFEDS). We trace and characterise outflows using SDSS spectroscopy, and explore the link between AGN luminosity and outflow properties. We find that the X-ray selection (eROSITA) is a powerful tool to select AGN in the feedback phase and this X-ray active phase is the best tracer of fast winds. We find a weak correlation between AGN bolometric luminosity and outflow velocity and ~30% of our sample have kinetic coupling efficiencies within 1-10%. In the second study, we perform a spatially resolved analysis of a red, X-ray obscured and X-ray luminous quasar, ID608 at a redshift of z=0.6031. Our analysis reveals that the quasar resides in an interacting system with 3 companion galaxies with outflows that extend up to 9.5 kpc and move at high velocities exceeding 1000 km/s.

Speaker: Blessing Musiimenta (University of Bologna)

51 The wind properties of the eROSITA-selected SDSS-V quasar sample

Quasar winds blown from the accretion disc are potentially capable of expelling large quantities of gas from their host galaxies and may thus be an important mechanism for feedback. These winds are evident in rest-frame UV spectra in the form of blueshifted broad emission lines. How such winds are launched – and how such launching relates to the physical state of the accretion disc system that powers quasars – remains a key open question. SDSS-V follow-up of eROSITA sources is yielding a large quasar sample for which we have access to their wind properties and (via measurements of the X-ray, UV and optical emission) constraints on their accretion state. I will present new measurements for a sample of redshift 1.5-3.5 quasars that quantify how the strength of quasar accretion disc-winds (from the CIV emission line) depends on both the strength of the UV ionising continuum (probed via HeII emission) and the X-ray properties derived from the eROSITA spectra (i.e., photon index and $N_H$ column density). We show that quasars with a given X-ray luminosity have a broad range of wind properties that appear to depend on the presence (or lack) of a strong, UV-bright inner accretion disc (revealed by the HeII emission line strength). A strong UV component can lead to over-ionisation of the wind such that the outflow strength is decreased. These results point to radiation-driven winds whose strength and presence are highly sensitive to the physical structure of the accretion system. With this information in hand we can investigate what, if any, X-ray conditions are required for driving winds and gain a new perspective of the wind properties of the X-ray selected quasars.

Speaker: Amy Rankine (University of Edinburgh)

52 Constraining CGM physics with eRASS and CAMELS

The circumgalactic medium (CGM) plays a key role in understanding the fundamental processes driving galaxy formation and evolution, such as feedback from supernovae (SN) and active galactic nuclei (AGN). The recent eRASS observations of X-ray surface brightness profiles and X-ray luminosity scaling relations at the CGM scale, present an exceptional opportunity for understanding CGM physics. Using the CAMELS simulation suite, which encompasses state-of-the-art cosmological simulations (IllustrisTNG, SIMBA, and Astrid) with varying SN and AGN feedback, we compare the the CGM X-ray surface brightness profiles and X-ray luminosity scaling relations with those from eRASS. Using emulators of the X-ray surface brightness profiles and scaling relations, we perform statistical inferences to obtain constraints on SN and AGN feedback physics. Our findings have important implications for CGM modeling in cosmological simulations, and pave the way for future multi-wavelength missions, such as CMB-S4, Athena, and LEM, to further improve our understanding of the CGM.

Speaker: Erwin Lau (CfA)

12:30 Lunch Break

X-ray View of the Milky Way: Stars

53 eROSITA and the stars

The eROSITA all-sky survey (eRASS) largely increased the number of well-characterized X-ray detected stars.
I will present our method, called HamStar, to identify these stars among all eROSITA sources. HamStar assigns
each eROSITA source a value, p_stellar, that describes the individual probability of the source to be stellar
in nature. HamStar uses a Bayesian-framework taking advantage of supplementary
all-sky information, mainly Gaia data, which allows us to generate samples with well-defined properties,
e.g., completeness and purity. I will describe the resulting sample properties
of the 140,000 stellar sources in eRASS1 and briefly provide an outlook to eRASS:4:5.
Finally, I will demonstrate how we use the HamStar results to discover new associations of
young stars in the solar neighborhood and how this will influence our picture of stellar evolution
in the critical time when planets are shaped.

Speaker: Christian Schneider (Hamburg Observatory)

54 Investigating the Solar-Stellar Analogy: The flare star AD Leo compared to our Sun

The solar corona is often invoked as a template for stellar ones, but the significant difference between solar and non-solar instruments and data makes direct comparison between X-ray observations of the Sun, which is usually of the resolved solar disk, and stellar point-source observations almost impossible. In order to overcome this hurdle, the research group at INAF Osservatorio Astronomico di Palermo has devised a method in which solar X-ray data is converted to a format which is virtually identical to that of actual stellar X-ray observations (called the Sun-as-an-Xray-star, SaXS, method; e.g., Peres+2000, ApJ 528).

First applications of the SaXS method have used a grid of synthetic "stellar-like" X-ray spectra based on emission measures for different types of solar coronal structures like background corona, active regions, cores of active regions and flares. The target star's coronal filling factor with these regions was then found by finding the grid-point closest to the observed X-ray spectrum of the star (e.g., Coffaro+2020). We have now further developed this method into spectral models using XSPEC that correspond to the different solar magnetic structures (background corona, active regions, cores of active regions and flares). Using these models, the hypothetical filling factors of these regions can be recovered by applying the fitting procedure in XSPEC.

For the first time, we apply the SaXS method to a star which significantly differs from our Sun, AD Leo, in order to investigate how far the postulated solar-stellar analogy can be stretched. The early-M dwarf AD Leo is the ideal benchmark for stellar activity in the low-mass regime and its influence on planet atmospheres due to its proximity (5 pc) and high activity level, allowing for high-signal X-ray observations. In this project, we aim to reconstruct an X-ray corona of AD Leo, assuming it is covered by solar magnetic structures. We apply the XSPEC implementation of the SaXS method to AD Leo spectra from eROSITA and XMM-Newton, and investigate the results of its application.

Speaker: Wilhelmina Maryann Joseph (Institut fuer Astronomie und Astrophysik Tuebingen (IAAT))

55 Systematic study of X-ray flaring in the benchmark planet host Proxima Centauri

Proxima Centauri has long been known as a flare star and, due to its proximity, is a very bright X-ray source. With the discovery of a planet in its habitable zone in 2016, Proxima Centauri has become a key target for studies of stellar activity and its impact on planets.
In this talk, I present a comprehensive study of the X-ray variability of this M dwarf with a dataset consisting of dedicated observations made with XMM-Newton from 2001 to 2017 combined with eROSITA data from the four all-sky surveys, counting two years of observations.

I discuss our method to identify the time intervals of quiescent and flaring coronal emission. From time-resolved spectral fitting we obtained the coronal temperature for the full range of activity states. This allowed us for the first time to study the relation between the coronal temperature and brightness on the basis of the variability of a single star. We compare our results to the literature, where time-averaged values for the coronal temperature and surface flux ($T_{\rm corona}$ and $F_{\rm x,surf}$, respectively) were presented for a range of stars from spectral type F to M.
We observe that coronal emission of Proxima Centauri is found at higher temperatures than solar-type stars and that at a specific value of $F_{\rm surf,x}$ or X-ray luminosity ($L_{\rm x}$) it exhibits a spread in $T_{\rm corona}$ (from $\sim 2-15$ MK) probably caused by the amount of electric current involved in the coronal heating mechanism. M dwarfs such as Proxima Centauri are known to have orbiting planets, whose formation and evolution is highly influenced by the host star's X-ray emission. Our results on the $L_{\rm x}-$distribution at different coronal temperatures, or activity levels, of Proxima Centauri play a crucial role in understanding whether the flaring activity of M~dwarfs tends to promote or destroy their habitability.
We also include our previous results for a sample of $10$ M dwarfs detected during eFEDS where we observed that the eROSITA datasets show a gap in the $T_{\rm corona}-F_{\rm surf,x}$ relation that might result from the poor time-sampling of eROSITA light curves. Since eROSITA observes the whole sky in 4-hour scans, i.e. one eRODAy, the chances of keeping track of consecutive (and perhaps fainter) flares emitted by one source is reduced.

Speaker: Enza Magaudda

56 X-ray activity & rotation in the M dwarf 10pc sample

Volume-complete samples are key to characterizing stellar populations. We have set out to constrain the activity and rotation rates of all 150 early-M dwarf stars from the `10pc sample in the era of Gaia' (from Reyle et al. 2021). Hereby, we uncover the full spread that such stars can exhibit, which we find to span three orders of magnitude both in X-ray brightness and rotation period.

At the extremes, we have identified a superflare on AD Leo, that was shown to be 10 000 times more energetic than a typical solar flare, and a star with no persistent X-ray emission, consistent with the properties of a solar coronal hole. To calibrate the M dwarf X-ray activity on our Sun, we use the Sun-as-an-Xray-star approach, which transforms solar Yohkoh observations to stellar-like data.

Our study makes use of a dedicated deep XMM-Newton survey complementing archival X-ray data and the recent eROSITA half-sky surveys, as well as newly determined TESS rotation periods combined with published rotation data. We find the majority of 10pc M dwarfs to be low-activity and slowly rotating stars, in contrast to flux-limited samples of M dwarfs where the majority are `saturated'. This underlines the importance of volume-limited studies for our understanding of biases in larger but incomplete samples.

Speaker: Beate Stelzer (IAAT)

Poster Viewing: Coffee break

The Future in Data Analysis

57 The SRG/eROSITA upper limits of the Galactic Western hemisphere.

We implemented the first half-sky SRG/eROSITA upper limit database to provide X-ray photometric data and flux upper limit to every position in the Western Galactic hemisphere. For variable objects, transients, or a large number of sources detected at wavelengths other than X-rays, eROSITA flux upper limits can be crucial to understanding their physical and statistical properties. Thus, the eROSITA flux upper limits can contribute to important scientific goals such as the investigation of long-term X-ray variability, the search and discovery of new transients, and timing analysis. We provide a detailed description of the process of retrieving SRG/eROSITA upper limits for a large set of input positions, as well as of the eROSITA data, the X-ray aperture photometry, the upper limit calculation via a Bayesian approach, and the final data products. We also characterize the architecture of the database and the web tool, which are designed to handle large queries of input positions.

Speaker: Dusán Tubín (Leibniz Institute for Astrophysics Potsdam)

58 The STATiX view of the transient X-ray sky

Our recently developed Space and Time Algorithm for Transients in X-rays (STATiX, Ruiz et al. 2024) builds upon tools from the image and signal processing fields and in particular the Multi-Scale Variance Stabilisation Transform (Starck et al. 2009) to provide a complete detection analysis pipeline optimised for finding transient sources on X-ray imaging observations. STATiX operates on 3-dimensional data cubes with 2-spatial and one temporal dimensions. Here we will present our first results for the systematic application of STATiX to the full XMM-Newton archive. We will show the rich variety of X-ray sources with interesting temporal behaviour (fast X-ray transients, flaring stars, QPEs, etc) and compare with other efforts to detect and characterize transient X-ray sources (eg. ExTRAS, EXODUS, STONK).

Speaker: Angel Ruiz (IAASARS, National Observatory of Athens)

59 Towards Bayesian Imaging of the eROSITA sky

The Early Data Release and eRASS1 data from the eROSITA space telescope have already revealed a remarkable number of previously undetected X-ray sources. Leveraging Bayesian inference and generative modeling techniques for X-ray imaging, we aim to enhance the sensitivity and scientific value of these observations by denoising, deconvolving, and decomposing the X-ray sky. Utilizing information field theory, we exploit the spatial and spectral correlation structures of various sky components with non-parametric prior models to improve their reconstruction.
By incorporating the instrument's point-spread function, exposure, and effective area information from the calibration database into our forward model, we seek to develop a comprehensive Bayesian imaging algorithm for the eROSITA Western Galactic Hemisphere. This approach aims to enhance the existing X-ray source catalogs therefore advancing our understanding of the X-ray universe.

Speakers: Vincent Eberle (Max Planck Institute for Astrophysics, LMU Munich), Matteo Guardiani (Max Planck Institute for Astrophysics), Margret Westerkamp (Max Planck Institute for Astrophysics; Ludwig-Maximilians-Universität München)

60 X-UBIK - The Universal Bayesian Imaging Kit Applied to X-ray data

The advent of the new generation of instrumentation in astrophysics like eROSITA poses several challenges due to the high-dimensional signals that vary in space, time, and energy. These typically have non-trivial correlation structures and are often a mixture of overlapping signal components that need to be separated. In order to facilitate multi-instrument analysis of correlated signals in general, we are developing the Universal Bayesian Imaging Kit (UBIK), a flexible and modular framework for high-fidelity Bayesian imaging. UBIK is designed to address these challenges using information field theory, which allows the consistent application of Bayesian logic to signal reconstruction, allowing uncertainties to be estimated. In particular, we use generative models to encode prior knowledge about the signals of interest in order to exploit spatial and spectral correlations and thereby improve their reconstruction from noisy data and enhance the component separation. Here, we show the application of UBIK to Poisson-noise-affected merged X-ray data of eROSITA, allowing data sets from different observations to be combined. This provides an enhanced and high quality visualisation of extended sources, point sources and background individually.

Speaker: Margret Westerkamp (Max Planck Institute for Astrophysics; Ludwig-Maximilians-Universität München)

61 Automatic Point Source Detection through Model Stress

For a deep understanding of the X-ray universe, it is crucial to rely on complete and accurate information on its primary constituents. These constituents, such as active galactic nuclei, galaxies, and other compact and diffuse objects display distinct features in the sky and therefore imprint differently on astronomical data. In this work, we leverage these differences to construct statistical models for their a priori independent spatial and spectral distributions in the sky. This not only enhances the overall observation reconstruction, but also allows to segregate the flux of the various components that populate the sky and more accurately study their individual features. Specifically, we introduce a new technique that uses a notion of model stress to automatically detect and separate point-like sources from diffuse, correlated structures. We showcase the benefits of this approach on publicly available eROSITA data.

Speaker: Matteo Guardiani (Max Planck Institute for Astrophysics)

Friday, 20 September

X-ray emission from galaxies

62 X-ray View of the Milky Way: Compact Objects

Compact stellar remnants like white dwarfs, neutron stars and black holes are important probes of matter under extreme conditions of gravity, density, temperature and magnetic fields. A large fraction of these stellar remnants reside in binary systems providing insights into massive star evolution, accretion and some outstanding questions in astrophysics like progenitors of SN 1a and gravitational waves.
A better knowledge of the origin, evolution, and feedback onto the interstellar medium from these systems is also essential to understand the evolution of the Galaxy as a whole. In this talk I will review our understanding of compact stellar remnants in the Milky Way. The field of study has been especially revolutionised with the eROSITA all-sky surveys (eRASSs), that will for the first time unveil the X-ray faint, most populated end of the Galactic population of these compact objects. Their identification has been further facilitated by several multi-wavelength follow-up campaigns and facilities that aided to pinpoint their nature.

Speaker: Chandreyee Maitra (MPE)

63 Ultraluminous X-ray sources in the first eROSITA all-sky survey

Ultraluminous X-ray sources (ULX) are thought to be dominated by the most luminous X-ray binaries, many of which accrete at super-Eddington rates, although some small fraction of the population may harbor intermediate-mass black holes. We present a catalog of ULX candidates identified in the first eROSITA all-sky survey (eRASS1) consisting of 89 strong ULX candidates which we use for further analysis and 260 weaker candidates which require confirmation by dedicated X-ray observations. Contrary to earlier works based on serendipitous detections in pointed observations, our catalog was created from the unbiased X-ray observations performed by eROSITA across all galaxies listed in the Heraklion Extragalactic Catalogue of the eROSITA-DE sky. We estimate the number of unknown background contaminants and the sensitivity of our identification procedure, after removing known contaminants such as foreground stars, AGN, and supernovae. We determine the X-ray luminosity function (XLF) of ULX and compare against extrapolations of the XLFs of other populations such as low- and high-mass X-ray binaries. We determine the fraction of ULXs identified in galaxies of different morphological types and find that ULXs with luminosities below 1e41 erg/s are concentrated in late-type galaxies. As for high luminosity ULXs, we identify an excess in early-type galaxies that cannot be attributed to the high mass X-ray binary population but is expected to arise from low-mass X-ray binaries or the elusive intermediate mass black hole population.

Speaker: Philipp Weber (EACP/FAU)

64 The first all-sky survey of star-forming galaxies with eROSITA: Scaling relations and a population of X-ray luminous starbursts

The eROSITA all-sky X-ray (0.2-8.0 keV) survey, provides the first unbiased census of the X-ray emission of galaxies, allowing us to study the emission from X-ray binaries (XRBs) and the hot interstellar medium in the full range of stellar population parameters present in the local Universe. By combining the updated version of the HECATE v2.0 value-added catalogue of nearby galaxies (z$<$0.048) with the X-ray data obtained from the eRASS1, we study the integrated X-ray emission from normal galaxies as a function of their stellar population parameters (i.e. star formation rate-SFR-; stellar mass-M$_{\star}$-; Metallicity, and stellar population age). This allows us to investigate the L$_{X}$-SFR-M$_{\star}$-Metallicity relation with a larger and less biased sample than any other previous study. Our analysis reveals a sub-population of very X-ray luminous starburst galaxies (up to ~2 dex excess with respect to that expected from the current scaling relations) with higher specific SFRs, lower metallicities, and younger stellar populations. We discuss the role of several different contributors to this excess (e.g. hot-gas, LMXBs, or stochastic sampling of the X-ray binary X-ray luminosity function) as well as the contamination from background AGN, and low-luminosity AGN (including tidal disruption events). These results demonstrate the power of large blind surveys such as eRASS which can provide a more complete picture of the X-ray emitting galaxy population and their diversity. In addition, the more sensitive observations of the eRASS:4, and the inclusion of galaxies beyond the volume covered by the HECATE, allow us to set better constraints on the L$_{X}$-SFR-M$_{*}$-Metallicity relation, and has the prospect of revealing rare populations of objects and recovering unbiased underlying correlations between X-ray emission and host galaxy properties.

Speaker: Elias Kyritsis (Institute of Astrophysics -FORTH/ Physics Department - UoC)

65 Shedding light on quiescent X-ray Binaries through population studies with eROSITA

X-ray binaries (XRBs) are our most accessible way to probe populations of Galactic compact objects in the X-ray regime, and offer crucial constraints on Galactic stellar evolution models. Previously reported distributions of XRBs plateau at intermediate luminosities of $10^{35}$ erg/s, and only reach fluxes down to a few $10^{-12}$ cgs. eROSITA’s improved sensitivity allows us to extend this further by a factor ≥ 10, reaching the little explored low luminosity regime, providing improved constraints on the log(N)-log(S) distributions of both persistently faint XRBs and transients in quiescence.
We present log(N)-log(S) distributions of high mass (HMXBs) and low mass X-ray binaries (LMXBs), as obtained using eROSITA detections of nearly 150 XRBs in the western Galactic hemisphere. We perform detailed comparisons to results from previous missions, which are reproduced after accounting for variability for the first time. We discuss the effects of including candidate XRBs identified during the survey. Since eROSITA detections are a proxy for persistent or quiescent activity, we report on neutron star HMXBs that remain observable at low luminosities even when not in outburst (long considered off states) and further remark on eROSITA’s discovery of several such neutron star HMXBs, providing new test cases for low luminosity accretion.

Speaker: Aafia Zainab Ansar (Dr. Karl Remeis Sternwarte Bamberg & ECAP, FAU)

10:30 Coffee break

X-ray emission from galaxies

66 Classification and study of Galactic symbiotic stars with eROSITA

Symbiotic stars are binary systems in which a compact object, usually a white dwarf, accretes matter from its red giant companion. Theoretical studies suggest a population of 10^4-10^5 symbiotic stars in the Milky Way (e.g, Corradi & Munari, 2003), which is noticeably inconsistent with the observational results of classified symbiotic stars of ~ 400 sources (Akras, 2018). Based on their X-ray properties, symbiotic stars are categorised into four types (luna et al, 2013), and the main accretion mechanism in the majority of symbiotic stars are wind accretion (Bondi-Hoyle; Bondi & Hoyle 1944). This implies a soft X-ray emission <2.0 keV with a luminosity < 10^34 erg/s . Therefore, the eROSITA all-sky survey is an ideal survey to search for missing symbiotic in X-rays. We have performed a multi-wavelength study using data of both eROSITA and available infrared and optical surveys (WISE and 2MASS, Gaia) to search for all candidates for different types of galactic symbiotic stars. Additional UV data (Galex) and deep X-ray observations (XMM-Newton) have been used for the study of peculiar systems.

Speaker: Sara Saeedi (ECAP)

Observatories and their Future

67 Results from SRG/eROSITA - XMM-Newton observations

XMM-Newton observed several SRG/eROSITA detection allowing detailed studies due to the longer exposure time. The talk will present some highlights resulting from such joint studies.

Speaker: Norbert Schartel (ESA)

eROSITA conference: Self-organised break-outs for launching collaborations