Conveners
Active Galactic Nuclei: I
- Elisabetta Cavazzuti (Italian Space Agency)
Active Galactic Nuclei: II
- Elisabetta Cavazzuti (Italian Space Agency)
Active Galactic Nuclei: III
- Eduardo Ros (MPI für Radioastronomie & Universitat de València)
The broadband emission from the lobes of radio galaxy Centaurus A (Cen A) has been widely considered to be produced via a simple leptonic model, i.e., synchrotron and inverse-Compton scattering. More recently, gamma-ray data from the Fermi Large Area Telescope (LAT) has hinted at a spatial mismatch with the radio and microwave images of the Cen A lobes, challenging the simple leptonic...
The $\gamma$-ray sky is strongly dominated by blazars, i.e. AGN with relativistic jets oriented closely with our line of sight. Radio galaxies are their misaligned counterparts, and make up about ∼ 1-2% of all AGN observed by Fermi-LAT. Nonetheless, they provide us with a view of AGN jets which is less biased by Doppler boosting effects, and allow us to test jet production and emission...
The RoboPol program has been monitoring an unbiased sample of blazars since 2013 with a cadence of less than a few days. The main drive has been to quantify their optical polarisation properties, understand its variability and gain insight in the mechanisms producing the smooth and long rotations of the polarisation angle, in a systematic and unbiased way. Here we focus on the magnitude of...
Ever since the revolutionary discovery by the Fermi mission that active galactic nuclei (AGN)produce copious amounts of high-energy emission, its origin has remained elusive. Using high-frequency radio interferometry (VLBI) polarization imaging, we could probe the magnetic field topology of the compact high-energy emission regions in blazars. A case study for blazar 3C 279 reveals presence of...
We report on recent results from a target-of-opportunity programme to
obtain spectropolarimetry observations of flaring gamma-ray blazars with
the Southern African Large Telescope (SALT). In tandem with this observational
program, we develop a leptonic blazar model for a self-consistent representation
of SEDs and optical (SALT) spectropolarimetry. Such modeling provides an accurate...
3C 84 is unique in that it is one of the few non-blazar AGN that is detected at ultra high energies (including MAGIC in addition to Fermi/LAT). The source itself has been increasing in flux density at both radio and Gamma-rays since approximately 2005 with VLBI observations showing that this rising emission is associated with a slowly moving component south of the jet-launching (C1) region,...
Radio Galaxies have emerged as unique gamma-ray emitting source class
on the extragalactic sky. With their jets believed to misaligned such
that Doppler boosting effects become moderate, they allow for new
insights into the nuclear source region. I will report on new results
for M87 and discuss implications of recent findings for Centaurus A. The
relevance of a magnetospheric gamma-ray...
The multi-wavelength photon spectrum from the BL Lac object AP Librae extends from radio to TeV gamma rays. The X-ray to very high energy gamma ray emission from the extended jet of this source has been modeled earlier with inverse Compton (IC) scattering of relativistic electrons off the CMB photons. The IC/CMB model requires the kpc scale extended jet to be highly collimated with bulk...
The Third Catalog of Hard Fermi-LAT Sources (3FHL), based on the first 7 years of LAT data using the Pass 8 event-level analysis, contains 1556 sources in the 10 GeV--2 TeV energy range. The sensitivity and angular resolution are improved by factors of 3 and 2 relative to the previous LAT catalog in this energy range (1FHL). Most 3FHL sources (79%) are extragalactic, including 16 sources...
Preliminary results of the fourth catalog of active galactic nuclei (AGNs, 4LAC) detected in gamma rays by the Fermi Large Area Telescope (LAT) in seven years of scientific operation are presented. This catalog will be similar to the previously released third LAT AGN catalog (3LAC). The 4LAC includes approximately 2500 gamma-ray sources located at high Galactic latitudes (|b| > 10°) that are...
Extreme blazars (EHBLs) are mostly characterized by a synchrotron peak located at energies > 1 keV, and by the hardness of the spectrum in the high-energy (HE, E > 100 MeV) range. So far, only a handful of these objects have been detected at very-high energies (VHE, E > 100 GeV) by Imaging Atmospheric Cherenkov Telescopes. Moreover, multi-wavelength observations of some of these blazars (like...
A broadband study of $z>3$ blazars enables us to understand the evolution of the properties of relativistic jets over cosmic time. It has been found in many studies that such high $z$ blazars host $>$billion solar mass black holes and thus shed a new light on the supermassive black hole formation in the early Universe. Here, we report the first gamma-ray detection of blazars beyond $z=3.1$...
The Fermi-LAT revealed that blazars dominate the census of the gamma-ray sky, and a significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range. However, the possible connection between radio and very high energy (VHE, E>0.1 TeV) emission still remains elusive, owing to the lack of a homogeneous VHE sky coverage.
With this work we aim to quantify and...
The spectral energy distributions (SEDs) of blazars are dominated by synchrotron and inverse Compton radiation. The origin of blazar variability can be investigated from the time variation of the SEDs. However, it is difficult to estimate the optimal model parameters of SEDs and their uncertainties because some parameters are strongly correlated and the standard method may be trapped by local...
In late 2016, the Fermi Large Area Telescope (LAT) observed a dramatic increase in the gamma-ray activity from
the blazar CTA 102. The enhanced state was monitored throughout the whole electromagnetic spectrum and persisted for several weeks, reaching daily gamma-ray fluxes (0.1-100 GeV) as high as 10^-5 ph cm^-2 s^-1. We present the analysis of these flares and discuss their characteristics...
The flux variability of blazars can shed light on the innermost region of jets well beyond the imaging capabilities of present generation telescopes in any part of the electromagnetic spectrum. The FSRQ, CTA 102 (z ~ 1) started showing activity in gamma-rays during 2016 with a few very bright flares. The detection of intra-day variability above 100 MeV using Fermi-LAT during these flares...
Because of their brightness and proximity (z=0.03), Mrk421 and Mrk501 are among the very-high-energy (>100 GeV) gamma-ray objects that can be studied with the greatest level of detail, and consequently they are excellent astrophysical high-energy physics laboratories to study the nature of blazars. Motivated by the extensive temporal exposure of Fermi-LAT, since 2008, there has been an...
The flat spectrum radio quasar (FSRQ) PKS 1510-089 (z=0.361) is known for its complex multi-wavelength behaviour. Since 2015, it has been very active across the entire electromagnetic spectrum. This has lead to joint observation campaigns including Fermi-LAT, Cherenkov telescopes and several instrument covering the synchrotron branch. Observations resulted in a range of remarkable...
Many $\gamma$-ray flares within blazars are highly correlated with flares detected at longer wavelengths; however, a small subset appears to occur in isolation. These orphan $\gamma$-ray flares challenge models of blazar variability. We have developed the 'Ring of Fire' model to explain the origin of these orphan flares. In this model, electrons contained within a blob of plasma moving...
We have analyzed data from the flat-spectrum radio quasars PKS 1510-089 collected over a period of eight years from 2008 August to 2016 December with the Fermi-LAT. We have identified several flares of this highly variable source, studied their temporal and spectral properties in detail, and compared with previous works on flares of PKS 1510-089. Five major flares and few sub-flares have...
By using deep radio source catalogs currently available, we present a new blazar candidate catalog, BROS, which includes 56314 sources located at declination δ>−40∘ and outside the Galactic Plane (|b|>10). We picked up flat-spectrum radio sources of α>−0.5 (α is defined as Fν∝ν^α) from 0.15 GHz TGSS and 1.4 GHz NVSS catalogs. Then, we identified their optical counterparts by cross-matching...
We present an analysis of 8 years of Fermi-LAT γ-ray data obtained for NGC 1275. By examining the changes in its flux and spectral shape over the entire dataset, we found that its spectral behavior changed around 2011 February. The γ-ray spectra at the early times evolves largely at high energies, while the photon indices were unchanged in the latter despite rather large flux variations. To...
The proper understanding of blazar and ither AGN flux variability at the various energy bands is one of the most important goals of multifrequency astrophysics. In this frame a peculiar and controversial phenomenology is the periodicity, claimed mostly in radio or optical flux light curves of a restricted group of blazars. Significance estimates are uncertain and the evidence for temporal...
The gamma-ray, short timescale variability in blazars ($\sim$ minutes to hours), which has been recently detected in GeV gamma-rays by Fermi-LAT, challenges existing blazar emission models. An excellent candidate for describing such events is the relativistic magnetic reconnection model, in which energy is transferred to compact regions, denoted as plasmoids, characterized by high...
Blazars are luminous extragalactic sources across the entire electromagnetic spectrum, but the spectral formation mechanisms in these sources are not well understood. We have developed a new theoretical model for simulating blazar spectra in which we numerically integrate the electron transport equation to generate the electron number distribution with respect to energy. Our transport model...
The Automatic Telescope for Optical Monitoring is a 75cm optical telescope located at the H.E.S.S. site in Namibia. Since 2005, it monitors around 250 gamma-ray emitting AGN and has observed numerous blazar flares in several colour bands with multiple measurements during any given night.
We will present recent flaring events together with corresponding multi-wavelength data including...
Active galactic nuclei (AGN) are one of the largest class of gamma-ray sources with blazars accounting for the vast majority of the AGN associations. Despite the advancements in angular resolution and localization Fermi has offered, roughly half of the detected sources are still unassociated with a lower-energy counterpart. We propose a new method for the association of gamma-ray loud blazars...
From 1997 we are monitoring of about 70 blazars in BVRI bands using ST6 and Apogee cameras attached to 70-cm meniscus telescope of Abastumani Observatory. During nearly twenty years over 300000 images have been obtained as a result over 3000 nights observations. Most dense coverage of selected FERMI/LAT brightest sources have been undertaken after lunch of FERMI satellite in 2008. We present...
Based on nine years of Fermi LAT observations, we have identified 30 projects for Active Galactic Nuclei (AGN) that appear to provide strong prospects for significant scientific advances. This Very Important Project (VIP) AGN list includes those AGNs that have good multiwavelength coverage, are regularly detected by the Fermi LAT, and offer scientifically interesting timing or spectral...
We report hints for the correlation between the X-ray and $\gamma$-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. We have divided Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low (L) and high (H) level of X-ray emission from the active nucleus of this galaxy, determined...
The compact radio source 2013+370 is a not yet well-studied blazar which is located close to the galactic plane (b = 1.2º) at a redshift of z = 0.859. We performed Very Long Baseline Interferometry (VLBI) monitoring observations at 15, 43, 86 GHz, which reveal the source kinematics on sub-parsec scales. The VLBI data are then combined with flux density variability measurements using the...
In this paper, we will present the analysis of Suzaku/XIS, Swift/XRT, and Fermi-LAT data. We analyzed Suzaku/XIS data (taken from the year 2006 to 2015) and Fermi-LAT data (taken from the year 2008 to 2015) of the gamma-ray emitting radio galaxy NGC 1275. Correlated brightening of the nucleus in both the X-ray and GeV gamma-ray energy bands was found for the period 2013–2015. This is the first...
