Speaker
Description
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.
