25 mai 2023Séminaire – David Homan (AIP Postdam)

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Extreme variability in active galactic nuclei: detection with eROSITA and results from multi-wavelength follow-up

Abstract

Active galactic nuclei (AGN) are known to be variable at all wavelengths 
and on many timescales. Large optical surveys have revealed a small 
number of AGN that show drastic changes in their optical luminosity, on 
a timescale of years. These changes are associated with large accretion 
changes in AGN. The repeated eROSITA all-sky X-ray surveys have provided 
the basis for a systematic, large-scale search for extreme X-ray 
variability in extragalactic objects. We have combined the survey 
dataset with a multi-wavelength follow-up campaign of the most variable 
objects. The follow-up observations include optical spectroscopy and 
X-ray and UV observations. This presentation will cover the results of 
our search for extremely variable AGN based on at least four epochs of 
available eROSITA data. Our sample consists of ~2200 vetted 
extragalactic sources with significant X-ray changes. As part of our 
follow-up, we have collected optical spectroscopic follow-up on ~350 
objects, including repeat spectroscopy for 40% of these. I will briefly 
introduce the challenges that extreme AGN variability pose for our 
understanding of accretion in AGN, next I will present our 
sample-selection criteria, discuss statistics on the detected X-ray 
variability. Finally, I will also briefly summarise our results for some 
of the most interesting individual sources.