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.