Tackling the soft X-ray excess in AGN with variability studies
  The origin of the soft X-ray excess in AGN has been a mystery ever since its
discovery. We present how the time variability of this spectral component can
point towards its origin. Using the powerful technique of multi-epoch fitting,
we study how the soft excess in a given object depends on other parameters of
the continuum and the accretion disk possibly hinting at its nature. As an
example, we present results from this technique applied to the Seyfert galaxy
Mrk 841. We study all (3) XMM and some of the Suzaku pointings available and
find that the source displays an impressive variability in the soft X-ray band
on the timescale of years. We study several common soft excess models and their
ability to physically consistently explain this spectral variability. Mrk 841
is found to show a distinct variability pattern that can be best explained by
the soft excess originating mostly from a thermal Comptonization component. The
variability timescale can be constrained to be on the order of a few days.
