Multiwavelength campaign on Mrk 509. IV. Optical-UV-X-ray variability
  and the nature of the soft X-ray excess
  We present the analysis of XMM-Newton and Swift optical-UV and X-ray
observations of the Seyfert-1/QSO Mrk 509, part of an unprecedented
multi-wavelength campaign, investigating the nuclear environment of this AGN.
The XMM-Newton data are from a series of 10 observations of about 60 ks each,
spaced from each other by about 4 days, taken in Oct-Nov 2009. During our
campaign, Mrk 509 was also observed with Swift for a period of about 100 days,
monitoring the behaviour of the source before and after the XMM-Newton
observations. With these data we have established the continuum spectrum in the
optical-UV and X-ray bands and investigated its variability on the timescale of
our campaign with a resolution time of a few days. In order to measure and
model the continuum as far as possible into the UV, we also made use of HST/COS
observations of Mrk 509 (part of our coordinated campaign) and of an archival
FUSE observation. We have found that in addition to an X-ray power-law, the
spectrum displays soft X-ray excess emission below 2 keV, which interestingly
varies in association with the thermal optical-UV emission from the accretion
disc. The change in the X-ray power-law component flux (albeit smaller than
that of the soft excess), on the other hand, is uncorrelated to the flux
variability of the soft X-ray excess and the disc component on the probed
timescale. The results of our simultaneous broad-band spectral and timing
analysis suggest that, on a resolution time of a few days, the soft X-ray
excess of Mrk 509 is produced by the Comptonisation of the thermal optical-UV
photons from the accretion disc by a warm (0.2 keV) optically thick (tau ~ 17)
corona surrounding the inner regions of the disc. This makes Mrk 509, with a
black hole mass of about 1-3 x 10^8 solar masses, the highest mass known system
to display such behaviour and origin for the soft X-ray excess.
