XMM-Newton long-look observation of the narrow line Seyfert 1 galaxy
  PKS0558-504; I: Spectral analysis
  We present results from the spectral analysis of a long XMM-Newton
observation of the radio-loud NLS1 galaxy PKS0558-504. The source is highly
variable, on all sampled time scales. We did not observe any absorption
features in either the soft or hard X-ray band. We found weak evidence for the
presence of an iron line at ~6.8 keV, which is indicative of emission from
highly ionized iron. The 2-10 keV band spectrum is well fitted by a simple
power law model, whose slope steepens with increasing flux, similar to what is
observed in other Seyferts as well. The soft excess is variable both in flux
and shape, and it can be well described by a low-temperature Comptonisation
model, whose slope flattens with increasing flux. The soft excess flux
variations are moderately correlated with the hard band variations, and we
found weak evidence that they are leading them by ~20 ksec. Our results rule
out a jet origin for the bulk of the X-ray emission in this object. The
observed hard band spectral variations suggest intrinsic continuum slope
variations, caused by changes in the "heating/cooling" ratio of the hot corona.
The low-temperature Comptonising medium, responsible for the soft excess
emission, could be a hot layer in the inner disc of the source, which appears
due to the fact that the source is accreting at a super-Eddington rate. The
soft excess flux and spectral variations could be caused by random variations
of the accretion rate.
