X-ray variability of the Narrow Line Seyfert 1 Galaxy PKS 0558-504

Abstract

We present results from several XMM-Newton observations of the radio-loud Narrow-line Seyfert 1 galaxy (NLS1) PKS 0558-504. We find evidence for strong and persistent X-ray variability, both on short and long time-scales. On short time scales of $\ga$2 hours the source varies smoothly by 15-20%; long-term variations by a factor $\ga$2 could not be resolved in the relatively short exposures: we find the source mostly in a “low” state, in 2 out of the 11 observations in a “high state”. X-ray flares seem to be recurrent with a time scale of ~24 ks which, if interpreted as the Keplerian time scale in the disc, would place the emission region just outside the last stable orbit. The X-ray spectrum of PKS 0558-504 can be well fitted by two Comptonization components, one at moderate temperatures of kT ~ 4.5 keV and optical depths of $\tau \sim 2$, the other at high temperatures ( kT $\ga$ 50 keV) and low optical depths ($\tau \la 1.0$). These parameters are, however, subject to large errors due to the inherent degeneracy of the Comptonization models. Flux variations of the source are caused by changes of the colder component only, the hot component with parameters very similar to those of BLS1 galaxies, stays constant. All results fit nicely the picture that NLS1 galaxies are lower mass objects, accreting close to the Eddington rate emitting X-rays from a very active magnetically powered accretion disc corona.