XMM-Newton long-look observation of the narrow-line Seyfert 1 galaxy PKS 0558-504. II. Timing analysis

Context. PKS 0558-504 has been observed repeatedly by XMM-Newton as a calibration and performance verification target. In this work, we present results from the power spectral analysis using data from a long XMM-Newton observation and archival RXTE data.
Aims: To search for periodic and/or quasi-periodic signals, to estimate the power spectrum over a broad range which covers almost six decades in frequency, and to compare the variability in the ultraviolet (UV), soft and hard X-ray bands.
Methods: We used the XMM-pn soft and hard band light curves to compute the high frequency power spectrum (PSD) and search for periodicities. We also used an archival, long RXTE light curve to constrain the power spectrum at low frequencies, and the XMM-OM light curve to estimate the UV PSD. We also used the results from the time-resolved spectral studies we presented in a recent paper to compare in more detail the X-ray, “soft-excess” and UV variations.
Results: We did not detect any significant periodic or quasi-periodic signals. The soft band power spectral slope is steeper than the hard band PSD slope, but consistent within the 1-σ uncertainties. The broad frequency band, 2-10 keV PSD follows a power-law like shape of an index ~0.5 up to a break frequency at ~ 7 × 10^-6 Hz (which corresponds to a time scale of 1.7 days). At higher frequencies, the PSD steepens to a slope of ~2.2. The OM power spectrum has a significantly smaller amplitude than the X-ray and the soft-excess PSDs.
Conclusions: The similarity between the PSD shape of PKS 0558-504 and other radio-quiet Seyferts 1 suggests a common variability (and emission) mechanism. Consequently any jet emission in the X-ray band should be unimportant in this source. Using the scaling relations between PSD break time scales and black hole mass/bolometric luminosity, we estimated a black hole mass of ~ 2 × 10⁸ M_⊙. Power spectral analysis of the light curves which resulted from the time-resolved spectroscopic study of the same data set indicates that the intrinsic power spectrum may steepen with increasing energy, contrary to what is currently believed for typical Seyferts. When scaled appropriately, the OM and soft-excess light curves show similar variability patterns. They may originate from the accretion disc, at a distance of ~20 and 5 Schwarzschild radii, respectively, and their variations could be due to accretion rate variations, which operate on the local viscosity time scale and propagate inwards.