Long-term monitoring of PKS0558-504 with Swift: the disc-corona link

TL;DR

This study uses 1.5 years of Swift observations to explore the variability and disc-corona connection in the highly variable, super-Eddington accreting galaxy PKS 0558-504, revealing a tight optical-UV link and corona seed photon origin.

Contribution

It provides new insights into the disc-corona relationship and variability propagation in a highly accreting, radio-loud Narrow-Line Seyfert 1 galaxy through multi-wavelength monitoring.

Findings

Optical and UV emission are tightly correlated.

X-ray variability is largest among bands.

Soft X-ray flux correlates with the hard photon index.

Abstract

PKS 0558-504 is a highly variable, X-ray bright, radio-loud, Narrow-Line Seyfert 1 galaxy with super-Eddington accretion rate and extended jets that do not dominate the emission beyond the radio band. Therefore, this source represents an ideal laboratory to shed some light on the central engine in highly accreting systems and specifically on the link between accretion disc and corona. Here we present the results from a 1.5 year monitoring with Swift XRT and UVOT. The simultaneous coverage at several wavelengths confirms that PKS 0558-504 is highly variable in any band from optical, to UV and X-rays, with the latter showing the largest amplitude changes but with the UV emission dominating the radiative output. A cross-correlation analysis reveals a tight link between the emission in the optical and UV bands and provides suggestive evidence in favor of a scenario where the variability originates in the outer part of the accretion flow and propagates inwards before triggering the activity of the X-ray emitting corona. Finally, a positive correlation between the soft X-ray flux and the hard photon index suggests that in PKS 0558-504 the seed photons are provided to the corona by the soft excess component.