Long-term monitoring of PKS 0558-504 with Swift: the disc-corona link

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 Swift-UVOT. The simultaneous coverage at several wavelengths confirms that PKS 0558-504 is highly variable in any band from the optical to ultraviolet (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 favour 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.