Long-term X-ray spectral variability of the radio-loud narrow-line Seyfert 1 galaxy PKS 0558-504

We present X-ray observations of the narrow-line Seyfert 1 (NLS1) galaxy PKS 0558-504 obtained with RXTE during a 1 yr monitoring campaign. This source, which is one of the very few radio-loud NLS1 galaxies, shows strong X-ray flux variability on timescales of weeks to months, accompanied by spectral variability. The main goal of this study is to investigate the spectral variability with model-independent methods and time-resolved spectroscopy in order to shed light on the origin of the X-rays. The main results can be summarized as follows: (1) The flux typically changes by a factor of 1.5Y2 on timescales of 10-30 days, with few extreme events where the flux increases by a factor of similar to 4 in 3 days. (2) We do not observe any large amplitude, flux- related spectral variations. During the flux variations, the spectrum varies mainly in normalization and not in shape. We do observe some small- amplitude spectral variations, which do not correlate with flux, although there is a hint of spectral hardening as the source brightens. (3) There is no evidence for reprocessing features, such as the Fe K alpha line or a Compton hump. We argue that PKS 0558-504 is a peculiar object that appears to be different from most of the radio-quiet and radio-loud active galactic nuclei (AGNs). If a jet is responsible for the bulk of the X rays, it must operate in an unusual way. If instead a corona is responsible for the X-rays, the system might be a large-scale analog of the Galactic black holes in the transient intermediate state.