Long-Term X-Ray Spectral Variability of the Radio-Loud Narrow-Line Seyfert 1 Galaxy PKS 0558-504
Abstract
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 1.5-2 on timescales of 10-30 days, with few extreme events where the flux increases by a factor of ~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 Kα 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2007
- DOI:
- 10.1086/510798
- arXiv:
- arXiv:astro-ph/0611348
- Bibcode:
- 2007ApJ...656..691G
- Keywords:
-
- Galaxies: Active;
- Galaxies: Jets;
- Galaxies: Nuclei;
- X-Rays: Galaxies;
- Astrophysics
- E-Print:
- 12 pages, 7 figures, accepted for publication in ApJ