XMM-Newton observation of the narrow-line QSO PHL 1092: detection of a high and variable soft component

Abstract

We present results based on an XMM-Newton observation of the high-luminosity narrow-line QSO PHL 1092 performed in 2003 January. The 0.3-10 keV spectrum is well described by a model that includes a power law (Γ~ 2.1) and two blackbody components (kT~ 130 eV and kT~50 eV). The soft X-ray excess emission is featureless and contributes ~80% to the total X-ray emission in the 0.3-10 keV band. The most remarkable feature of the present observation is the detection of X-ray variability at a very short timescale; the X-ray emission varied by 35% in about 5000 s. We find that this variability can be explained by assuming that only the overall normalization varied during the observation. There was no evidence for any short-term spectral variability, and the spectral shape was similar even during the ASCA observation carried out in 1997. Considering the high intrinsic luminosity (~2 x 10⁴⁵ ergs s^-1) and the large inferred mass of the putative black hole (~1.6 x 10⁸ M_⊙), the observed timescale of variability indicates emission at close to the Eddington luminosity arising from very close to the black hole. We suggest that PHL 1092 in particular (and narrow-line Seyfert galaxies in general) is a fast-rotating black hole emitting close to its Eddington luminosity, and the X-ray emission corresponds to the high-soft state seen in Galactic black hole sources.