Extreme X-ray Variability In The Narrow-Line QSO PHL 1092

Abstract

A ROSAT observation of the narrow-line Fe II QSO PHL 1092 shows rapid variability that requires an efficiency of at least 0.13, exceeding the theoretical maximum for an accretion disk around a non-rotating black hole. Plausible explanations for this high efficiency incorporate anisotropic emission and/or accretion onto a rapidly rotating black hole, the latter recently suggested by Kwan et al. as a mechanism for generating PHL 1092's strong Fe II lines by mechanical heating in an accretion disk. The soft X-ray luminosity of PHL 1092 had also increased by a factor of 21 over the weak Einstein detection, to more than 5 x 10^46 ergs/s. Its photon spectral index of 4.2 is among the steepest of any AGN. These X-ray properties are characteristic of narrow-line Seyfert 1 galaxies, of which PHL 1092 is evidently a very luminous member. Narrow-line QSOs also extend a significant correlation between X-ray luminosity and X-ray spectral index which we have found among a large sample of optically-selected, narrow-line Seyfert 1 galaxies observed by ROSAT.