Insights on the X-ray weak quasar phenomenon from XMM–Newton monitoring of PHL1092

Abstract

PHL 1092 is a z ∼ 0.4 high-luminosity counterpart of the class of Narrow-Line Seyfert 1 galaxies. In 2008, PHL 1092 was found to be in a remarkably low X-ray flux state during an XMM–Newton observation. Its 2 keV flux density had dropped by a factor of ∼260 with respect to a previous observation performed 4.5 yr earlier. The ultraviolet (UV) flux remained almost constant, resulting in a significant steepening of the optical-to-X-ray slope [alpha][subscript ox] from −1.57 to −2.51, making PHL 1092 one of the most extreme X-ray weak quasars with no observed broad absorption lines (BALs) in the UV. We have monitored the source since 2008 with three further XMM–Newton observations, producing a simultaneous UV and X-ray data base spanning almost 10 yr in total in the activity of the source. Our monitoring programme demonstrates that the [alpha][subscript ox] variability in PHL 1092 is entirely driven by long-term X-ray flux changes. We apply a series of physically motivated models with the goal of explaining the UV-to-X-ray spectral energy distribution and the extreme X-ray and [alpha][subscript ox] variability. We consider three possible models. (i) A breathing corona scenario in which the size of the X-ray-emitting corona is correlated with the X-ray flux. In this case, the lowest X-ray flux states of PHL 1092 are associated with an almost complete collapse of the X-ray corona down to the marginal stable orbit. (ii) An absorption scenario in which the X-ray flux variability is entirely due to intervening absorption. If so, PHL 1092 is a quasar with standard X-ray output for its optical luminosity, appearing as X-ray weak at times due to absorption. (iii) A disc-reflection-dominated scenario in which the X-ray-emitting corona is confined within a few gravitational radii from the black hole at all times. In this case, the intrinsic variability of PHL 1092 only needs to be a factor of ∼10 rather than the observed factor of ∼260. We discuss these scenarios in the context of non-BAL X-ray weak quasars.