PHL 1092 is a z \sim 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 \sim 260 with respect to a previous observation performed 4.5 yr earlier .
The UV flux remained almost constant , resulting in a significant steepening of the optical–to–X–ray slope \alpha _ { 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 database spanning almost 10 yr in total in the activity of the source .
Our monitoring program demonstrates that the \alpha _ { 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 ( SED ) and the extreme X–ray and \alpha _ { 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 \sim 10 rather than the observed factor of \sim 260 .
We discuss these scenarios in the context of non–BAL X–ray weak quasars .