Insights on the X-ray weak quasar phenomenon from XMM-Newton monitoring
  of PHL 1092
  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 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 ~10 rather than
the observed factor of ~260.
