An Extreme X-ray Variability Event of a Weak-Line Quasar
  We report the discovery of an extreme X-ray flux rise (by a factor of > 20)
of the weak-line quasar SDSS J153913.47+395423.4 (hereafter SDSS J1539+3954) at
z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1
AGNs where such dramatic X-ray variability has been observed. Before the X-ray
flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation
from its UV flux; after the rise, the ratio of its X-ray flux and UV flux is
consistent with the majority of the AGN population. We also present a
contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV
continuum level remains generally unchanged despite the dramatic increase in
the X-ray flux, and its C iv emission line remains weak. The dramatic change
only observed in the X-ray flux is consistent with a shielding model, where a
thick inner accretion disk can block our line of sight to the central X-ray
source. This thick inner accretion disk can also block the nuclear ionizing
photons from reaching the high-ionization broad emission-line region, so that
weak high-ionization emission lines are observed. Under this scenario, the
extreme X-ray variability event may be caused by slight variations in the
thickness of the disk. This event might also be explained by gravitational
light-bending effects in a reflection model.
