A Rapid and Large-Amplitude X-ray Dimming Event in a z ~ 2.6 Radio-Quiet
  Quasar
  We report a dramatic fast X-ray dimming event in a z=2.627 radio-quiet type 1
quasar, which has an estimated supermassive black hole (SMBH) mass of
$6.3\times 10^{9} M_\odot$. In the high X-ray state, it showed a typical level
of X-ray emission relative to its UV/optical emission. Then its 0.5-2 keV
(rest-frame 1.8-7.3 keV) flux dropped by a factor of $\approx7.6$ within two
rest-frame days. The dimming is associated with spectral hardening, as the 2-7
keV (rest-frame 7.3-25.4 keV) flux dropped by only $17\%$ and the effective
power-law photon index of the X-ray spectrum changed from $\approx2.3$ to
$\approx0.9$. The quasar has an infrared (IR)-to-UV spectral energy
distribution and a rest-frame UV spectrum similar to those of typical quasars,
and it does not show any significant long-term variability in the IR and
UV/optical bands. Such an extremely fast and large-amplitude X-ray variability
event has not been reported before in luminous quasars with such massive SMBHs.
The X-ray dimming is best explained by a fast-moving absorber crossing the line
of sight and fully covering the X-ray emitting corona. Adopting a
conservatively small size of $5 {G} M_{\rm BH}/c^2$ for the X-ray corona, the
transverse velocity of the absorber is estimated to be $\approx 0.9c$. The
quasar is likely accreting with a high or even super-Eddington accretion rate,
and the high-velocity X-ray absorber is probably related to a powerful
accretion-disk wind. Such an energetic wind may eventually evolve into a
massive galactic-scale outflow, providing efficient feedback to the host
galaxy.
