SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with
  Extreme X-ray Variability
  We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS
J$075101.42+291419.1$. It has a black hole mass of $1.6\times 10^7~\rm M_\odot$
measured from reverberation mapping (RM), and the black hole is accreting with
a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May
reveals a flux drop by a factor of $\sim 22$ with respect to the Swift
observation in 2013 May when it showed a typical level of X-ray emission
relative to its UV/optical emission. The lack of correlated UV variability
results in a steep X-ray-to-optical power-law slope ($\alpha_{\rm OX}$) of
-1.97 in the low X-ray flux state, corresponding to an X-ray weakness factor of
36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mild
UV/optical continuum and emission-line variability also suggest that the
accretion rate did not change significantly. A single power-law model modified
by Galactic absorption describes well the $0.3-10$ keV spectra of the X-ray
observations in general. The spectral fitting reveals steep spectral shapes
with $\Gamma\approx3$. We search for active galactic nuclei (AGNs) with such
extreme X-ray variability in the literature and find that most of them are
narrow-line Seyfert 1 galaxies and quasars with high accretion rates. The
fraction of extremely X-ray variable objects among super-Eddington accreting
AGNs is estimated to be $\approx 15-24\%$. We discuss two possible scenarios,
disk reflection and partial covering absorption, to explain the extreme X-ray
variability of SDSS J$075101.42+291419.1$. We propose a possible origin for the
partial covering absorber, which is the thick inner accretion disk and its
associated outflow in AGNs with high accretion rates.
