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We report the discovery of extreme X-ray variability in a type 1 quasar: SDSSJ$075101.42+291419.1$. 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 witha super-Eddington accretion rate. Its XMM-Newton observation in 2015 Mayreveals a flux drop by a factor of $\sim 22$ with respect to the Swiftobservation in 2013 May when it showed a typical level of X-ray emissionrelative to its UV/optical emission. The lack of correlated UV variabilityresults 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 of36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mildUV/optical continuum and emission-line variability also suggest that theaccretion rate did not change significantly. A single power-law model modifiedby Galactic absorption describes well the $0.3-10$ keV spectra of the X-rayobservations in general. The spectral fitting reveals steep spectral shapeswith $\Gamma\approx3$. We search for active galactic nuclei (AGNs) with suchextreme X-ray variability in the literature and find that most of them arenarrow-line Seyfert 1 galaxies and quasars with high accretion rates. Thefraction of extremely X-ray variable objects among super-Eddington accretingAGNs is estimated to be $\approx 15-24\%$. We discuss two possible scenarios,disk reflection and partial covering absorption, to explain the extreme X-rayvariability of SDSS J$075101.42+291419.1$. We propose a possible origin for thepartial covering absorber, which is the thick inner accretion disk and itsassociated outflow in AGNs with high accretion rates.

SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-ray Variability