Disappearance of a narrow Mg II absorption system with a measured
  velocity up to $\rm 166,000 km s^{-1}$
  Quasar J152645.61+193006.7 ($z_{\rm e}=2.5771$) was observed by the Sloan
Digital Sky Survey (SDSS) on 2006 May 31, and again on 2011 April 9. The time
interval of the SDSS two observations is 497.4 days at the quasar rest frame.
From the spectra of this quasar, we detect a phenomenon of disappearance of a
narrow $\rm Mg II \lambda\lambda2796,2803$ absorption system with a velocity up
to $\rm 166,129 km s^{-1}$ with respect to the quasar. This disappearance event
can be caused by changes in the ionization state of absorbing gas or by the
bulk motion of the absorbing gas across the quasar sightline. The coverage
fraction analysis shows that this absorber partially covers the background
emission sources with an effective coverage fraction of $C_{\rm
f}=0.40\pm0.06$. The time variation analysis and the coverage fraction analysis
imply that this absorber might be intrinsic to the quasar. However, the
scenario of a cosmologically separated foreground object located at $z=0.9170$
accounting for the phenomenon cannot be ruled out according to current
available data.
