The disappearance of a narrow Mg II absorption system in quasar SDSS
  J165501.31+260517.4
  In this letter, we present for the first time, the discovery of the
disappearance of a narrow Mg II $\lambda\lambda2796,2803$ absorption system
from the spectra of quasar SDSS J165501.31+260517.4 ($z_{\rm e}=1.8671$). This
absorber is located at $z_{\rm abs} =1.7877$, and has a velocity offset of
$8,423\rm ~km~s^{-1}$ with respect to the quasar. According to the velocity
offset and the line variability, this narrow Mg II $\lambda\lambda2796,2803$
absorption system is likely intrinsic to the quasar. Since the corresponding UV
continuum emission and the absorption lines of another narrow Mg II
$\lambda\lambda2796,2803$ absorption system at $z_{\rm abs}=1.8656$ are very
stable, we think that the disappearance of the absorption system is unlikely to
be caused by the change in ionization of absorption gas. Instead, it likely
arises from the motion of the absorption gas across the line of sight.
