Published online by Cambridge University Press: 25 May 2016
We report the results of the search for the change in the warm absorber of the quasar MR 2251-178 with ASCA. We detected its change along with the X-ray intensity change. In the bright phase in 1993, its X-ray spectra showed the absorption edge features by OVII and OVIII edges. In contrast, in the medium intensity (in 1996), the absorption apparently increased and the edge energy was shifted to the lower energy. These results are well explained by the ionization change of the same absorber. The timescale of the change requires moderately high density and the location to be nearer to the center than the Narrow Line Region. We briefly discuss the stability of the warm absorber.