The variable X-ray absorption and soft X-ray excess of the QSO MR 2251-178.

Abstract

We present the results from a programme of 15 EXOSAT observations in which we monitored the QSO MR2251 - 178. Over the period 1983 October-1984 December the hard X-ray flux varied by a factor of 2. While there is an overall correlation between the ME(2-10 keV) and LE(0.1-2 keV) fluxes the pattern of variability is such that it cannot be described by simple intensity, absorption or slope variations. The source spectrum also shows, when bright, an excess at soft energies above that which would be predicted by extrapolating the power law and absorption found from the ME spectrum. We find that it is possible to explain all the observed features by adopting the 'warm' absorber model in which the absorbing material is partially ionized by the flux of extreme ultraviolet and X-ray photons from the central continuum source. The opacity profile of the absorbing material is then a function of the source luminosity. Using a photo-ionization code and adopting appropriate parameters for MR2251 - 178, we find that the observed correlation of opacity with source luminosity can be reproduced. The preferred location of the absorbing material is close to the central continuum source. The recent evidence for 'cool' material in the very centre of Seyfert galaxies is thus extended to include an object of significantly higher luminosity.