Persistence and Change in the Soft X-Ray Spectrum of the Quasar PG 1211+143

Abstract

Two Einstein and three EXOSAT observations of PG 1211 + 143 over a 6 year baseline show that strong steep-spectrum low-energy X-ray emission is a persistent feature of this quasar. EXOSAT observations of PG 1211 + 143 detected a factor of 2.3 increase in its steep soft X-ray (C-band) flux in 18 days. The bulk of the soft X-ray emission of PG 1211 + 143 therefore comes from a region less than 5 x 10^16^ cm across. In another (quasar rest frame) interval of 193 days the soft X- rays decreased by a factor of 3.7. Either an intrinsic emission variation or an increase of NH from 3.4 to 7.6 x 10^20^ atoms cm^-2^ could produce the same change. In the same time interval the hard (2-10 keV) X-rays decreased by a factor 1.6 +/- 0.05. This suggests a connection between the two energy regimes and argues against variable absorption causing the soft X-ray variations. In order to decrease in luminosity so rapidly a thermal source in PG 1211 + 143 would have to be optically thick to both electron scattering and free-bound absorption. The same argument applies to six of the seven other rapidly variable soft excesses reported in the literature, which suggests that optically thin thermal models are ruled out in general.