Elvis, MartinWilkes, BelindaGiommi, P.McDowell, Jonathan2017-06-081991Elvis, Martin, Belinda J. Wilkes, P. Giommi, and Jonathan McDowell. 1991. “Persistence and Change in the Soft X-Ray Spectrum of the Quasar PG 1211 + 143.” The Astrophysical Journal 378 (September): 537. doi:10.1086/170454.0004-637Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:32955263The present study examines two Einstein and three Exosat observations of PG 1211 + 143 over a 6-yr baseline which show that strong steep-spectrum low-energy X-ray emission is a persistent feature of this quasar. Exosat observations of PG 1211 + 143 detected an increase of a factor of 2.3 in its steep soft X-ray flux during 18 days. It is concluded that the bulk of the soft X-ray emission of PG 1211 + 143 comes from a region less than 5 x 10 to the 16th cm across. In another interval of 193 days, the soft X-rays decreased by a factor of 3.7. In the same interval the hard X-rays decreased by a factor of 1.6 + or - 0.05, which suggests a connection between the two energy regimes and argues against variable absorption causing the soft X-ray variations. It is contended that 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.en-USquasarsX-rays: sourcesX-rays: spectraJournal Article2017-02-0819912017-06-0810.1086/170454