The X-ray spectrum of QSO 1821+643.

Abstract

Pravdo & Marshall have reported an unusually soft X-ray spectrum for the high-luminosity QSO 1821 + 643, based on measurements from the A2 experiment on HEAO-1. However, in a series of EXOSAT observations of this object we have found the medium-energy X-ray spectrum to be consistent with the canonical form observed in low-luminosity active galaxies. The most plausible interpretation is that two continuum components are present, a soft component which declined by a factor of ~10 over the period 1977-85 and a medium-energy component of standard slope which showed only a factor of ~2 decrease over the same period. This source may provide an important test of the hypothesis that the soft excesses present in the X-ray spectra of many active galaxies originate in the hot innermost regions of an accretion disc.