The X-ray spectrum and variability of Markarian 421.

Abstract

The X-ray bright BL Lac object Markarian 421 (Mrk 421) was observed by Exosat four times over a period of 6 days in February 1984. The source exhibited a very soft power-law X-ray spectrum (photon spectral index is about 2.8) with an extremely low intrinsic column density (less than or equal to 10 to the 20th/sq cm). Significant X-ray variability (about 40 percent) was apparent on a time scale of the order of a day with an indication of a softening of the X-ray spectrum as the source flux decreased. The relatively rapid X-ray variability and overall (radio to X-ray) spectrum of Mrk 421 are discussed in terms of the synchrotron self-Compton model. A high degree of relativistic beam is not a requirement for this source.