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 1984 February. The source exhibited a very soft power-law X-ray spectrum (photon spectral index Γ∼2.8) with an extremely low intrinsic column density (|$n_\text H \lesssim10^{20} \enspace\text {cm}^{-2}$|). Significant X-ray variability (∼40 per cent) 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.
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