X-ray and ultraviolet observations of Markarian 421

We present the results of a detailed study of the BL Lacertæ type object Markarian 421 (Mrk 421) based upon EXOSAT and IUE observations made during 1984 and 1985. In the X-ray band Mrk 421 exhibits significant variability on time-scales typically of the order of a day with a minimum e-folding time-scale, t_v, of $$\sim5\,\times\,{10}^{4}$$ s. There is some indication of a dichotomy of X-ray states. The majority of the observations showed the source to be in a ‘low’, soft state in which the spectral index and 2–6 keV flux are well correlated in the sense that the source hardens as it brightens. However, during a period of at least 3 days in 1984 December the source entered a hard, ‘outburst’ state during which the spectral index remained at ~1. In contrast, at ultraviolet wavelengths Mrk 421 exhibits only slow drifts in flux on a time-scale of weeks and little change in spectral index. We show that a canonical relativistic jet model can adequately explain the multi-waveband spectra at all epochs if the upper cut-off in the synchrotron emitting electron population has a variable radial dependence. During outburst the X-ray emission may emanate predominantly from a very narrow region, possibly a knot, within the jet.