X-ray and ultra-violet observations of BL Lacertae type objects.

We present the results from EXOSAT and IUE observations of five BL Lac type objects, Mrk 421, Mrk 501, 1218+304, Mrk 180 and 0414+009. We find that at least for the relatively well studied sources Mrk 421 and Mrk 501, the behaviour in the X-ray and ultra-violet differs significantly. In the majority of cases, the X-ray spectra are well represented by a simple power-law model over the 1-6 keV band. In the case of the high state observations of Mrk 421, however, a marked downward curvature of the spectrum is required above ~3 keV, which can be modelled as an exponential decrease. Similar convex spectra are also suggested, although not statistically required, in a number of other cases. We found no evidence in any of the observations for a hardening of the X-ray spectra above a few keV as has been previously reported. In all cases we find low energy cut-offs in the X-ray spectra consistent with absorption in the line-of-sight gas column density through out own galaxy implying that the intrinsic column density of cold gas in these objects is small (< 1 X 1020 cm-2). All five sources exhibit significant X-ray variability with minimum variability timescales in the range ~3 hr to ~7 days. Furthermore, the X-ray flux and spectral index for each object appears to be correlated in the sense that the X-ray spectrum hardens as the source brightens. In contrast, the ultra-violet spectra are generally consistent with a simple power-law of index ~1, and exhibit only slow drifts in flux on a timescale ~weeks. The ultra-violet to X-ray continuum can therefore be modelled as a power-law of index ~1 below about 0.1 keV, above which the source steepens. The spectral variability in the X-ray band can then be described in terms of a 'pivoting' of the high-energy continuum about the break-point. We have investigated two specific SSC models for the continuum emission. It was found that whilst a simplistic homogeneous disk-shaped emission region can provide an acceptable explanation of single epoch snap-shots of the multi-waveband spectrum, it experiences some difficulty accounting for the high energy spectral variability. In contrast, by allowing a variable radial dependence in the upper cut-off in the synchrotron emitting electron population in an inhomogeneous relativistic jet model, we have demonstrated that the multi-waveband spectrum of the most demanding source, Mrk 421, at all epochs can be reproduced. Thus we conclude that a jet model, broadly following the current paradigm for BL Lac type objects can give an acceptable explanation of present multi-waveband measurements.