The ultraviolet to X-ray continua of BL Lac objects.

Abstract

We present the results from EXOSAT observations of three X-ray bright BL Lacertae objects, Mrk 501, 1218+304 and MRK 180. All three sources have soft power-law X-ray spectra with low-energy cut-offs consistent with absorption in the line-of-sight gas column density through our own galaxy. The three objects also exhibit significant spectral variability in the X-ray band on time-scales ranging from a few days (in the case of 1218+304) to a year (Mrk 501). In each case, X-ray flux and spectral index appear to be correlated, in the sense that the X-ray spectrum hardens as the source brightens. The intrinsic ultraviolet to X-ray spectrum of these and several other X-ray bright BL Lac objects can be modelled as a power-law continuum of energy index ~1.0 below about 0.1 keV, above which the spectral slope steepens. The X-ray spectral variability can then be described in terms of a 'pivoting' of the high-energy continuum about the 0.1 keV break-point. In the context of the standard synchrotron self-Compton models, such behaviour favours a direct synchrotron origin for the soft X-ray continuum and inhomogeneous geometries for the emitting region.