Annu. Rev. Astron. Astrophys. 1997. 35: 445-502 Copyright © 1997 by Annual Reviews. All rights reserved

6.4. X Rays: The Crossing of Different Emission Components

For HBL, the medium energy X-ray emission (2-6 keV) is typically steep, extrapolating smoothly from the UV and comprising part of the downward curving synchrotron spectrum. In LBL, the synchrotron component curves down well below the X-ray band, which is then dominated by a much flatter component (see Figure 6). Emission line blazars tend to fall in the LBL category. Borderline objects may have both components contributing in the X-ray range with the steep one prevailing in the soft X-ray range.

In HBL, rapid large-amplitude X-ray variability is the rule (flux doubling on time scales of hours). The spectra harden systematically with increasing intensity (Urry et al 1986, Treves et al 1989, George et al 1988b, Giommi et al 1990, Sembay et al 1993, Sambruna et al 1994). Comparing the UV and X-ray variability of HBL suggests that both spectral changes and variability amplitude are greater beyond the synchrotron peak, which is in the soft X rays for these objects.

ROSAT observations of a complete sample of radio-selected BL Lac objects (mostly LBL) show, in general, flatter spectra than for HBL and different variability behavior as well. In three cases there were "inverse" spectral changes, that is, a softening of the spectrum with increasing intensity (Cappi et al 1994, Urry et al 1996). This can be understood in terms of the relative variation of two spectral components that intersect each other in the X-ray range: a soft, highly variable one that swamps (high soft intensity) or uncovers (low soft intensity) a less-variable flatter component (see Figure 6). Such objects should be intermediate between LBL and HBL.

Less is known about the X-ray variability of emission line blazars because they are relatively weak X-ray sources. Einstein observations revealed extremely hard spectra in the 0.2-4 keV band (Worrall & Wilkes 1990), much flatter than the extrapolation of the optical-UV spectrum. Short-term variations were not detected in 3C 279 and NRAO 140, the only two blazars with sufficient intensity in the Ginga data base (2-20 keV), while long-term variations were seen in three out of four sources observed repeatedly (Tashiro 1995).

In summary, when a flat component is present in the X-ray band (as for LBL), it appears to vary on longer time scales and with lower amplitude than the steep X-ray component that is thought to be an extension of the longer wavelength synchrotron emission. However, this statement may be partly biased by the lower X-ray fluxes of LBL compared to those of the better-observed HBL.