ASCA observations of blazars and multiband analysis

We present data for 18 blazars observed with the X-ray satellite ASCA, half of which were also observed contemporaneously with the EGRET instrument on board Compton Gamma Ray Observatory as parts of multiwavelength campaigns. The observations show a clear difference in the spectra between three subcl asses of blazars, namely, high-energy peaked BL Lacertae objects (HBLs), lo w-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). Th e ASCA X-ray spectra of HBLs are the softest, with the power-law energy ind ex alpha similar to 1-2, and they form the highest observable energy tail o f the low-energy (LE, synchrotron) component. The X-ray spectra of the QHBs are the hardest (alpha similar to 0.6) and are consistent with the lowest observable energy end of the high-energy (HE, Compton) component. For LBLs, the X-ray spectra are intermediate. We find that the radiation process res ponsible for the HE peak for HBLs can be explained solely by Doppler-booste d synchrotron self-Compton (SSC) emission, with the Doppler factor delta co nsistent with the VLBI and/or gamma-ray variability data. For many QHBs, on the other hand, the gamma-rays cannot be solely a result of the SSC mechan ism unless delta is significantly in excess of that inferred from VLBI data . We consider an alternative scenario consistent with the measured values o f delta, where the SSC component is still present in QHBs and dominates in the X-ray band but is below the observed gamma-ray spectrum. With an assump tion that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting a value of 10 for delta, we infer the magnetic fiel d B to be 0.1-1 G and Lorentz factors gamma(b) of electrons radiating at th e peak of the nu F(nu) spectrum of similar to 10(3) for QHBs; this is much lower than gamma(b) similar to 10(5) for HBLs, even though the Values of B are comparable in the two subclasses. This difference of gamma(b) is most l ikely due to the large photon density expected in QHBs (e.g., from thermal components visible in these objects) as compared with that of HBLs; Compton upscattering of these photons may well provide the observed GeV flux.