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 onboard Compton Gamma Ray Observatory as parts of multiwavelength campaigns. The observations show a clear difference in the spectra between three subclasses of blazars, namely, high-energy peaked BL Lacertae objects (HBLs), low-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). The ASCA X-ray spectra of HBLs are the softest, with the power-law energy index α ~ 1-2, and they form the highest observable energy tail of the low-energy (LE, synchrotron) component. The X-ray spectra of the QHBs are the hardest (α ~ 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 responsible for the HE peak for HBLs can be explained solely by Doppler-boosted synchrotron self-Compton (SSC) emission, with the Doppler factor δ consistent with the VLBI and/or γ-ray variability data. For many QHBs, on the other hand, the γ-rays cannot be solely a result of the SSC mechanism unless δ is significantly in excess of that inferred from VLBI data. We consider an alternative scenario consistent with the measured values of δ, where the SSC component is still present in QHBs and dominates in the X-ray band but is below the observed γ-ray spectrum. With an assumption that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting a value of 10 for δ, we infer the magnetic field B to be 0.1-1 G and Lorentz factors γb of electrons radiating at the peak of the νF(ν) spectrum of ~103 for QHBs; this is much lower than γb ~ 105 for HBLs, even though the values of B are comparable in the two subclasses. This difference of γb is most likely 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.

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