ASCA Observations of Blazars and Multiband Analysis

Abstract

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. The observations show a clear difference in the spectra between three subclasses of blazars, namely the High-energy peaked BL Lac objects (HBLs), Low-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission, with the Doppler factor consistent with the VLBI and/or gamma-ray variability data. For many QHBs, on the other hand, the gamma-rays cannot be solely due to the SSC mechanism unless the Doppler factor is significantly in excess of that inferred from VLBI data. We consider an alternative scenario consistent with the measured values of the Doppler factor, where the SSC component is still present in QHBs and it dominates in the X-ray band, but it is below the observed gamma-ray spectrum. With an assumption that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting the Doppler factor of 10, we infer the magnetic field to be 0.1 - 1 Gauss, and Lorentz factors of electrons radiating at the peak of the nu F(nu) spectrum of 10^3 for QHBs; this is much lower than 10^5 for HBLs. This difference is most likely due to the large photon density expected in QHBs.