We have observed 18 blazars with the X-ray satellite ASCA, half of which were also observed contemporaneously with the γ-ray detector EGRET onboard the CGRO satellite as parts of multiband campaigns. Multiband analysis was made based on above data and published results to study the radiation process of blazars.
The ASCA observations showed a clear difference in variability and spectra between three subclasses, the X-ray selected BL Lac objects(XBLs), radio-seledted BL Lac objects(RBLs), and quasar-hosted blazars(QHBs). We found that XBLs have the steep X-ray spectra with photon indices 2.0~2.7, and their spectra tend to be gradually steepening with increasing energy. QHBs are found to have flatter spectra with photon indices 1.4~1.7, which are well represented by the power law shape. RBLs have spectra with photon indices 1.7~2.0. We detected time variability: XBLs varied on timescale as short as hours, while QHBs varied on longer timescale of days or months.
We detected a simultaneous X-ray/TeV γ-ray flare from XBL Mkn421 with no apparent variability in the radio, optical, UV, and GeV γ-ray bands. The correlation suggests that a single electron population is responsible for both X-rays and TeV γ-rays. We observed the soft X-ray flare lagged behind the hard X-ray flare by 1 hr. We interpret the lag is due to energy loss of electrons by the synchrotron radiation: the magnetic field inferred by this picture is ~0.2 gauss.
We studied two broad pronounced peaks in the spectral energy distribution νF(ν) of blazars: the low energy (LE) peak and the high energy (HE) peak. ASCA spectra clearly shows that the X-ray band is a higher energy tail of the LE peak for XBLs and a lower energy tail of the HE peak for QHBs. For RBLs the X-ray band is found to be a mixture of the LE and HE peak. We found the negative correlation between the luminosity ratio of two peaks and the peak frequency of the LE peak. The LE peak reaches the maximum at 1013-1014Hz for QHBs and 1016-1017Hz for XBLs.
We found that the radiation process of HE peak for γ-ray dominant blazars commonly cannot be explained by only Synchrotron-Self-Compton(SSC) process. By utilizing the ASCA spectra and the LE peak which is thought to be synchrotron radiation, we estimated the γ-ray luminosity via SSC process. The luminosity of the synchrotron and the SSC radiation gave the energy density of the synchrotron photon and the magnetic field. The magnetic field is inferred to be 0.1~1 gauss. From the derived magnetic field and the observed cutoff frequency of the LE peak. We estimated the maximum Lorentz factor γ of electron is ~104 for QHBs. ~106 for XBLs. Our analysis suggests that this difference of γ is due to the large photon density of QHBs compared with that of XBLs.