7.2.2 Multiwavelength Spectral Continuity of BL Lac Objects and FSRQ

The multiwavelength spectra of HBL, LBL, and FSRQ form a continuous sequence that suggests a common physical mechanism (Sambruna 1994; Sambruna et al. 1995; Maraschi et al. 1995). The spectra of LBL (RBL) and FSRQ have relatively low peak wavelengths while the HBL (XBL) have higher peak wavelengths. Very roughly, along the sequence from HBL to LBL to FSRQ, the wavelength of the peak synchrotron emission decreases ⁽¹⁷⁾ , the X-ray spectral index flattens, the ratio of gamma-ray to bolometric luminosity increases, the bolometric luminosity itself increases, and the mean redshift increases (luminosity and redshift are automatically correlated in flux-limited samples). These trends are illustrated in part by the multiwavelength spectra of a typical FSRQ (Fig. 4) and a typical HBL and LBL (Fig. 15).

Analyzing the spectra of these and other blazars in terms of synchrotron models, Sambruna (1994) finds that a simple transformation in terms of angle between HBL and LBL can not explain the differences in their multiwavelength spectra. To account for the shorter peak wavelength of the synchrotron emission in HBL compared to LBL, it is necessary to invoke both higher electron Lorentz factors (not to be confused with the bulk Lorentz factor of the jet) and stronger magnetic fields in the X-ray-emitting region. Similarly, the FSRQ have still lower electron energies and weaker magnetic fields than the RBL. The lower electron energies mean that in the X-ray band, the synchrotron component is relatively less important than any Compton-scattered emission (whether from scattered synchrotron photons or other ambient photons), leading to the flatter X-ray spectra and perhaps the stronger gamma-ray emission.

Thus it is plausible that all blazars are dominated by synchrotron plus Compton-scattered continuum emission, with mean relativistic electron energies and magnetic fields systematically declining with increasing bolometric luminosity (Sambruna 1994). To establish this connection among BL Lac objects and FSRQ definitively, via analysis of the physical parameters in blazar jets, requires extensive multiwavelength monitoring of complete samples of blazars. Unfortunately it is difficult to correlate variability between gamma-ray and other wavelengths because current detector sensitivity precludes variability studies for all but a few of the brightest blazars (Kniffen et al. 1993; Hunter et al. 1993).