The multiwavelength Doppler factors for a sample of gamma-ray loud blazars

Abstract

The high luminosity and rapid variability detected in γ-ray loud blazars imply that the beaming effect plays an important role in these sources. The Doppler factor, depending on two unobservable parameters (Lorentz factor and viewing angle), is very important for understanding the basic properties of blazars. Although the viewing angle is unobservable, increasing direct and indirect evidence has shown up to indicate that the jet is bent, and therefore the viewing angle also varies. In this sense, the Doppler factor, which is dependent on the viewing angle and the Lorentz factor, should also vary. In the present paper, using available data of blazars at different (radio, optical, X-ray, and γ-ray) wavebands and assuming that multi-waveband radiation is produced by the radiation of accelerated particles in a jet, we discuss the properties of the Doppler factors of blazars at different wavebands. Our results indicate that the Doppler factor of a blazar is a function of frequency. The Doppler factor decreases with frequency from radio to X-ray bands, but increases with frequency from X-ray to γ-ray bands. The γ-ray Doppler factors found in the present paper are correlated with those by Ghisellini et al. (1998, AAA070.159.155), suggesting that our estimation is reasonable. Furthermore, BL Lacertae objects are found to show lower Doppler factors and higher synchrotron peak frequencies compared with FSRQs; the promising TeV sources are BL Lacertae objects.