Abstract We collect a sample of 51 Fermi-detected gamma-ray loud blazars with known radio Doppler factors and study properties of the Doppler factors of blazars at optical, X-ray and gamma-ray bands. A basic assumption is that the emission from the radio to gamma-ray bands of the blazars are produced by the nonthermal radiation of accelerated particles in a jet. Our results show that (1) the Doppler factors of blazars are a function of frequency, with the Doppler factor decreasing with frequency from the radio to X-ray regions, and then increasing from the X-ray to γ-ray regions which are similar to results given by Zhang et al., and (2) there are marginal correlations between the Doppler factors at radio and X-ray bands and the synchrotron peak fre quency, and a strong correlation between the Doppler factor in the gamma-ray band and the synchrotron peak frequency, but no correlation in the optical band.

Keywords galaxies: active — galaxies: nuclei — galaxies: jets — radiation mecha- nisms: non-thermal

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