Estimation of the Doppler Factors for Blazars Using a Bayesian Hierarchical Model

Based on the long-term monitoring data at 1 mm wavelength from the Submillimeter Array (SMA), we used a Bayesian hierarchical model to fit the light curves of 155 blazars and estimated their radio brightness temperatures and variability Doppler factors. The brightness temperature and Doppler factor distributions between BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs), as well as between Fermi and non-Fermi blazars, were compared using the Wilcoxon rank-sum test. The results show that (1) FSRQs have higher average Doppler factor than BL Lacs; (2) Fermi blazars have higher average brightness temperature and Doppler factor than non-Fermi blazars. After comparing the brightness temperatures and variability Doppler factors at 1 mm and 15 GHz wavelengths, we found a tendency of decreasing towards higher frequencies for these two parameters.