Superluminal Motions of AGNs and GRBs at Multiple-Frequencies

Abstract

Superluminal motion has been found to occur in many kinds of celestial bodies with the relativistic ejecta, especially for some active galactic nuclei (AGNs), gamma-ray bursts (GRBs) and micro-quasars, although these objects are largely different in their lifetimes, sizes and features. We compare the apparent superluminal motions of GRBs with AGNs and different sub-classes of AGNs (e.g., Radio galaxies, BL Lac objects, and Quasars). Particularly, we focus on two low luminosity GRBs, namely 060218 and 170817A, that are ultra-long and short bursts associated with supernova and gravitational wave, respectively. The apparent transverse velocity ($\beta_{app}$) of Swift and pre-Swift GRBs are tightly correlated with the Doppler factor $\delta$ as $\beta_{app}\propto\delta^{0.5}$, while all AGNs distribute around the line of $\beta_{app}= \delta$ behaving a weak correlation of $\beta_{app}$ with $\delta$ . In contrast, $\beta_{app}$ is positively correlated with Lorenz factor $\gamma$ or $\gamma^2(1+z)^{-1}$, not for GRBs but for AGNs. $\beta_{app}$ and $1+z$ are independent for neither GRBs nor AGNs, but apparently exhibit a positive correlation of $\beta_{app}$ with $1+z$ from AGNs to GRBs, showing a cosmological effect of evolution with redshift. I also found that GRB 170817A and GRB 060218 are outliers of the above correlations. Superluminal properties of GRBs are significantly different from those of AGNs. However, there are no obvious differences between radio galaxies, BL Lac objects, and quasars in terms of their superluminal motion. In despite of radiation mechanism, beaming effect and cosmological expanding would play the same roles on the superluminal motion for AGNs and GRBs.