Exploring the central sub-pc region of the \gamma-ray bright radio galaxy 3C 84 with the VLBA at 43 GHz in the period of 2002-2008

TL;DR

This study analyzes the kinematics and radio emission of the central region of 3C 84 from 2002-2008, revealing the formation and acceleration of a new radio component C3 and its relation to gamma-ray activity.

Contribution

It provides the first detailed kinematic analysis of the sub-pc scale jet in 3C 84 during 2002-2008, linking radio component evolution to gamma-ray emission.

Findings

The new component C3 was formed by 2003 and brightened significantly after 2008.

C3 showed moderate acceleration from 0.10c to 0.47c, remaining sub-relativistic.

A one-zone SSC model can reproduce gamma-ray emission but not the radio spectral index.

Abstract

Following the discovery of a new radio component right before the GeV \gamma-ray detection since 2008 August by Fermi Gamma-ray Space Telescope, we present a detailed study of the kinematics and lightcurve on the central sub-pc scale of 3C 84 using the archival VLBA 43-GHz data covering the period between 2002 January to 2008 November. We find that the new component "C3", previously reported by the observations with the VLBI Exploration of Radio Astrometry (VERA), was already formed in 2003. The flux density of C3 increases moderately until 2008, and then it becomes brighter rapidly after 2008. The radio core, C1, also shows a similar trend. The apparent speed of C3 with reference to the core C1 shows moderate acceleration from 0.10c to 0.47c between 2003 November to 2008 November, but is still sub-relativistic. We further try to fit the observed broadband spectrum by the one-zone synchrotron self-Compton (SSC) model using the measured apparent speed of C3. The fit can reproduce the observed \gamma-ray emission, but does not agree with the observed radio spectral index between 22 and 43 GHz.