Multi-Epoch VERA Observations of Sagittarius A*: I. Images and Structural Variability

TL;DR

This study presents multi-epoch VLBI observations of Sgr A* at 43 GHz, revealing flux and size variations, a prolonged flaring event, and suggesting intrinsic changes in the source's properties rather than scattering effects.

Contribution

First multi-epoch VLBI imaging of Sgr A* at 43 GHz showing flux and size variability, and proposing a model involving electron heating for flaring events.

Findings

Detected 11% flux variation and 19% size variation.

Identified a flaring event lasting over 10 days.

Flaring likely caused by intrinsic electron heating, not scattering.

Abstract

We report the results of multi-epoch observations of Sgr A* with VLBI Exploration of Radio Astrometry (VERA) at 43 GHz, carried out from 2004 to 2008. We detected a time variation of flux at 11 % level and intrinsic size at 19 %. In addition, comparisons with previous Very Long Baseline Array (VLBA) results shows that Sgr A* underwent the flaring event at least longer than 10 days in May 2007. The intrinsic size of Sgr A* remained unchanged within 1 $\sigma$ level from the size before/after the flaring event, indicating that the brightness temperature of Sgr A* was increased. The flaring event occurred within 31 d, which is shorter than the refractive time scale. Moreover it is difficult to explain the increase in the spectral index at the flaring event by the simple interstellar scattering model. Hence, the flaring event is most likely associated with the changes in intrinsic properties of Sgr A*. We considered the origin of the brightness temperature variation, and concluded that the flaring event of Sgr A* could be explained by the continuous heating of electrons, such as a standing shock in accretion flow.