The variability of Sagittarius A* at 3 millimeter

TL;DR

This study monitored Sgr A* at 3 mm over several years, revealing intra-day variability and supporting models involving expanding plasma or orbiting hot spots, with implications for understanding its physical behavior.

Contribution

First long-term 3-mm variability monitoring of Sgr A* with detailed intra-day event analysis and model implications.

Findings

Detected intra-day flux variations of up to 33% within a few hours.

Supported the expanding plasmon and hot spot models for variability.

Estimated a minimum mass-loss rate of 9.7×10^{-10} solar masses per year.

Abstract

We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling can not be ignored, and a minimum mass-loss rate of 9.7*10^{-10}M_sun /yr is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*.