Radio and Millimeter Monitoring of Sgr A*: Spectrum, Variability, and Constraints on the G2 Encounter

TL;DR

This study presents multi-frequency radio observations of Sagittarius A* over two years, finding variability but no significant impact from the G2 object, and constraining the nature of the emission region.

Contribution

It provides the first comprehensive multi-frequency monitoring of Sgr A* during the G2 encounter, constraining models of G2's interaction and the emission mechanisms.

Findings

No evidence of increased accretion or emission due to G2.

G2's cross-sectional area is less than 2 x 10^29 cm^2.

Spectral index remains flat between 230 and 690 GHz.

Abstract

We report new observations with the Very Large Array, Atacama Large Millimeter Array, and Submillimeter Array at frequencies from 1.0 to 355 GHz of the Galactic Center black hole, Sagittarius A*. These observations were conducted between October 2012 and November 2014. While we see variability over the whole spectrum with an amplitude as large as a factor of 2 at millimeter wavelengths, we find no evidence for a change in the mean flux density or spectrum of Sgr A* that can be attributed to interaction with the G2 source. The absence of a bow shock at low frequencies is consistent with a cross-sectional area for G2 that is less than $2 \times 10^{29}$ cm$^2$. This result fits with several model predictions including a magnetically arrested cloud, a pressure-confined stellar wind, and a stellar photosphere of a binary merger. There is no evidence for enhanced accretion onto the black hole driving greater jet and/or accretion flow emission. Finally, we measure the millimeter wavelength spectral index of Sgr A* to be flat; combined with previous measurements, this suggests that there is no spectral break between 230 and 690 GHz. The emission region is thus likely in a transition between optically thick and thin at these frequencies and requires a mix of lepton distributions with varying temperatures consistent with stratification.