The Intrinsic Shape of Sagittarius A* at 3.5-mm Wavelength

TL;DR

This paper presents high-resolution VLBI observations of Sagittarius A* at 3.5 mm, revealing its intrinsic size and shape, and shows that observed asymmetries are due to interstellar scattering rather than intrinsic features.

Contribution

First successful 3.5 mm VLBI detection of Sgr A* with an expanded array, providing new measurements of its intrinsic size and shape, and analyzing scattering effects.

Findings

Measured intrinsic source size: 147x120 microarcseconds.

Detected non-zero closure phases explained by scattering, not intrinsic asymmetry.

Confirmed source orientation at approximately 88 degrees east of north.

Abstract

The radio emission from Sgr A$^\ast$ is thought to be powered by accretion onto a supermassive black hole of $\sim\! 4\times10^6~ \rm{M}_\odot$ at the Galactic Center. At millimeter wavelengths, Very Long Baseline Interferometry (VLBI) observations can directly resolve the bright innermost accretion region of Sgr A$^\ast$. Motivated by the addition of many sensitive, long baselines in the north-south direction, we developed a full VLBI capability at the Large Millimeter Telescope Alfonso Serrano (LMT). We successfully detected Sgr A$^\ast$ at 3.5~mm with an array consisting of 6 Very Long Baseline Array telescopes and the LMT. We model the source as an elliptical Gaussian brightness distribution and estimate the scattered size and orientation of the source from closure amplitude and self-calibration analysis, obtaining consistent results between methods and epochs. We then use the known scattering kernel to determine the intrinsic two dimensional source size at 3.5 mm: $(147\pm7~\mu\rm{as}) \times (120\pm12~\mu\rm{as})$, at position angle $88^\circ\pm7^\circ$ east of north. Finally, we detect non-zero closure phases on some baseline triangles, but we show that these are consistent with being introduced by refractive scattering in the interstellar medium and do not require intrinsic source asymmetry to explain.