First Space-VLBI Observations of Sagittarius A*

TL;DR

This paper reports the first Earth-space VLBI observations of Sgr A* using the RadioAstron project, achieving unprecedented resolution but not detecting fringes on longer baselines, highlighting the need for submillimeter observations.

Contribution

First space VLBI observations of Sgr A* demonstrating the potential and limitations of space VLBI at centimeter wavelengths for black hole imaging.

Findings

Consistent with anisotropic Gaussian scattering at short baselines.

Detection of persistent image substructure at intermediate baselines.

No fringes detected on long ground or space baselines.

Abstract

We report results from the first Earth-space VLBI observations of the Galactic Center supermassive black hole, Sgr A*. These observations used the space telescope Spektr-R of the RadioAstron project together with a global network of 20 ground telescopes, observing at a wavelength of 1.35cm. Spektr-R provided baselines up to 3.9 times the diameter of the Earth, corresponding to an angular resolution of approximately 55 microarcseconds and a spatial resolution of $5.5 R_{\rm Sch}$ at the source, where $R_{\rm Sch} \equiv 2 G M/c^2$ is the Schwarzschild radius of Sgr A*. Our short ground baseline measurements (<80 M\lambda) are consistent with an anisotropic Gaussian image, while our intermediate ground baseline measurements (100-250 M\lambda) confirm the presence of persistent image substructure in Sgr A*. Both features are consistent with theoretical expectations for strong scattering in the ionized interstellar medium, which produces Gaussian scatter-broadening on short baselines and refractive substructure on long baselines. We do not detect interferometric fringes on any of the longer ground baselines or on any ground-space baselines. While space VLBI offers a promising pathway to sharper angular resolution and the measurement of key gravitational signatures in black holes, such as their photon rings, our results demonstrate that space VLBI studies of Sgr A* will require sensitive observations at submillimeter wavelengths.