First Space-VLBI Observations of Sagittarius A*

Michael D Johnson,Carl R Gwinn,Mikhail M Lisakov,Petr A Voitsik,Gabriele Bruni,Yuri Y Kovalev

doi:10.3847/2041-8213/ac3917

Abstract

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 1.35 cm. Spektr-R provided baselines up to 3.9 times the diameter of the Earth, corresponding to an angular resolution of approximately 55 μas and a spatial resolution of 5.5R Sch at the source, where R Sch ≡ 2GM/c 2 is the Schwarzschild radius of Sgr A*. Our short ground baseline measurements ( ≲ 80 Mλ) are consistent with an anisotropic Gaussian image, while our intermediate ground baseline measurements (100–250 Mλ) 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.

Highlights

Very long baseline interferometry (VLBI) provides the highest direct angular resolution in astronomy, recently culminating in the first image of a black hole (The Event Horizon Telescope Collaboration et al 2019)
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
Space-VLBI is an exciting frontier for black hole astrophysics, with the potential to resolve the gravitationally lensed “photon rings” of nearby supermassive black holes (Johnson et al 2020), to measure the masses of thousands of supermassive black holes via their “shadow” diameters (Pesce et al 2021), and to track the orbits of many supermassive black hole binaries (D’Orazio & Loeb 2018)

Summary

INTRODUCTION

Very long baseline interferometry (VLBI) provides the highest direct angular resolution in astronomy, recently culminating in the first image of a black hole (The Event Horizon Telescope Collaboration et al 2019). Gwinn et al (2014) discovered compact image substructure in Sgr A∗ at λ = 1.3 cm, with approximately 1% of the total flux density measured on baselines up to 3000 km (≈ 250×106λ, or 0.8 mas resolution) This signal is consistent with expected “refractive substructure” produced by scattering in the ionized interstellar medium Using the VLBI software processing package PIMA , we performed baseline-based fringe fitting, which included determination of the phase acceleration term to statistically separate observations with significant interferometric signal and http://astrogeo.org/pima/. For our subsequent analysis and discussion, we use results from both these software packages, relying on PFDs and upper limits from PIMA for baselines without detected fringes, and using the highest possible signal-to-noise ratio of ground antenna-based fringe fitting solutions in AIPS. We did not detect fringes on any baselines to Spektr-R

ANALYSIS AND RESULTS

F S -VLBI S

Short-Baseline Analysis

Long-Baseline Analysis

Consistency with Expected Properties

SUMMARY