Masses of Nearby Supermassive Black Holes with Very-Long Baseline Interferometry

TL;DR

This paper explores how combining VLBI imaging with dynamical measurements can significantly improve mass estimates of nearby supermassive black holes, especially Sgr A*, by reducing the mass-distance correlation.

Contribution

It demonstrates that future VLBI observations can halve the uncertainty in black hole mass measurements and identifies optimal targets for such imaging.

Findings

VLBI imaging can improve mass accuracy by a factor of two for Sgr A*.

Simulations show the potential of 1 mm VLBI to refine black hole mass estimates.

Other nearby supermassive black holes are suitable targets for future VLBI imaging.

Abstract

Dynamical mass measurements to date have allowed determinations of the mass M and the distance D of a number of nearby supermassive black holes. In the case of Sgr A*, these measurements are limited by a strong correlation between the mass and distance scaling roughly as M ~ D^2. Future very-long baseline interferometric (VLBI) observations will image a bright and narrow ring surrounding the shadow of a supermassive black hole, if its accretion flow is optically thin. In this paper, we explore the prospects of reducing the correlation between mass and distance with the combination of dynamical measurements and VLBI imaging of the ring of Sgr A*. We estimate the signal to noise ratio of near-future VLBI arrays that consist of five to six stations, and we simulate measurements of the mass and distance of Sgr A* using the expected size of the ring image and existing stellar ephemerides. We demonstrate that, in this best-case scenario, VLBI observations at 1 mm can improve the error on the mass by a factor of about two compared to the results from the monitoring of stellar orbits alone. We identify the additional sources of uncertainty that such imaging observations have to take into account. In addition, we calculate the angular diameters of the bright rings of other nearby supermassive black holes and identify the optimal targets besides Sgr A* that could be imaged by a ground-based VLBI array or future space-VLBI missions allowing for refined mass measurements.