The S2 star as a probe of the accretion disk of Sgr A*

TL;DR

This paper proposes that the interaction between the S2 star's stellar wind and the accretion disk of Sgr A* produces observable X-ray emission, which can be used to probe the disk's density near the black hole.

Contribution

It introduces a model of how the S2 star's wind interacts with the accretion disk, providing a method to measure disk density through observed shock emission during pericenter passage.

Findings

Shocked wind reaches ~1 keV temperature and cools via bremsstrahlung.

X-ray luminosity peaks near S2's pericenter, comparable to Sgr A*'s quiescent emission.

Detection of this emission constrains the accretion disk density at several thousand gravitational radii.

Abstract

How accretion proceeds around the massive black hole in the Galactic center and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic center passes through the accretion disk of the massive black hole and any observational signature from such interaction may be used as an accretion probe. Because of its early stellar type, S2 is expected to possess a fairly powerful wind. We show here that the ram pressure of the accretion disk shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of ~ 1 keV and cools efficiently through optically thin, thermal bremsstrahlung emission. The radiation from the shocked wind peaks around the epoch of the pericenter passage of the star at a luminosity potentially comparable to the quiescent emission detected from Sgr A*. Detection of shocked wind radiation can constrain the density of the accretion disk at a distance of several thousands of gravitational radii from the black hole.