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

How accretion proceeds around the massive black hole in the Galactic Centre and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic Centre passes through the accretion disc 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 posses a fairly powerful wind. We show here that the ram pressure of the accretion disc shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of similar to 1 keV and cools efficiently through optically thin, thermal bremsstrahlung emission. The radiation from the shocked wind peaks around the epoch of the pericentre 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 disc at a distance of several thousands of gravitational radii from the black hole.