Simulating the pericentre passage of the Galactic centre star S2

Abstract

Context. Our knowledge of the density distribution of the accretion flow around Sgr A* – the massive black hole (BH) at our Galactic centre (GC) – relies on two measurements only: one at a distance of a few Schwarzschild radii (Rs) and one at roughly 105 Rs, which are usually bridged by a power law, which is backed by magnetohydrodynamical simulations. The so-called S2 star reached its closest approach to the massive BH at around 1500 Rs in May 2018. It has been proposed that the interaction of its stellar wind with the high-density accretion flow at this distance from Sgr A* will lead to a detectable, month-long X-ray flare. Aims. Our goal is to verify whether or not the S2 star wind can be used as a diagnostic tool to infer the properties of the accretion flow towards Sgr A* at its pericentre (an unprobed distance regime), putting important constraints on BH accretion flow models. Methods. We run a series of three-dimensional adaptive mesh refinement simulations with the help of the RAMSES code which include the realistic treatment of the interaction of S2’s stellar wind with the accretion flow along its orbit and – apart from hydrodynamical and thermodynamical effects – include the tidal interaction with the massive BH. These are post-processed to derive the X-ray emission in the observable 2–10 keV window. Results. No significant excess of X-ray emission from Sgr A* is found for typical accretion flow models. A measurable excess is produced for a significantly increased density of the accretion flow. This can, however, be ruled out for standard power-law accretion flow models as in this case the thermal X-ray emission without the S2 wind interaction would already exceed the observed quiescent luminosity. Only a significant change of the wind parameters (increased mass loss rate and decreased wind velocity) might lead to an (marginally) observable X-ray flaring event. Conclusion. Even the detection of an (month-long) X-ray flare during the pericentre passage of the S2 star would not allow for strict constraints to be put on the accretion flow around Sgr A* due to the degeneracy caused by the dependence on multiple parameters (of the accretion flow model as well as the stellar wind).