Modelling the thermal X-ray emission around the Galactic Centre from colliding Wolf–Rayet winds

Abstract

The Galactic Centre is a hotbed of astrophysical activity, with the injection of wind material from $\sim$30 massive Wolf-Rayet (WR) stars orbiting within 12 arcsec of the supermassive black hole (SMBH) playing an important role. Hydrodynamic simulations of such colliding and accreting winds produce a complex density and temperature structure of cold wind material shocking with the ambient medium, creating a large reservoir of hot, X-ray-emitting gas. This work aims to confront the 3 Ms of Chandra X-ray Visionary Program observations of this diffuse emission by computing the X-ray emission from these hydrodynamic simulations of the colliding WR winds, amid exploring a variety of SMBH feedback mechanisms. The major success of the model is that it reproduces the spectral shape from the 2-5 arcsec ring around the SMBH, where most of the stellar wind material that is ultimately captured by Sgr A* is shock-heated and thermalised. This naturally explains that the hot gas comes from colliding WR winds, and that the wind speeds of these stars are, in general, well constrained. The flux level of these spectra, as well as 12 $\times$ 12-arcsec$^2$ images of 4-9 keV, shows that the X-ray flux is tied to the SMBH feedback strength; stronger feedback clears out more hot gas, thereby decreasing the thermal X-ray emission. The model in which Sgr A* produced an intermediate-strength outflow during the last few centuries best matches the observations to within about 10 per cent, showing that SMBH feedback is required to interpret the X-ray emission in this region.