Modelling Accretion Disk and Stellar Wind Interactions: the Case of Sgr A*

Abstract

Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disk, the ram and thermal pressures of the disk terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in $\sim 10^8$ cm s${}^{-1}$ range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericenter passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of $L\approx 4\times 10^{33}$ erg s${}^{-1}$ for a stellar mass-loss rate, disk number density, and thermal pressure strength of ${\dot{M}}_{\mathrm{w}}=10^{-7}{M}_{\odot }{\mathrm{yr}}^{-1}$, $n_{\mathrm{d}}=10^5$ cm${}^{-3}$, and $\alpha =0.1$, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disk at a distance of $\sim 3000$ gravitational radii from the supermassive black hole.

12 pages, 9 figures, accepted by MNRAS

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• First observed interaction of the circumstellar envelope of an S-star with the environment of Sgr A*
• High-sensitivity radio study of the non-thermal stellar bow shock EB27
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• X3: a high-mass Young Stellar Object close to the supermassive black hole Sgr~A*
• The formation and stability of a cold disc made out of stellar winds in the Galactic Centre
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• Runaway O-star Bow Shocks as Particle Accelerators? The Case of AE Aur revisited
• Non-thermal emission in hyper-velocity and semi-relativistic stars
• 3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2
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