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

TL;DR

This paper models the interaction between stellar winds and the accretion disk around Sgr A*, predicting X-ray flares during star passages that can inform about the disk's properties.

Contribution

It introduces a semi-analytical model verified by hydrodynamic simulations to describe wind-disk interactions near Sgr A* and predicts observable X-ray signatures.

Findings

Predicted X-ray flare with luminosity ~4×10^{33} erg/s during S2 star's pericenter passage.

Model matches observed quiescent X-ray emission levels from Sgr A*.

Detection of such flares can constrain disk density and thickness at ~3000 gravitational radii.

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 $\sim10^{8}$ 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 $\dot{M}_{\rm w}= 10^{-7} M_\odot\, {\rm yr}^{-1}$, $n_{\rm d} = 10^{5}$ 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.