Simulating the pericentre passage of the Galactic centre star S2
  The so-called S2 star reached its closest approach to the massive black hole
(BH) at around 1500 $R_\mathrm{s}$ 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. 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. 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.
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. 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).
