Simultaneous HESS and Chandra observations of Sagitarius A(star) during an X-ray flare

The rapidly varying (~10 min timescale) non-thermal X-ray emission observed from Sgr A$^{\star}$ implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV $\gamma$-ray source HESS J1745-290 is coincident with Sgr A$^{\star}$ and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of $\approx$9 increase in the X-ray flux of Sgr A$^{\star}$, no evidence is found for an increase in the TeV $\gamma$-ray flux from this region. We find that an increase in the $\gamma$-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the $\gamma$-ray emission is produced within ~1014 cm (~100 $\,R_{\rm S}$) of the supermassive black hole.