Chandra Observation of the Unidentified TeV Gamma-Ray Source HESS J1303−631 in the Galactic Plane

The imaging atmospheric Cherenkov array H.E.S.S. recently discovered an extended source in the 0.4−10 TeV energy range, HESS J1303−631. We obtained a 5 ks observation with the ACIS-I array on the Chandra X-ray observatory that does not reveal an obvious compact or diffuse X-ray counterpart. Archival ROSAT images are also blank in this region. Although there are several radio pulsars within the field of HESS J1303−631, none is detected in X-rays to a flux limit of < 5 × 10 ergs cm s, and none is a likely counterpart on energetic grounds. Over the entire 17×17 ACIS-I field, we place an upper limit of < 5.4× 10 ergs cm s on the excess diffuse flux in the 2− 10 keV band. One hard point-source with flux ≈ 4× 10 ergs cm s lies within 0.5 of the centroid of the TeV emission. These exploratory observations suggest that deeper pointings with Chandra and XMM-Newton are needed before we can learn more about the nature of HESS J1303−631. Its similarity to the unidentified source TeV J2032+4130 indicates the probable existence of a new class of high-energy source in the Galactic plane that originates from young, massive stars or their supernova remnants. Subject headings: gamma-rays: individual (HESS J1303−631) — gamma-rays: observations — X-rays: stars