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

The imaging atmospheric Cerenkov array HESS 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^-14 ergs cm^-2 s^-1, 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^-12 ergs cm^-2 s^-1 on the excess diffuse flux in the 2-10 keV band. One hard point source with flux ~4×10^-14 ergs cm^-2 s^-1 lies within 0.5 arcmin 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.