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 \times 10 ^ { -14 } ergs cm ^ { -2 } s ^ { -1 } , and none is a likely counterpart on energetic grounds .
Over the entire 17 ^ { \prime } \times 17 ^ { \prime } ACIS-I field , we place an upper limit of < 5.4 \times 10 ^ { -12 } ergs cm ^ { -2 } s ^ { -1 } on the excess diffuse flux in the 2 - 10 keV band .
One hard point-source with flux \approx 4 \times 10 ^ { -14 } ergs cm ^ { -2 } s ^ { -1 } lies within 0. ^ { \prime } 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 .