ChandraObservation 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 anextended source in the 0.4$-$10 TeV energy range, HESS J1303-631. We obtained a5 ks observation with the ACIS-I array on the Chandra X-ray observatory thatdoes not reveal an obvious compact or diffuse X-ray counterpart. Archival ROSATimages are also blank in this region. Although there are several radio pulsarswithin the field of HESS J1303-631, none is detected in X-rays to a flux limitof $<5 \times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$, and none is a likelycounterpart on energetic grounds. Over the entire $17^{\prime} \times17^{\prime}$ 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. Onehard 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. Theseexploratory observations suggest that deeper pointings with Chandra and XMM areneeded before we can learn more about the nature of HESS J1303-631. Itssimilarity to the unidentified source TeV J2032+4130 indicates the probableexistence of a new class of high-energy source in the Galactic plane thatoriginates from young, massive stars or their supernova remnants.Comment: Accepted for publication in The Astrophysical Journa