Deep X-ray Observations of the Gamma-ray Bright Pulsar Wind Nebula HESS J1303-631

HESS J1303$-$631 is an evolved, TeV-bright pulsar wind nebula (PWN) powered by the energetic radio pulsar PSR~J1301$-$6305, offset by $\approx 7^{\mathrm{\prime }}$ northwest of the TeV centroid, with an energy-dependent gamma-ray morphology. We present new, deep joint Chandra (108 ks) and XMM-Newton (102 ks) observations of this field. High-resolution Chandra ACIS imaging resolves the pulsar from a neighboring point source that XMM-Newton cannot separate. The pulsar’s 2-10 keV spectrum is well described by an absorbed power law with $\mathrm{\Gamma }=2.02\pm 1.04$ and an unabsorbed luminosity of $3.44\times {10}^{31}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$ at $d=4.7$ kpc. Chandra also reveals faint diffuse emission east of the pulsar: a clumpy compact nebula on arcsecond scales ($\mathrm{\Gamma }=2.98\pm 0.53$, $L_X=4.15\times {10}^{31}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$) and a fainter extended nebula on arcminute scales ($\mathrm{\Gamma }=1.20\pm 1.25$, $L=4.36\times {10}^{31}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$). This morphology, the hard GeV counterpart, and the energy-dependent TeV emission together support an evolved, synchrotron-underluminous PWN whose X-ray nebula is far more compact than its TeV nebula. We also serendipitously discovered a transient X-ray source, CXOU~J130131.0$-$625919, in one of the nine new Chandra observations. Its hard spectrum ($\mathrm{\Gamma }=1.4\pm 0.3$) and 0.5-7,keV luminosity of $3.34\times {10}^{32}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$ at a fiducial distance of 3 kpc, together with a plausible near-infrared counterpart, most closely match a flare from a young pre-main-sequence star, though an active binary or compact-object binary is not excluded. Its large offset from HESS J1303$-$631 makes association with the TeV source unlikely.