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 $\approx7'$ 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 $Γ=2.02\pm1.04$ and an unabsorbed luminosity of $3.44\times10^{31}\,{\rm erg\,s^{-1}}$ at $d=4.7$ kpc. Chandra also reveals faint diffuse emission east of the pulsar: a clumpy compact nebula on arcsecond scales ($Γ=2.98\pm0.53$, $L_X=4.15\times10^{31}\,{\rm erg\,s^{-1}}$) and a fainter extended nebula on arcminute scales ($Γ=1.20\pm1.25$, $L=4.36\times10^{31}\,{\rm erg\,s^{-1}}$). 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$-$625919, in one of the nine new Chandra observations. Its hard spectrum ($Γ=1.4\pm0.3$) and 0.5-7,keV luminosity of $3.34\times10^{32}\,{\rm erg\,s^{-1}}$ 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.