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 about7' 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 plus or minus1.04 and an unabsorbed luminosity of 3.44 times10 31, 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 plus or minus0.53, L X=4.15 times10 31, erg,s-1) and a fainter extended nebula on arcminute scales (Γ=1.20 plus or minus1.25, L=4.36 times10 31, 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 plus or minus0.3) and 0.5-7,keV luminosity of 3.34 times10 32, 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.