Chandra Detection of SN 2010da Four Months After Outburst: Evidence for a High-mass X-Ray Binary in NGC 300

We present the results of a 63 ks Chandra observation of the "supernova impostor" SN 2010da four months after it was first observed on 2010 May 24. We detect an X-ray source at ~7σ confidence coincident with the optical location of the outburst. Our imaging analysis has revealed a hard central point source, surrounded by soft diffuse emission extending ~8'' north of the central source. The diffuse emission has a hardness ratio (~-0.4), 0.35-2 keV luminosity (~6 × 10³⁵ erg s^-1), and size (~70 pc) consistent with that of a supernova remnant, although the low number of counts prohibits detailed spectral modeling. The 0.5-6 keV spectrum of the central point source is well described by both a power law with Γ ~0 and a blackbody with kT ~ 1.8 keV, with no evidence for intrinsic absorption beyond the Galactic column. We estimate the 0.3-10 keV luminosity to be ~2 × 10³⁷ erg s^-1, a factor of ~25 decrease since the initial outburst. The high X-ray luminosity and slow fading rate is not consistent with this object being a single massive star undergoing an outburst. When combined with the optical and infrared properties of the outburst and progenitor systems, our X-ray observations suggest the SN 2010da system may be a Be X-ray binary system which experienced an extremely bright optical outburst near simultaneously with a bright, Type II X-ray outburst.