Recurring X-ray Outbursts in the Supernova Impostor SN~2010da in NGC~300

We present new observations of the "supernova impostor" SN 2010da using the Chandra X-ray Observatory and the Hubble Space Telescope. During the initial 2010 outburst, the 0.3-10 keV luminosity was observed by Swift to be \(\sim5\times10^{38}\) erg s\(^{-1}\) and faded by a factor of \(\sim\)25 in a four month period. Our two new Chandra observations show a factor of \(\sim\)10 increase in the 0.35-8 keV X-ray flux, from \(\sim\)4\(\times10^{36}\) erg s\(^{-1}\) to \(4\times10^{37}\) erg s\(^{-1}\) in \(\sim\)6 months, and the X-ray spectrum is consistent in both observations with a power law photon index of \(\Gamma\sim0\). We find evidence of X-ray spectral state changes: when SN 2010da is in a high-luminosity state, the X-ray spectrum is harder (\(\Gamma\sim0\)) compared to the low-luminosity state (\(\Gamma\sim1.2\pm0.8\)). Using our Hubble observations, we fit the color magnitude diagram of the coeval stellar population to estimate a time since formation of the SN 2010da progenitor system of \(\lesssim\)5 Myr. Our observations are consistent with SN 2010da being a high-mass X-ray binary (HMXB) composed of a neutron star and a luminous blue variable-like companion, although we cannot rule out the possibility that SN 2010da is an unusually X-ray bright massive star. The \(\lesssim\)5 Myr age is consistent with the theoretically predicted delay time between the formation of a massive binary and the onset of the HMXB phase. It is possible that the initial 2010 outburst marked the beginning of X-ray production in the system, making SN 2010da possibly the first massive progenitor binary ever observed to evolve into an HMXB.