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 $\sim 5\times {10}^{38}$ 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$4$\times {10}^{36}$ erg s${}^{-1}$ to $4\times {10}^{37}$ erg s${}^{-1}$ in $\sim$6 months, and the X-ray spectrum is consistent in both observations with a power law photon index of $\mathrm{\Gamma }\sim 0$. We find evidence of X-ray spectral state changes: when SN 2010da is in a high-luminosity state, the X-ray spectrum is harder ($\mathrm{\Gamma }\sim 0$) compared to the low-luminosity state ($\mathrm{\Gamma }\sim 1.2\pm 0.8$). 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.