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We present new observations of the "supernova impostor" SN 2010da using theChandra X-ray Observatory and the Hubble Space Telescope. During the initial2010 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 fourmonth period. Our two new Chandra observations show a factor of $\sim$10increase 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 isconsistent 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 ahigh-luminosity state, the X-ray spectrum is harder ($\Gamma\sim0$) compared tothe low-luminosity state ($\Gamma\sim1.2\pm0.8$). Using our Hubbleobservations, we fit the color magnitude diagram of the coeval stellarpopulation to estimate a time since formation of the SN 2010da progenitorsystem of $\lesssim$5 Myr. Our observations are consistent with SN 2010da beinga high-mass X-ray binary (HMXB) composed of a neutron star and a luminous bluevariable-like companion, although we cannot rule out the possibility that SN2010da is an unusually X-ray bright massive star. The $\lesssim$5 Myr age isconsistent with the theoretically predicted delay time between the formation ofa massive binary and the onset of the HMXB phase. It is possible that theinitial 2010 outburst marked the beginning of X-ray production in the system,making SN 2010da possibly the first massive progenitor binary ever observed toevolve into an HMXB.

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