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 ∼5 × 10³⁸ erg s^-1 and faded by a factor of ∼25 in a four month period. Our two new Chandra observations show a factor of ∼10 increase in the 0.35-8 keV X-ray luminosity, from ∼4 × 10³⁶ to 4 × 10³⁷ erg s^-1 in ∼6 months, and the X-ray spectrum is consistent in both observations with a power-law with a photon index of Γ ∼ 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 (Γ ∼0) compared to the low-luminosity state (Γ ∼ 1.2 ± 0.8). Using our Hubble observations, we fit the colour-magnitude diagram of the coeval stellar population to estimate a time since formation of the SN 2010da progenitor system of ≲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 ≲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.