Discovery of a Red Supergiant Donor Star in SN2010da/NGC 300 ULX-1

SN2010da/NGC 300 ULX-1 was first detected as a supernova impostor in 2010 May and was recently discovered to be a pulsating ultraluminous X-ray source. In this Letter, we present Very Large Telescope/X-shooter spectra of this source obtained in 2018 October, covering the wavelength range 350-2300 nm. The J- and H-bands clearly show the presence of a red supergiant (RSG) donor star that is best matched by a MARCS stellar atmosphere with T _eff = 3650-3900 K and log(L _bol/L _o) = 4.25 ± 0.10, which yields a stellar radius R = 310 ± 70R _o. To fit the full spectrum, two additional components are required: a blue excess that can be fitted either by a hot blackbody (T ⪆ 20,000 K) or a power law (spectral index α ≈ 4) and is likely due to X-ray emission reprocessed in the outer accretion disk or the donor star; and a red excess that is well fitted by a blackbody with a temperature of ∼1100 K, and is likely due to warm dust in the vicinity of SN2010da. The presence of an RSG in this system implies an orbital period of at least 0.8-2.1 yr, assuming Roche-lobe overflow. Given the large donor-to-compact object mass ratio, orbital modulations of the radial velocity of the RSG are likely undetectable. However, the radial velocity amplitude of the neutron star is large enough (up to 40-60 km s^-1) to potentially be measured in the future, unless the system is viewed at a very unfavorable inclination.