The donor star of NGC 300 ULX-1/SN2010da

SN2010da was a bright transient event in the nearby galaxy NGC 300. Discovered in 2010, it was initially classified as a supernova but quickly identified as a supernova impostor. After the 2010 outburst the system showed up as an increasingly bright X-ray source (Binder+2011), and it has been in the ULX regime for the past few years. The origin of the 2010 outburst is still unclear; the dusty progenitor has been identified as a luminous blue variable (Chornock+2010, Elias-Rosa+2010), a yellow supergiant transitioning onto a blue loop (Villar+2016) or a supergiant B[e] star (Lau+2016). X-ray observations with XMM-Newton, NuSTAR and Swift revealed that the accretor is a rapidly up-spinning neutron star (Carpano+2018, Walton+2018, Binder+2018).We have obtained deep optical through near-infrared spectra of SN2010da with X-shooter in 2018 that for the first time reveal the massive star in this system to be a red supergiant. I will discuss the implications of this discovery for the origin of the 2010 outburst and the resulting X-ray binary.