Multi-wavelength study of 1eRASS J085039.9-421151 with eROSITA, NuSTAR, and X-shooter

The eROSITA instrument on board Spectrum-Roentgen-Gamma (SRG) has completed four scans of the X-ray sky, leading to the detection of almost one million X-ray sources in eRASS1 alone, including multiple new X-ray binary candidates. We report on analysis of the X-ray binary 1eRASS J085039.9-421151, using a similar to 55 ks long NuSTAR observation, following its detection in each eROSITA scan. An analysis of the eROSITA and NuSTAR X-ray spectra in combination with X-shooter data of the optical counterpart provide evidence of an X-ray binary with a red supergiant (RSG) companion, confirming previous results. However, we did determine a cooler spectral type for M2-3, owing to the presence of TiO bands in the optical and near-infrared spectra. The X-ray spectrum is well-described by an absorbed power law with a high-energy cutoff typically applied for accreting high mass X-ray binaries. In addition, we detected a strong fluorescent neutral iron line with an equivalent width of similar to 700 eV and an absorption edge, the latter indicating strong absorption by a partial covering component. It is unclear whether the partial absorber is ionised. There is no significant evidence of a cyclotron resonant scattering feature. We did not detect any pulsations in the NuSTAR light curves, possibly on account of a large spin period that has gone undetected due to insufficient statistics at low frequencies or potentially large absorption that causes pulsations to be smeared out. Even so, the low persistent luminosity, the spectral parameters observed (photon index, Gamma < 1.0), and the minuscule likelihood of detection of RSG-black hole systems suggest that the compact object is a neutron star.