Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA NuSTAR and X-shooter

TL;DR

This multiwavelength study of the X-ray binary 1eRASS J085039.9-421151 confirms its nature as a neutron star system with a red supergiant companion, using combined X-ray and optical data to analyze its spectral features and properties.

Contribution

The paper provides a detailed multiwavelength analysis confirming the neutron star nature of the X-ray binary with a red supergiant companion, including spectral characterization and absorption features.

Findings

X-ray spectrum fits an absorbed power law with high energy cutoff.

Detection of a strong fluorescent neutral iron line (~700 eV).

No pulsations detected, suggesting a large spin period or absorption effects.

Abstract

The eROSITA instrument on board Spectrum-Roentgen-Gamma has completed four scans of the X-ray sky, leading to the detection of almost one million X-ray sources in eRASS1 only, including multiple new X-ray binary candidates. We report on analysis of the X-ray binary 1eRASS J085039.9-421151, using a ~55\,ks long NuSTAR observation, following its detection in each eROSITA scan. 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, although we determine a cooler spectral type of 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 detect a strong fluorescent neutral iron line with an equivalent width of ~700\,eV and an absorption edge, the latter indicating strong absorption by a partial covering component. It is unclear if the partial absorber is ionised. There is no significant evidence of a cyclotron resonant scattering feature. We do not detect any pulsations in the NuSTAR lightcurves, possibly on account of a large spin period that goes 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.