Speaker
Description
X-ray binaries with red supergiant companions (RSGs) are extremely rare, and probe a very short-lived phase of binary evolution, just before the system is expected to undergo a second supernova. eRASS J085039.9-421151 (also Swift J0850.8-4219) was detected as a persistent source in the eROSITA survey at a relatively lower luminosity of 1e35 erg/s. The source was subsequently followed-up by NuSTAR, which showed a strong FeK-alpha line with an equivalent width of 0.7keV, and required a partial covering absorber with column density ~1e24 cm^-2 to yield a good fit. No pulsations were detected, but binary evolution scenarios lean towards a slowly rotating neutron star as the likely accretor. The system is one of only two RSG X-ray binaries in the Milky Way, with the other also showing similarly persistent behaviour and comparable X-ray luminosity, but markedly less obscuration. VLT/X-shooter data of the optical companion of eRASS J085039.9-421151, revealed its semi-variable nature as a red supergiant with multiple emission lines that vary in strength between the two available observations. In addition to emission lines attributed to the red supergiant's radial pulsations, there are also higher-ionisation lines that require the presence of hot plasma, likely indicating the accretion stream onto the neutron star. The obscuration was proposed to be due to the dusty environments that red supergiants are typically embedded in. However, recent XMM-Newton data do not show such strong levels of obscuration, making inferences on the absorber non-trivial, complicating the previous proposed picture. Multiwavelength study of such a system not only expands our current understanding of binary evolution, but also offers a window into the poorly understood stellar environment of red supergiants. The vastly improved spectral resolution and sensitivity offered by NewAthena makes the study of such a system an ideal science case, as it can pave the way for uncovering new such probes into late stage binary evolution.
