2023/09/14 by Kishalay De, De, Kishalay, F. Daly +3 · 2 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #Solar and Stellar Astrophysics (astro-ph.SR) #Stellar, planetary, and galactic studies
paper · pdf · doi:10.48550/arxiv.2309.07833
openalex publication_date 2023/09/14 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
High mass X-ray binaries hosting red supergiant (RSG) donors are a rare but crucial phase in massive stellar evolution, with only one source previously known in the Milky Way. In this letter, we present the identification of the second Galactic RSG X-ray binary SWIFT J0850.8-4219. We identify the source 2MASS 08504008-4211514 as the likely infrared counterpart with a chance coincidence probability ≈ 5 × 10-6. We present a 1.0 - 2.5 μm spectrum of the counterpart, exhibiting features characteristic of late-type stars and an exceptionally strong He I emission line, corroborating the identification. Based on i) the strength of the 12CO(2,0) band, ii) strong CN bandheads and absent TiO bandheads at ≈ 1.1 μm and iii) equivalent width of the Mg I 1.71 μm line, we classify the counterpart to be a K3-K5 type RSG with an effective temperature of 3820 ± 100 K, located at a distance of ≈ 12 kpc. We estimate the source X-ray luminosity to be (4 ± 1) × 1035 erg s-1, with a hard photon index (Γ< 1), arguing against a white dwarf accretor but consistent with a magnetized neutron star in the propeller phase. Our results highlight the potential of systematic NIR spectroscopy of Galactic hard X-ray sources in completing our census of the local X-ray binary population.