Infrared spectroscopy of SWIFT J0850.8-4219: Identification of the second red supergiant X-ray binary in the Milky Way

Abstract

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 $\approx 5\times 10^{-6}$. We present a $1.0-2.5\mu$m 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 ${}^{12}$CO(2,0) band, ii) strong CN bandheads and absent TiO bandheads at $\approx 1.1\mu$m and iii) equivalent width of the Mg I $1.71\mu$m line, we classify the counterpart to be a K3$-$K5 type RSG with an effective temperature of $3820\pm 100$ K, located at a distance of $\approx 12$ kpc. We estimate the source X-ray luminosity to be $(4\pm 1)\times 10^{35}$ erg s${}^{-1}$, with a hard photon index ($\Gamma <1$), 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.

6 pages, 4 figures, 1 table. Submitted to MNRAS letters. Comments welcome!