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High mass X-ray binaries hosting red supergiant (RSG) donors are a rare butcrucial phase in massive stellar evolution, with only one source previouslyknown in the Milky Way. In this letter, we present the identification of thesecond Galactic RSG X-ray binary SWIFT J0850.8-4219. We identify the source2MASS 08504008-4211514 as the likely infrared counterpart with a chancecoincidence probability $\approx 5 \times 10^{-6}$. We present a $1.0 -2.5\,\mu$m spectrum of the counterpart, exhibiting features characteristic oflate-type stars and an exceptionally strong He I emission line, corroboratingthe 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 tobe 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-rayluminosity to be $(4 \pm 1) \times 10^{35}$ erg s$^{-1}$, with a hard photonindex ($\Gamma < 1$), arguing against a white dwarf accretor but consistentwith a magnetized neutron star in the propeller phase. Our results highlightthe potential of systematic NIR spectroscopy of Galactic hard X-ray sources incompleting our census of the local X-ray binary population.

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