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

TL;DR

This study reports the discovery of the second red supergiant X-ray binary in the Milky Way, using infrared spectroscopy to identify the companion star and analyze the system's properties, contributing to understanding massive stellar evolution.

Contribution

The paper presents the first identification of a Galactic RSG X-ray binary using infrared spectroscopy, expanding the known population and providing detailed spectral classification and system parameters.

Findings

Identified 2MASS 08504008-4211514 as the RSG counterpart.

Classified the companion as a K3-K5 type RSG at ~12 kpc.

Estimated X-ray luminosity of (4 ± 1) × 10^{35} erg/s, indicating a neutron star accretor.

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.