Identification of a Population of X-ray-emitting Massive Stars in the Galactic Plane

Abstract

We present X-ray, infrared, optical, and radio observations of four previously unidentified Galactic plane X-ray sources: AX J163252–4746, AX J184738–0156, AX J144701–5919, and AX J144547–5931. Detection of each source with the Chandra X-ray Observatory has provided sub-arcsecond localizations, which we use to identify bright infrared counterparts to all four objects. Infrared and optical spectroscopy of these counterparts demonstrate that all four X-ray sources are extremely massive stars, with spectral classifications: Ofpe/WN9 (AX J163252–4746), WN7 (AX J184738–0156 = WR121a), WN7–8h (AX J144701–5919), and OIf^+ (AX J144547–5931). AX J163252–4746 and AX J184738–0156 are both luminous, hard, X-ray emitters with strong Fe xxv emission lines in their X-ray spectra at ~6.7 keV. The multi-wavelength properties of AX J163252–4746 and AX J184738–0156 are not consistent with isolated massive stars or accretion onto a compact companion; we conclude that their X-ray emission is most likely generated in a colliding-wind binary (CWB) system. For both AX J144701–5919 and AX J144547–5931, the X-ray emission is an order of magnitude less luminous and with a softer spectrum. These properties are consistent with a CWB interpretation for these two sources also, but other mechanisms for the generation ofX-rays cannot be excluded. There are many other as yet unidentified X-ray sources in the Galactic plane, with X-ray properties similar to those seen for AX J163252–4746, AX J184738–0156, AX J144701–5919, and AX J144547–5931. This may indicate a substantial population of X-ray-emitting massive stars and CWBs in the Milky Way.

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