@article {2011ApJ...727..105A, title = {{Identification of a Population of X-ray-emitting Massive Stars in the Galactic Plane}}, journal = {\apj}, volume = {727}, year = {2011}, note = {n/a}, pages = {105}, 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 of X-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. } }, doi = {10.1088/0004-637X/727/2/105}, author = {{Anderson}, G.~E. and {Gaensler}, B.~M. and {Kaplan}, D.~L. and {Posselt}, B. and {Slane}, P.~O. and {Murray}, S.~S. and {Mauerhan}, J.~C. and {Benjamin}, R.~A. and {Brogan}, C.~L. and {Chakrabarty}, D. and {Drake}, J.~J. and {Drew}, J.~E. and {Grindlay}, J.~E. and {Hong}, J. and {Lazio}, T.~J.~W. and {Lee}, J.~C. and {Steeghs}, D.~T.~H. and {van Kerkwijk}, M.~H.} }