University of Hertfordshire

From the same journal

From the same journal


  • G.E. Anderson
  • B.M. Gaensler
  • D.L. Kaplan
  • B. Posselt
  • P.O. Slane
  • J.C. Mauerhan
  • R.A. Benjamin
  • C.L. Brogan
  • D. Chakrabarty
  • J. Drake
  • J.E. Drew
  • J.E. Grindlay
  • J. Hong
  • T.J.W. Lazio
  • J.C. Lee
  • D. Steeghs
  • M.H. Van Kerkwijk
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Original languageEnglish
Pages (from-to)105-117
JournalThe Astrophysical Journal
Publication statusPublished - 2011


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.


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