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

• Main Authors: Anderson, Gemma E. (Author), Gaensler, B. M. (Author), Kaplan, David L. (Author), Posselt, Bettina (Author), Slane, Patrick O. (Author), Murray, Stephen S. (Author), Mauerhan, Jon C. (Author), Benjamin, Robert A. (Author), Brogan, Crystal L. (Author), Chakrabarty, Deepto (Contributor), Drake, Jeremy J. (Author), Drew, Janet E. (Author), Grindlay, Jonathan E. (Author), Hong, Jaesub (Author), Lazio, T. Joseph W. (Author), Lee, Julia C. (Author), Steeghs, Danny T. H. (Author), van Kerkwijk, Marten H. (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Physics (Contributor), MIT Kavli Institute for Astrophysics and Space Research (Contributor)
• Format: Article
• Language: English
• Published: Institute of Physics Publishing, 2012-04-13T15:59:44Z.
• Subjects: Article
• Online Access: Get fulltext

 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.