The binary nature of the Galactic centre X-ray source CXOGC J174536.1-285638

¹ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK e-mail: jsc@star.ucl.ac.uk
² Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
³ Department of Physics Astronomy, Louisiana State University, 273 Nicholson Hall, Tower Drive, Baton Rouge, LA 70803, USA

Received: 30 June 2009
Accepted: 27 August 2009

Abstract

Context. The combination of X-ray and near-IR surveys of the central 2°0.8° of the Galactic centre have revealed a population of X-ray bright massive stars. However, the nature of the X-ray emission, originating in wind collision zones or via accretion onto compact objects, is uncertain.

Aims. In order to address this we investigated the nature of one of the most luminous X-ray sources - CXOGC J174536.1-285638.

Methods. This was accomplished by an analysis of the near-IR spectrum with a non-LTE model atmosphere code to determine the physical parameters of the primary.

Results. This was found to be an highly luminous WN9h star, which is remarkably similar to the most massive stars found in the Arches cluster, for which comparison to evolutionary tracks suggest an age of 2–2.5 Myr and an initial mass of ~110 . The X-ray properties of CXOGC J174536.1-285638 also resemble those of 3 of the 4 X-ray detected WN9h stars within the Arches and in turn other very massive WNLh colliding wind binaries, of which WR25 forms an almost identical “twin”. Simple analytical arguments demonstrate consistency between the X-ray emission and a putative WN9h+mid O V-III binary, causing us to favour such a scenario over an accreting binary. However, we may not exclude a high mass X-ray binary interpretation, which, if correct, would provide a unique insight into the (post-SN) evolution of extremely massive stars. Irrespective of the nature of the secondary, CXOGC J174536.1-285638 adds to the growing list of known and candidate WNLh binaries. Of the subset of WNLh stars subject to a radial velocity survey, we find a lower limit to the binary fraction of ~45%; of interest for studies of massive stellar formation, given that they currently possess the highest dynamically determined masses of any type of star.