Near-infrared spectroscopy of 20 new Chandra sources in the Norma arm

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: frahoui@eso.org
² Harvard University, Department of Astronomy, 60 Garden street, Cambridge MA 02138, USA
³ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley CA 94720-7450, USA
⁴ Astronomy Department, University of California, 601 Campbell Hall, Berkeley CA 94720, USA
⁵ Department of Chemistry, Physics, and Astronomy, Georgia College & State University CBX 82, Milledgeville GA 31061, USA
⁶ Instituto de Astrofísica, Facultad de Física, Pontifica Universidad Católica de Chile, 306 Santiago 22, Chile
⁷ Millennium Institute of Astrophysics  
⁸ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder CO 80301, USA

Received: 16 April 2014
Accepted: 4 June 2014

Abstract

We report on CTIO/NEWFIRM and CTIO/OSIRIS photometric and spectroscopic observations of 20 new X-ray (0.5–10 keV) emitters discovered in the Norma arm Region Chandra Survey (NARCS). NEWFIRM photometry was obtained to pinpoint the near-infrared counterparts of NARCS sources, while OSIRIS spectroscopy was used to help identify 20 sources with possible high mass X-ray binary properties. We find that (1) two sources are WN8 Wolf-Rayet stars, maybe in colliding wind binaries, part of the massive star cluster Mercer 81; (2) two are emission-line stars, possibly in X-ray binaries, that exhibit near- and mid-infrared excesses either due to free-free emission from the decretion discs of Be stars or warm dust in the stellar winds of peculiar massive stars such as B[e] supergiants or luminous blue variables; (3) one is a B8-A3 IV-V star that could be in a quiescent high mass X-ray binary system; (4) two are cataclysmic variables including one intermediate polar; (5) three may be neutron star symbiotic binaries; (6) five are most likely white dwarf symbiotic binaries; and (7) five exhibit properties more consistent with isolated giant/dwarf stars. The possible detection of one to three high mass X-ray binaries is in good agreement with our predictions. However, our study illustrates the difficulty of clearly differentiating quiescent or intermediate X-ray luminosity systems from isolated massive stars, which may lead to an underestimation of the number of known high mass X-ray binaries.