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

TL;DR

This study uses near-infrared spectroscopy and photometry to classify 20 new X-ray sources in the Norma Arm, identifying various stellar types and potential high-mass X-ray binaries, highlighting classification challenges.

Contribution

It provides detailed classifications of 20 new X-ray sources in the Norma Arm, including potential high-mass X-ray binaries, using combined near-infrared observations, which was not previously done for these sources.

Findings

Identified two Wolf-Rayet stars, possibly in colliding wind binaries.

Detected two emission-line stars with infrared excesses, likely in X-ray binaries.

Found one star that could be in a quiescent high-mass X-ray binary system.

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.