Probing the nature of the X-ray source IGR J16327-4940 with Chandra

Abstract

We report on the results of a Chandra observation of the source IGR J16327-4940, suggested to be a high-mass X-ray binary hosting a luminous blue variable star (LBV). The source field was imaged by ACIS-I in 2023 to search for X-ray emission from the LBV star and eventually confirm this association. No X-ray emission is detected from the LBV star, with an upper limit on the X-ray luminosity of L$_{\rm 0.5-10 \,keV}\lt 2.9(^{+1.6} _{-1.1})\times 10^{32}$ erg s^-1 (at the LBV distance d = 12.7$^{+3.2} _{-2.7}$ kpc). We detected 21 faint X-ray sources, 8 of which inside the INTEGRAL error circle. The brightest one is the best candidate soft X-ray counterpart of IGR J16327-4940, showing a hard power-law spectrum and a flux corrected for the absorption UF_{0.5-10 keV} = 2.5 × 10^-13 erg cm^-2 s^-1, implying a luminosity of 3.0 × 10³³ d$_{10~{\rm kpc}}^2$ erg s^-1. No optical/near-infrared counterparts have been found. Previous X-ray observations of the source field with Swift/XRT and ART-XC did not detect any source consistent with the INTEGRAL position. These findings exclude the proposed LBV star as the optical association, and pinpoint the most likely soft X-ray counterpart. In this case, the source properties suggest a low-mass X-ray binary, possibly a new member of the very faint X-ray transient class.