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Infrared identification of hard X-ray sources in the Galaxy

The nature of the low- to intermediate-luminosity (Lx$\,\sim 10^{32-34}$ erg s$^{-1}$) source population revealed in hard band (2-10 keV) X-ray surveys of the Galactic Plane is poorly understood. To overcome such problem we cross-correlated the XMM-Newton 3XMM-DR4 survey with the infrared 2MASS and GLIMPSE catalogues. We identified reliable X-ray-infrared associations for 690 sources. We selected 173 sources having hard X-ray spectra, typical of hard X-ray high-mass stars (kT$\,>\,5\,$keV), and 517 sources having soft X-ray spectra, typical of active coronae. About $18\,\%$ of the soft sources are classified in the literature: $\sim\,91\%$ as stars, with a minor fraction of WR stars. Roughly $15\,\%$ of the hard sources are classified in the literature: $\sim\,68\%$ as high-mass X-ray stars single or in binary systems (WR, Be and HMXBs), with a small fraction of G and B stars. We carried out infrared spectroscopic pilot observations at the William Herschel Telescope for five hard X-ray sources. Three of them are high-mass stars with spectral types WN7-8h, Ofpe/WN9 and Be, and Lx$\sim\,10^{32}-10^{33}$erg s$^{-1}$. One source is a colliding-wind binary, while another source is a colliding-wind binary or a supergiant fast X-ray transient in quiescence. The Be star is a likely $γ$-Cas system. The nature of the other two X-ray sources is uncertain. The distribution of hard X-ray sources in the parameter space made of X-ray hardness ratio, infrared colours and X-ray-to-infrared flux ratio suggests that many of the unidentified sources are new $γ$-Cas analogues, WRs and low Lx HMXBs. However, the nature of the X-ray population with Ks $\geq$ 11 and average X-ray-to-infrared flux ratio remains unconstrained.