Publication Date:
2015-07-15
Description:
The nature of the low- to intermediate-luminosity ( L X ~ 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 Two Micron All Sky Survey and Galactic Legacy Infrared Mid-Plane Survey Extraordinaire 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 per cent of the soft sources are classified in the literature: ~91 per cent as stars, with a minor fraction of Wolf–Rayet (WR) stars. Roughly 15 per cent of the hard sources are classified in the literature: ~68 per cent as high-mass X-ray stars single or in binary systems (WR, Be and high-mass X-ray binaries – 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 L X ~ 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 -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 -Cas analogues, WRs and low L X HMXBs. However, the nature of the X-ray population with K s ≥ 11 and average X-ray-to-infrared flux ratio remains unconstrained.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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