The Galactic plane at faint X‐ray fluxes – I. Properties and characteristics of the X‐ray source population

We investigate the serendipitous X-ray source population revealed in XMM–Newton observations targeted in the Galactic plane within the region 315° 1032 erg s−1, whereas some will be extragalactic interlopers. A high fraction (≳90 per cent) of the soft sources have potential near-infrared (NIR) (Two-Micron All-Sky Survey and/or United Kingdom Infrared Deep Sky Survey) counterparts inside their error circles, consistent with the dominant soft-X-ray-source population being relatively nearby coronally-active stars. These stellar counterparts are generally brighter than J= 16, a brightness cut-off which corresponds to the saturation of the X-ray coronal emission at LX= 10−3 Lbol. In contrast, the success rate in finding likely IR counterparts to the hard X-ray sample is no more than ≈15 per cent down to J= 16 and ≈25 per cent down to J= 20, set against a rapidly rising chance coincidence rate. The make-up of the hard-X-ray-source population, in terms of the known classes of accreting and non-accreting systems, remains uncertain.