2011MNRAS.413..595W

We investigate the serendipitous X-ray source population revealed in XMM-Newton observations targeted in the Galactic plane within the region 315° < l < 45° and|b| < 2{img}. Our study focuses on a sample of 2204 X-ray sources at intermediate to faint fluxes, which were detected in a total of 116 XMM-Newton fields and are listed in the Second XMM-Newton Serendipitous Source Catalog. We characterize each source as spectrally soft or hard on the basis of whether the bulk of the recorded counts have energies below or above 2 keV and find that the sample divides roughly equally (56 per cent:44 per cent) into these soft and hard categories. The X-ray spectral form underlying the soft sources may be represented as either a power-law continuum with Γ∼ 2.5 or a thermal spectrum with kT∼ 0.5 keV, with N_H ranging from 10²⁰ to 10²²/cm2. For the hard sources, a significantly harder continuum form is likely, that is, Γ∼ 1, with N_H= 10²²-10²⁴/cm2. For ∼50 per cent of the hard sources, the inferred column density is commensurate with the total Galactic line-of-sight value; many of these sources will be located at significant distances across the Galaxy, implying a hard-band luminosity L_X> 10³² erg/s, 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 L_X= 10^–3 L_bol. 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.