We investigate the serendipitous X-ray source population revealed in XMM-Newton observations targeted in the Galactic Plane within the region 315 ^ { \circ } < l < 45 ^ { \circ } and |b| < 2.5 ^ { \circ } .
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 2XMMi catalogue .
We characterise 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 % :44 % ) 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 \Gamma \sim 2.5 or a thermal spectrum with kT \sim 0.5 keV , with N _ { H } ranging from 10 ^ { 20 - 22 } ~ { } cm ^ { -2 } .
For the hard sources , a significantly harder continuum form is likely , i.e. , \Gamma \sim 1 , with N _ { H } = 10 ^ { 22 - 24 } ~ { } cm ^ { -2 } .
For \sim 50 % 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 } > 10 ^ { 32 } ~ { } erg~ { } s ^ { -1 } , whereas some will be extragalactic interlopers .
A high fraction ( ^ { > } _ { \sim } 90 % ) of the soft sources have potential near-infrared ( 2MASS and/or UKIDSS ) 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 cutoff 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 infrared counterparts to the hard X-ray sample is no more than \approx 15 \% down to J=16 and \approx 25 \% 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 .