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<l<45 and |b|<2.5 deg. 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
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~2.5 or a thermal spectrum with kT~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~1 with N_H=10^{22-24} cm^{-2}. For ~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,
whereas some will be extragalactic interlopers. >90% of the soft sources have
potential NIR (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 IR counterparts to the hard X-ray sample is no more than
~15% down to J=16 and ~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.
