X-ray source populations in the Galactic plane

We present the first results from the XMM–Newton Galactic Plane Survey (XGPS). In the first phase of the programme, 22 pointings were used to cover a region of approximately 3 deg2 between 19° and 22° in Galactic longitude and ±0.6° in latitude. In total we have resolved over 400 point X-ray sources, at ≥ 5σ significance, down to a flux limit of ∼2 × 10−14 erg s−1 cm−2 (2–10 keV). The sources exhibit a very wide range of spectral hardness, with interstellar absorption identified as a major influence. The source populations detected in the soft (0.4–2 keV) band and hard (2–6 keV) band show surprisingly little overlap. The majority of the soft sources appear to be associated with relatively nearby stars with active stellar coronae, judging from their high coincidence with bright stellar counterparts. The combination of the XGPS measurements in the hard X-ray band with the results from earlier surveys carried out by ASCA and Chandra reveals the form of the low-latitude X-ray source counts over 4 decades of flux. It appears that extragalactic sources dominate below ∼10−13 erg s−1 cm−2 (2–10 keV), with a predominantly Galactic source population present above this flux threshold. The nature of the faint Galactic population observed by XMM–Newton remains uncertain, although cataclysmic variables and RS CVn systems may contribute substantially. XMM–Newton observes an enhanced surface brightness in the Galactic plane in the 2–6 keV band associated with Galactic ridge X-ray emission (GRXE). The integrated contribution of Galactic sources plus the breakthrough of extragalactic signal accounts for up to 20 per cent of the observed surface brightness. The XGPS results are consistent with the picture suggested from a deep Chandra observation in the Galactic plane, namely that the bulk of the GRXE is truly diffuse.