Many different classes of X-ray sources contribute to the Galactic landscape at high energies .
Although the nature of the most luminous X-ray emitters is now fairly well understood , the population of low-to-medium X-ray luminosity ( L _ { \mathrm { X } } = 10 ^ { 27 - 34 } \mathrm { erg s ^ { -1 } } ) sources remains much less studied , our knowledge being mostly based on the observation of local members .
The advent of wide field and high sensitivity X-ray telescopes such as XMM-Newton now offers the opportunity to observe this low-to-medium L _ { \mathrm { X } } population at large distances .
We report on the results of a Galactic plane survey conducted by the XMM-Newton Survey Science Centre ( SSC ) .
Beyond its astrophysical goals , this survey aims at gathering a representative sample of identified X-ray sources at low latitude that can be used later on to statistically identify the rest of the serendipitous sources discovered in the Milky Way .
The survey is based on 26 XMM-Newton observations , obtained at |b| < 20 deg , distributed over a large range in Galactic longitudes and covering a summed area of 4 deg ^ { 2 } .
The flux limit of our survey is 2 \times 10 ^ { -15 } \mathrm { erg cm ^ { -2 } s ^ { -1 } } in the soft ( 0.5 – 2 keV ) band and 1 \times 10 ^ { -14 } \mathrm { erg cm ^ { -2 } s ^ { -1 } } in the hard ( 2 – 12 keV ) band .
We detect a total of 1319 individual X-ray sources .
Using optical follow-up observations supplemented by cross-correlation with a large range of multi-wavelength archival catalogues we identify 316 X-ray sources .
This constitutes the largest group of spectroscopically identified low latitude X-ray sources at this flux level .
The majority of the identified X-ray sources are active coronae with spectral types in the range A – M at maximum distances of \sim 1 kpc .
The number of identified active stars increases towards late spectral types , reaching a maximum at K. Using infrared colours we classify 18 % of the stars as giants .
The observed distributions of F _ { \mathrm { X } } /F _ { \mathrm { V } } , X-ray and infrared colours indicates that our sample is dominated by a young ( 100 Myr ) to intermediate ( 600 Myr ) age population with a small contribution of close main sequence or evolved binaries .
We find other interesting objects such as cataclysmic variables ( d \sim 0.6 - 2 kpc ) , low luminosity high mass stars ( likely belonging to the class of \gamma -Cas-like systems , d \sim 1.5 - 7 kpc ) , T Tauri and Herbig-Ae stars .
A handful of extragalactic sources located in the highest Galactic latitude fields could be optically identified .
For the 20 fields observed with the EPIC pn camera , we have constructed log N ( > S ) – log S curves in the soft and hard bands .
In the soft band , the majority of the sources are positively identified with active coronae and the fraction of stars increases by about one order of magnitude from b = 60 ^ { \circ } to b = 0 ^ { \circ } at an X-ray flux of 2 \times 10 ^ { -14 } \mathrm { erg cm ^ { -2 } s ^ { -1 } } .
The hard band is dominated by extragalactic sources , but there is a small contribution from a hard Galactic population formed by CVs , HMXB candidates or \gamma -Cas-like systems and by some active coronae that are also detected in the soft band .
At b = 0 ^ { \circ } the surface density of hard sources brighter than 1 \times 10 ^ { -13 } \mathrm { erg cm ^ { -2 } s ^ { -1 } } steeply increases by one order of magnitude from l = 20 ^ { \circ } to the Galactic centre region ( l = 0.9 ^ { \circ } ) .