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Description
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<sub>X</sub> = 10<sup>27</sup> - 10<sup>34</sup> erg/s) 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<sub>X</sub> population at large distances. This study reports 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 degrees, distributed over a large range in Galactic longitudes and covering a summed area of 4 deg<sup>2</sup>. The flux limit of this survey is 2 x 10<sup>-15</sup> erg/cm<sup>2</sup>/s in the soft (0.5 - 2 keV) band and 1 x 10<sup>-14</sup> erg/cm<sup>2</sup>/s in the hard (2 - 1 2keV) band. A total of 1319 individual X-ray sources have been detected. Using optical follow-up observations supplemented by cross-correlation with a large range of multi-wavelength archival catalogs, the authors 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 to M at maximum distances of ~1 kpc. The number of identified active stars increases towards late spectral types, reaching a maximum at K. Using infrared colors, the authors classify 18% of the stars as giants. The observed distributions of F<sub>X</sub>/F<sub>V</sub>, X-ray and infrared colors indicates that their 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. The authors find other interesting objects such as cataclysmic variables (d ~ 0.6 - 2 kpc), low-luminosity high-mass stars (likely belonging to the class of Gamma-Cas-like systems, d ~ 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, the authors 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 degrees to b = 0 degrees at an X-ray flux of 2 x 10<sup>-14</sup> erg/cm<sup>2</sup>/s. 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 coronal stars that are also detected in the soft band. At b = 0 degrees, the surface density of hard sources brighter than 1 x 10<sup>-13</sup> erg/cm<sup>2</sup>/s steeply increases by one order of magnitude from l = 20 degrees to the Galactic center region (l = 0.9 degrees). This HEASARC table contains 739 X-ray sources detected in the 26 different fields observed in this study and listed in Tables 8 - 33, inclusive, of the reference paper. These 739 sources have the best XMM quality, i.e. the summary flag sum_flag which contains information about flags set automatically and manually for a given source is zero, meaning that there are no negative flags for the source detection, have either a 2MASS, USNO, GSC, or SDSS counterpart, whatever the probability of identification is, or have some information via SIMBAD or the authors own imaging or spectroscopic observations. For each X-ray source, its X-ray parameters are summarized, listing the pn count rates, and information on optical and infrared counterparts is provided. The properties of the 26 target fields are given in Table 1 of the reference paper, along with the breakdown of source classes in each field. This table was created by the HEASARC in May 2013 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/A+A/553/A12">CDS Catalog J/A+A/553/A12</a>, the 26 files table8.dat to table33.dat, inclusive. This is a service provided by NASA HEASARC .
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