Abstract
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 (Lx = 10^27-10^34 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 Lx 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 deg2. The flux limit of our survey is 2 x 10-15 erg/cm^2/s in the soft (0.5 - 2 keV) band and 1 x 10^-14 erg/cm^2/s 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 ~ 1 kpc. The number of identified active stars increases towards late spectral types, reaching a maximum at K. (abridged)
Highlights
Non-solar X-ray emission was discovered in the early 1960s using collimating instruments
In this paper we present results from an optical identification campaign conducted in the Galactic plane, at low and intermediate Galactic latitudes (|b| < 20◦ ) and covering a wide range of Galactic longitudes, by the XMM-Newton Survey Science Centre (Watson et al 2001)
At the chosen cutoff for the individual identification probability, the number of spurious associations is lower than 2%, so we expect a maximum of 5 spurious matches, a value not sufficient to explain the number of sources detected in the HR4 bands
Summary
Non-solar X-ray emission was discovered in the early 1960s using collimating instruments. Similar to other wavelength ranges, flux limited X-ray surveys allow us to gather large and homogeneous samples of different species of high energy sources such as stars or AGN X-ray sources are useful tracers of their parent stellar populations and their study can shed light on the evolutionary mechanisms of single and binary stars in remote parts of our Galaxy. They may as well help to constrain the past stellar formation rate, in particular the early formation stages during which many massive compact remnants were created.
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