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Description
The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed about 85 deg<sup>2</sup> of sky in the 4.5 - 10 keV band down to a flux of 4 - 5 x 10<sup>-14</sup> erg/cm<sup>2</sup>/s using 142 high Galactic latitude (|b| > 20<sup>o</sup>) observations made by the BeppoSAX Medium Energy Concentrator Spectrometer (MECS). The source surface density of 16.9 +/- 6.4 deg<sup>2</sup> at the survey limit corresponds to a resolved fraction of the 5 - 10 keV X-ray background (XRB) of the order of 20-30 per cent. Hardness ratio analysis indicates that the spectra of a substantial fraction of the HELLAS sources (at least one-third) are harder than a alpha<sub>E</sub> = 0.6 power law. This hardness may be caused by large absorbing columns. The hardness ratio analysis also indicates that many HELLAS sources may have a spectrum more complex than a single absorbed power law. A soft component, superimposed on a strongly cut-off power law, is likely to be present in several sources. There is no overlap among the 142 fields used and, wherever possible, multiple observations of the same field have been merged in one single pointing to increase the sensitivity. Fields were selected among public data (such as that of March 1999) and the authors' proprietary data. Fields centered on bright extended sources and bright Galactic sources were excluded from the survey, as were fields close to the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC) and M 33. Most of the fields have exposures between 30 and 100 ks, and 20 fields have an exposure higher than 80 ks (see Fig. 1 of reference paper). Sources were detected in images accumulated between 4.5 and 10 keV. Source count rates in four bands (1.3 - 10 keV, total or T; 1.3 - 2.5 keV, low band or L; 2.5 - 4.5 keV, middle band or M; 4.5 - 10 keV, high band or H) were extracted and corrected for the energy-dependent vignetting and for the MECS PSF. The count rates were converted to fluxes using a conversion factor of 7.8 x 10<sup>-11</sup> erg cm<sup>-2</sup> s<sup>-1</sup> (5 - 10 keV flux) per one '3 ECS count' (4.5 - 10 keV) appropriate for a power-law spectrum with alpha<sub>E</sub> = 0:6. The factor is not strongly sensitive to the spectral shape, owing to the narrow band: thus, for alpha<sub>E</sub> = 0.4 and 0.8 it is 8.1 and 7.6 x 10<sup>-11</sup> erg cm<sup>-2</sup> s<sup>-1</sup>, respectively. A conversion factor of 9:9 x 10<sup>-11</sup> erg cm<sup>-2</sup> s<sup>-1</sup> per one '3 MECS count' has been used for sources under the 550 micron (µm) beryllium strongback supporting the MECS window to account for the reduced detector sensitivity. This table was created by the HEASARC in July 2014 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/MNRAS/327/771">CDS Catalog J/MNRAS/327/771</a> file table2.dat. This is a service provided by NASA HEASARC .
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