The ASCA Medium-Sensitivity Survey (The GIS Catalog Project): Source Counts and Evidence for Emerging Population of Hard Sources

Details The ASCA Medium-Sensitivity Survey (The GIS Catalog Project): Source Counts and Evidence for Emerging Population of Hard Sources The ASCA Medium-Sensitivity Survey (The GIS Catalog Project): Source Counts and Evidence for Emerging Population of Hard Sources

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...524l..11u&link_type=abstract

The Astrophysical Journal, Volume 524, Issue 1, pp. L11-L14.

Astronomy and Astrophysics

Astronomy

47

Cosmology: Diffuse Radiation, Galaxies: Active, Surveys, X-Rays: Galaxies

Scientific paper

We present first results from the ASCA Medium-Sensitivity Survey (or the GIS catalog project). From the serendipitous fields amounting to 106 deg^2, we determined the logN-logS relations in the 0.7-7 and 2-10 keV bands with the best statistical accuracy obtained so far, over the flux range from 1x10^-11 to 5x10^-14 and 1x10^-11 to 7x10^-14 ergs s^-1 cm^-2, respectively. When the sources detected in the 0.7-7 keV band are divided into two subsamples with higher and lower spectral hardness, the former exhibits a significantly steeper slope than the latter at fluxes below ~10^-12 ergs s^-1 cm^-2 (0.7-7 keV). The average spectrum of sources becomes continuously harder toward fainter fluxes, from a photon index of 2.1 in the 0.7-10 keV range at the flux of ~10^-11 to 1.6 at ~10^-13 ergs s^-1 cm^-2 (0.7-7 keV). This is consistent with the comparison of source counts between the 2-10 and the 0.7-2 keV bands and solves the puzzle of their discrepancy reported previously. Our results demonstrate rapid emergence of hard X-ray sources with a decreasing flux from ~10^-12 to ~10^-13 ergs s^-1 cm^-2 (2-10 keV).

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