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

TL;DR

This paper reports on the ASCA Medium Sensitivity Survey, revealing the evolution of source spectra and counts, indicating a rising population of hard X-ray sources at lower flux levels, with implications for understanding cosmic X-ray backgrounds.

Contribution

It provides the first detailed Log N - Log S relations in multiple bands and shows spectral hardening of sources at fainter fluxes, highlighting an emerging population of hard X-ray sources.

Findings

Steeper Log N - Log S slope for harder sources below 10E-12 erg sE-1 cmE-2

Spectral hardness increases as flux decreases, from photon index 2.1 to 1.6

Rapid emergence of hard X-ray sources at lower flux levels

Abstract

We present first results from the ASCA Medium Sensitivity Survey (AMSS; or the GIS catalog project). From the serendipitous fields amounting to 106 degE-2, we determined the Log N - Log S relations in the 0.7-7 keV and 2-10 keV bands with the best statistical accuracy obtained so far, over the flux range from 1 x 10E-11 to 5 x 10E-14 and 7 x 10E-14 erg sE-1 cmE-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 ~10E-12 erg sE-1 cmE-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 ~10E-11 to 1.6 at ~10E-13 erg sE-1 cmE-2 (0.7-7 keV). This is consistent with the comparison of source counts between the 2-10 keV and the 0.7-2 keV band, and solves the puzzle of their discrepancy reported previously. Our results demonstrate rapid emergence of hard X-ray sources with a decreasing flux from \~10E-12 to ~10E-13 erg sE-1 cmE-2 (2-10 keV).