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Title:
ASCA observations of deep ROSAT fields - I. The nature of the X-ray source populations
Authors:
Georgantopoulos, I.; Stewart, G. C.; Blair, A. J.; Shanks, T.; Griffiths, R. E.; Boyle, B. J.; Almaini, O.; Roche, N.
Affiliation:
AA(Department of Physics and Astronomy, The University of Leicester, Leicester LE1 7RH ), AB(Department of Physics and Astronomy, The University of Leicester, Leicester LE1 7RH ), AC(Department of Physics and Astronomy, The University of Leicester, Leicester LE1 7RH ), AD(Physics Department, University of Durham, South Road, Durham DH1 3LE ), AE(Department of Physics, Carnegie Mellon University, Wean Hall, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA ), AF(Anglo-Australian Observatory, PO Box 296, Epping NSW 2121, Australia ), AG(Institute of Astronomy, Madingley Road, Cambridge CB3 0HA ), AH(Johns Hopkins University, Department of Physics and Astronomy, Baltimore, MD 21218, USA )
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 291, Issue 1, pp. 203-210. (MNRAS Homepage)
Publication Date:
10/1997
Origin:
MNRAS
MNRAS Keywords:
SURVEYS, GALAXIES: ACTIVE, GALAXIES: GENERAL, X-RAYS: GALAXIES, X-RAYS: GENERAL
Bibliographic Code:
1997MNRAS.291..203G

Abstract

We present ASCA GIS observations (total exposure ~100-200 ks) of three fields which form part of our deep ROSAT survey. We detect 26 sources down to a limiting flux (2-10 keV) of ~5x10^-14 erg cm^-2 s^-1. Sources down to this flux level contribute ~30 per cent of the 2-10 keV X-ray background. The number-count distribution, logN-logS, is a factor of 3 above the ROSAT counts, assuming a spectral index of Gamma=2 for the ROSAT sources. This suggests the presence at hard energies of a population other than the broad-line AGN which contribute to the ROSAT counts. This is supported by spectroscopic observations that show a large fraction of sources that are not obvious broad-line AGN. The average 1-10 keV spectral index of these sources is flat with Gamma=0.92+/-0.16, significantly different from that of the broad-line AGN (Gamma=1.78+/-0.16). Although some of the narrow-emission-line galaxies which are detected with ROSAT are also detected here, the nature of the flat-spectrum sources remains as yet unclear.

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