The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

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Abstract

In this paper we study the X-ray energy distribution of a large sample of faint extragalactic X-ray sources. The sample used is extracted from the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) and consists of 599 sources with flux in the range 10^-13^ to 10^-11^ ergs cm^-2^ s^-1^ (0.3-3.5 keV) which are either identified with or classified as extragalactic objects. The spectral analysis of these sources is carried out on a statistical basis using two independent approaches. First we exploit the fact that for a given source the decrease in the observed count rate produced by interstellar absorption is a function of the intrinsic slope of the source spectrum. We thus analyze the distribution of detected sources as a function of the hydrogen column density which characterizes different regions of the survey in order to place constraints on the sources' spectral slopes. We then study the hardness ratio distribution of the sample as a whole and of various subsamples to extract further spectral information on extragalactic X-ray sources. We find that the distribution of power-law slopes of the whole extragalactic sample has a mean of 0.95 +/- 0.04 (energy index) with an intrinsic dispersion of about 0.36. This is the first detailed analysis of the spectral properties of a large number of faint (fx >= 10^-13^ ergs cm^-2^ s^-1^) extragalactic X-ray sources. Our results indicate that these sources are characterized, in the 0.3-3.5 keV band, by a spectrum significantly steeper than that which characterizes the diffuse extragalactic X-ray background radiation (in the 2-10 keV band). Preliminary results on the spectral properties of individual classes of sources indicate that X-ray selected active galactic nuclei are characterized by a variety of spectral indices with average 1.03^+0.05^_-0.06_ and intrinsic dispersion ~0.36 and suggest that the spectral properties of X-ray selected normal galaxies are similar to those of clusters of galaxies. The range of galactic hydrogen column density values which characterize the IPC images used in the EMSS is rather limited, 85% of the sky surveyed is observed through an hydrogen column density varying from 1.2 x 10^20^ to 7.7 x 10^20^ cm^2^. This makes the predicted bias in source selection, resulting from observing with a broadband detector through a nonuniform medium, entirely negligible. Such would not be the case for an X-ray survey extending into the Galactic plane or performed with a detector sensitive to much "softer" X-rays.

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