1999ApJ...524..674D

A complete sample of 60 serendipitous hard X-ray sources with flux in the range ∼1x10^–13 ergs.cm^–2.s^–1 to ∼4x10^–12 ergs.cm^–2.s^–1 (2-10 keV), detected in 87 ASCA GIS2 images, was recently presented in the literature. Using this sample it was possible to extend the description of the 2-10 keV logN(>S)-logS down to a flux limit of ∼6x10^–14 ergs.cm^–2.s^–1 (the faintest detectable flux), resolving about a quarter of the cosmic X-ray background (CXB). In this paper we have combined the ASCA GIS2 and GIS3 data of these sources to investigate their X-ray spectral properties using the hardness ratios and the stacked-spectra method. Because of the sample statistical representativeness, the results presented here, which refer to the faintest hard X-ray sources that can be studied with the current instrumentation, are relevant to the understanding of the CXB and of the active galactic nucleus (AGN) unification scheme. The ``stacked'' spectra show that the average source's spectrum hardens toward fainter fluxes; it changes from an energy spectral index α<sub>E</sub>=0.87±0.08 for the 20 brightest sources (2-10 keV count rate ≥3.9x10<sup>–3</sup> counts.s<sup>–1</sup>, the ``bright'' sample) to α_E=0.36±0.14 for the remaining 40 fainter sources (the ``faint'' sample). The dividing line of 3.9x10<sup>–3</sup> counts.s<sup>–1</sup> corresponds to unabsorbed 2-10 keV fluxes in the range ∼5.4x10<sup>–13</sup> to ∼3.1x10<sup>–13</sup> ergs.cm<sup>–2</sup>.s<sup>–1</sup> for a source described by a power-law model with energy spectral index between 0.0 and 2.0. It thus seems that we are now beginning to detect those sources that have the ``correct'' spectral shape to be responsible for the 2-10 keV CXB. The hardness-ratio analyses indicate that this flattening is due to a population of sources with very hard spectra showing up in the faint sample; about half of the sources in this sample require α_E≲0.5, while only ∼10% of the sources in the bright sample are consistent with an energy spectral index so flat. A number of sources (∼30%) in the faint sample seem to be characterized by an apparently ``inverted'' X-ray spectrum (i.e., α_E≲0.0). These objects are probably extremely absorbed sources, as expected from the CXB synthesis models based on the AGN unification scheme, if not a new population of very hard serendipitous sources. The broadband (0.7-10 keV) spectral properties of the selected sources, as inferred from the hardness-ratios diagram, seem to be more complex than is expected from a simple absorbed power-law model. We have thus investigated more complex models, in line with the AGN unification scheme, and we find that these models seem to be able to explain the overall spectral properties of the present sample; this result also seems to be suggested by a comparison of the hardness-ratio diagram of the serendipitous ASCA sources with that obtained using a sample of nearby and well-known Seyfert 1 and Seyfert 2 galaxies observed with ASCA.