Surveys
Extragalactic
The origin of the Cosmic X-ray background (CXB) has been one of the key issues of X-ray astronomy. In the soft X-ray band below 2 keV it is dominated by the integrated emission of discrete sources, many of which are AGN (see e.g. Shanks et al. 1991 Nature 353, 315; Hasinger 1996 A&AS 120, 607). In the hard X-ray band above 2 keV its origin is far from understood. Until recently, only a few per cent of the 2-10 keV background had been resolved into discrete sources. All the major classes of X-ray sources resolved with ROSAT have X-ray spectra which are significantly softer than that of the X-ray background (leading to the so-called X-ray spectral paradox). It is clear that a population of objects with significantly harder X-ray spectra than the AGN and narrow-line galaxies currently identified in deep ROSAT surveys must exist and will begin to dominate at energies above 2 keV. Plausible candidates for such hard X-ray sources are highly absorbed objects, such as Seyfert II galaxies and other AGN/QSO (e.g., Comastri et al. 1995 A&A 296, 1) or, as recently proposed (di Matteo & Fabian 1996 MNRAS 286, 393) objects with extremely flat intrinsic spectra due to advection dominated accretion disks.ASCA has performed the first sensitive non-biased sky survey in the 2-10 keV band with an imaging instrument. More than 100 sources have been detected from the large 6-deg^2 field (LSS field). Roughly 30% of the CXB in the 2-10 keV band has been directly resolved into discrete sources (Ueda et al. 1998 Nature 391, 866) at a flux level of ~10^-13 erg/cm^2/s. Analysis of the fluctuations in the x-ray background based on ASCA SIS data (Gendreau et al. 1998 MNRAS 297, 41) indicates that > 50% of the XRB is resolved at a flux level of 4x10^-14 erg/cm2/s and that the logN-logS in the 2-10 keV band is consistent with the Euclidean slope of -3/2 down to the faintest levels reachable by ASCA and lies on the extrapolation from the previous results obtained with Ginga and HEAO-1. Similar results based on ASCA data have been obtained by Georgantopoulos et al. (1997 MNRAS 291, 203) and Cagnoni et al. (1997 ApJ 493, 54). The average spectrum of the sources with fluxes less than 4x10^-13 erg/cm^2/s in the LSS fields shows a photon index of 1.5+/-0.2 in the 2-10 keV band (Ueda et al. 1998 Nature 391, 866) consistent with the slope of the CXB. These weak sources are thus significantly harder than that seen at fluxes 1-2 orders of magnitude brighter in the Ginga fluctuation analysis (see e.g. Hayashida 1990 Phd thesis) or in the large number of targeted source spectra obtained by ASCA.
Several faint hard ASCA sources have recently been identified as highly absorbed AGN, classified optically as type II Seyferts (Akiyama et al. 1998 ApJ 500, 173; Boyle et al. 1998 MNRAS 296, 1). To extend these results, Takahashi et al. (1998 in preparation) have been constructing a catalog of serendipitous sources seen using the GIS. This catalog, which will soon be made public, contains over 1000 sources. Using the catalog, the logN-logS distribution has been obtained over a factor of 500 in flux range. Because the ASCA error circles are typically 25'' in size or larger, optical/IR/radio identification of most of these objects has proven very difficult. By using ASCA as a finding chart, AXAF and XMM will be able to rapidly identify these moderate flux objects, which, with an areal density of ~1-3 per square degree, will be rare objects in the small AXAF and XMM fields.
Galactic Plane
The largest single ASCA observing programs has been the galactic plane survey with over 150 pointings. The ASCA GIS with its 20' radius field of view is able, in moderately short exposures, to survey the entire galactic plane over the life of the ASCA mission. This will provide the first unbiased sample of many categories of X-ray sources. One of the most important products will be a Galactic source catalog with a limiting flux of 2x10^-13 erg/cm^1/s. The source density is 3 per square degree and one half of the objects detected so far are uncatalogued and unidentified. These have included some of the pulsars and SNR discussed earlier. Other results have included the first detailed broad band spectrum of the Galactic ridge emission (Kaneda et al. 1997 ApJ 491, 638). Recent RXTE results (Valinia and Marshall 1998 ApJ, in press) combined with ASCA data strongly favor a nonthermal origin for the hard x-ray emission indicating a very large injection of energy into low energy cosmic rays in the galaxy.
Last modified: Tuesday, 26-Jun-2001 14:22:36 EDT
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