The Swift /BAT High-Latitude Survey: First Results

Abstract

We present preliminary results from the first 3 months of the Swift Burst Alert Telescope (BAT) high Galactic latitude survey in the 14-195 keV band. The survey reaches a flux of {\textasciitilde}10-11 ergs cm-2 s-1 and has {\textasciitilde}2.7 arcmin (90% confidence) positional uncertainties for the faintest sources. This represents the most sensitive survey to date in this energy band. These data confirm the conjectures that a high-energy-selected active galactic nucleus (AGN) sample would have very different properties from those selected in other bands and that it represents a ``true'' sample of the AGN population. We have identified 86% of the 66 high-latitude sources. Twelve are Galactic-type sources, and 44 can be identified with previously known AGNs. All but five of the AGNs have archival X-ray spectra, enabling us to estimate the line-of-sight column densities and other spectral properties. Both of the z>0.11 objects are blazars. The median redshift of the others (excluding radio-loud objects) is 0.012. We find that the column density distribution of these AGNs is bimodal, with 64% of the nonblazar sources having column densities NH>=1022 cm-2. None of the sources with logLX>43.5 (cgs units) show high column densities, and very few of the lower LX sources have low column densities. Based on these data, we expect the final BAT catalog to have >200 AGNs and reach fluxes of less than {\textasciitilde}10-11 ergs cm-2 s-1 over the entire sky.