SwiftBAT Survey of AGNs

Abstract

We present the results of the analysis of the first 9 months of data of the Swift BAT survey of AGNs in the 14-195 keV band. Using archival X-ray data or follow-up Swift XRT observations, we have identified 129 (103 AGNs) of 130 objects detected at | b| > 15° and with significance >4.8 σ. One source remains unidentified. These same X-ray data have allowed measurement of the X-ray properties of the objects. We fit a power law to the log N–log S distribution, and find the slope to be 1.42 ± 0.14. Characterizing the differential luminosity function data as a broken power law, we find a break luminosity log L*(erg s−1) = 43.85 ± 0.26, a low-luminosity power law slope a = 0.84+ 0.16−0.22, and a high-luminosity power law slope b = 2.55+ 0.43−0.30, similar to the values that have been reported based on INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with an rms spread of 0.27. Integration of our luminosity function gives a local volume density of AGNs above 1041 erg s−1 of 2.4 × 10−3 Mpc−3, which is about 10% of the total luminous local galaxy density above M* = − 19.75. We have obtained X-ray spectra from the literature and from Swift XRT follow-up observations. These show that the distribution of log nH is essentially flat from nH = 1020 to 1024 cm−2, with 50% of the objects having column densities of less than 1022 cm−2. BAT Seyfert galaxies have a median redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have log nH > 22.