Swift BAT 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 n_H is essentially flat from n_H = 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 n_H > 22.