The power output of local obscured and unobscured AGN: crossing the absorption barrier with Swift/ BAT and IRAS

• Published in: Monthly notices of the Royal Astronomical Society 2010-02, Vol.402 (2), p.1081-1098
• Main Authors: Vasudevan, R. V., Fabian, A. C., Gandhi, P., Winter, L. M., Mushotzky, R. F.
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics; black hole physics; Black holes; Earth, ocean, space; Exact sciences and technology; galaxies: active; galaxies: Seyfert; Infrared radiation; Telescopes
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.15936.x

The Swift/Burst Alert Telescope (BAT) 9-month catalogue of active galactic nuclei (AGN) provides an unbiased census of local supermassive black hole accretion, and probes to all but the highest levels of absorption in AGN. We explore a method for characterizing the bolometric output of both obscured and unobscured AGN by combining the hard X-ray data from the Swift/BAT instrument (14–195keV) with the reprocessed infrared (IR) emission as seen with the Infrared Astronomical Satellite (IRAS) all-sky surveys. This approach bypasses the complex modifications to the spectral energy distribution introduced by absorption in the optical, UV and 0.1–10 keV regimes and provides a long-term, average picture of the bolometric output of these sources. We broadly follow the approach of Pozzi et al. for calculating the bolometric luminosities by adding nuclear IR and hard X-ray luminosities, and consider different approaches for removing non-nuclear contamination in the large-aperture IRAS fluxes. Using mass estimates from the black hole mass-host galaxy bulge luminosity relation, we present the Eddington ratios λEdd and 2–10 keV bolometric corrections for a subsample of 63 AGN (35 obscured and 28 unobscured) from the Swift/BAT catalogue, and confirm previous indications of a low Eddington ratio distribution for both samples. Importantly, we find a tendency for low bolometric corrections (typically 10–30) for the obscured AGN in the sample (with a possible rise from ∼15 for λEdd < 0.03 to ∼32 above this), providing a hitherto unseen window on to accretion processes in this class of AGN. This finding is of key importance in calculating the expected local black hole mass density from the X-ray background since it is composed of emission from a significant population of such obscured AGN. Analogous studies with high-resolution IR data and a range of alternative models for the torus emission will form useful future extensions to this work.