The Northern Galactic Cap AGN from the 58-month BAT Catalogue: A Comprehensive X-ray Spectral Study
Abstract
The all-sky hard X-ray Swift/BAT survey has provided the most complete census of local AGN activity to date, unbiased to all but the most heavy absorption levels. Continual monitoring in the 14--195 keV band has allowed the assembly of hard X-ray detected AGN catalogues after 9, 22, 36, 58 and 70 months of operation, increasing the sample size by probing to fainter fluxes. The seminal study of Winter et al. (2009) presented a comprehensive X-ray analysis of the AGN in the 9-month catalogue, providing their absorption and luminosity distributions, characterising the spectral shape for each source in the catalogue, and allowing the determination of sample-wide properties for an unbiased AGN sample. We present a timely revision of this exercise for the latest publicly available 58-month BAT catalogue (flux limit 4 x 10^-12 erg/s/cm^2 in the 14-195 keV band), focusing on the Northern Galactic Cap (b>50 degrees). This sky area has excellent potential for further dedicated study due to a wide range of multi-wavelength data that are already available, and we propose it as a low-redshift analogue to the ‘deep field’ observations of AGN at higher redshifts. We consistently fit all the 100 objects in this sky region with a suite of models to determine the best fitting column densities, luminosities and spectral features (Iron lines, soft excesses and warm absorber edges). Comparison with previous works on the 9-month and 36-month catalogues now allows a better understanding of whether the deepening exposure of the BAT catalogue uncovers progressively different AGN properties. We find that ~60% of the sample is absorbed above logNH=22, 9% is Compton thick, and Compton reflection is significant for the sample overall (average reflection amplitude <R> = 2.7). The sample is complete down to fluxes 4 times fainter than the 9-month catalogue in the 2--10 keV band. We emphasise the utility of this Northern Galactic Cap sample for a wide variety of future studies on AGN, and outline one such current project on the stacked emission from this sample and the connections with X-ray background synthesis models.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1310906V