BAT AGN Spectroscopic Survey (BASS) – VI. The ΓX–L/LEdd relation
We study the relation between accretion rate (in terms of L/L_(Edd)) and shape of the hard X-ray spectral energy distribution (namely the photon index Γ_x) for a large sample of 228 hard X-ray-selected, low-redshift active galactic nuclei (AGNs), drawn from the Swift/BAT AGN Spectroscopic Survey (BASS). This includes 30 AGNs for which black hole mass (and therefore L/L_(Edd)) is measured directly through masers, spatially resolved gas or stellar dynamics, or reverberation mapping. The high-quality and broad energy coverage of the data provided through BASS allow us to examine several alternative determinations of both Γ_x and L/L_(Edd). For the BASS sample as a whole, we find a statistically significant, albeit very weak correlation between Γ_x and L/L_(Edd). The best-fitting relations we find, Γ_x ≃ 0.15 log L/L_(Edd) + const., are considerably shallower than those reported in previous studies. Moreover, we find no corresponding correlations among the subsets of AGN with different MBH determination methodology. In particular, we find no robust evidence for a correlation when considering only those AGN with direct or single-epoch MBH estimates. This latter finding is in contrast to several previous studies which focused on z > 0.5 broad-line AGN. We discuss this tension and conclude that it can be partially accounted for if one adopts a simplified, power-law X-ray spectral model, combined with L/L_(Edd) estimates that are based on the continuum emission and on single-epoch broad-line spectroscopy in the optical regime. We finally highlight the limitations on using Γ_x as a probe of supermassive black hole evolution in deep extragalactic X-ray surveys.
Additional Information© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 May 5. Received 2017 May 4; in original form 2017 March 27. We thank the anonymous referee, as well as M. Brightman, O. Shemmer, and R. Bär for their constructive comments, which helped us to improve the manuscript. This work made use of the MATLAB package for astronomy and astrophysics (Ofek 2014). This research has made use of the NED, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of NASA's Astrophysics Data System. CR acknowledges financial support from the China-CONICYT fund, FONDECYT 1141218 and Basal-CATA PFB-06/2007. MK acknowledges support from the Swiss National Science Foundation (SNSF) through the Ambizione fellowship grant PZ00P2_154799/1, and from NASA through ADAP award NNH16CT03C. KS and KO acknowledge support from the Swiss National Science Foundation (SNSF) through grants 200021_157021, PP00P2_138979, and PP00P2_166159. ET acknowledges support from CONICYT-Chile grants Basal-CATA PFB-06/2007 and FONDECYT Regular 1160999. The work of DS was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. MB acknowledges support from NASA Headquarters under the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H.
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