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.