BAT AGN Spectroscopic Survey (BASS) - VI. The Γ_X-L/L_Edd 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 M_BH determination methodology. In particular, we find no robust evidence for a correlation when considering only those AGN with direct or single-epoch M_BH 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.