The ΓX-L/LEdd relation in BAT AGN Spectroscopic Survey (BASS)

We present a study of 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 over 200 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. We find very weak correlation between Γ_x and L/L_Edd for the BASS sample as a whole, with best-fitting relations that 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, and in particular 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 conclude that this tension 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. Given these findings, we highlight the limitations of using Γ_x as a probe of supermassive black hole evolution in deep extragalactic X-ray surveys.