X-ray photon index as a function of Log(λ_Edd). Only WISSH QSOs with ≥20 photons (i.e. with X-ray spectral analysis), for which an estimate of M_BH is available are considered. The symbols are the same as in Figure 5, with the exception of WISSH sources, which are represented as blue stars. The black solid, dashed, dotted, and dash-dotted lines correspond to the Liu et al. (2021) and Brightman et al. (2013) relations, and two models (BCES bisector and FITEXY method) from Trakhtenbrot et al. (2017), respectively. Specifically, Liu et al. (2021) consider 47 AGN with both super- and sub-Eddington accretion rates from the sample compiled by the SEAMBH collaboration (Du et al. 2018). Brightman et al. (2013) use 69 broad-line AGN from the extended Chandra Deep Field South and COSMOS surveys. Trakhtenbrot et al. (2017) refer to 228 hard X-ray selected AGN at 0.01 < z < 0.5, drawn from the Swift/BAT AGN Spectroscopic Survey (BASS, Baumgartner et al. 2013).