Universality of the critical Eddington ratio

Determine whether the critical Eddington ratio for the accretion-state transition in active galactic nuclei is universal or instead scales systematically with black-hole mass.

Background

The paper argues that the current DESI–eROSITA sample does not decisively probe the low-accretion-rate regime, particularly below approximately $\lambda_{\rm Edd}\lesssim10^{-3}$, where radiatively inefficient accretion flows and a harder X-ray spectral branch may emerge. Establishing the transition threshold is complicated by selection effects, luminosity-estimator covariance, non-simultaneous observations, and uncertain accretion-rate proxies.

The authors note that recent work suggests the critical accretion rate may depend on black-hole mass rather than having a single universal value. If so, the transition luminosity would broaden or shift across populations with different black-hole masses. Future observations combining low-Eddington-ratio AGN, independent accretion-rate estimates, and response-aware X-ray spectroscopy are proposed as a means of resolving this question.