The Radiative Efficiency of Accretion Flows in Individual AGN

TL;DR

This study measures the radiative efficiency of individual AGN using spectral fits and black hole mass estimates, revealing a correlation between efficiency and black hole mass that suggests higher spins in more massive black holes.

Contribution

It provides the first direct measurements of radiative efficiency in a sample of 80 quasars, linking efficiency to black hole mass and spin, and compares results with the Soltan argument.

Findings

Radiative efficiency increases with black hole mass.

Efficiency correlates with Eddington ratio and black hole spin.

Results support the hypothesis of higher spins in more massive black holes.

Abstract

The radiative efficiency of AGN is commonly estimated based on the total mass accreted and the total AGN light emitted per unit volume in the universe integrated over time (the Soltan argument). In individual AGN, thin accretion disk model spectral fits can be used to deduce the absolute accretion rate Mdot, if the black hole mass M is known. The radiative efficiency {\eta} is then set by the ratio of the bolometric luminosity L_bol to Mdot c^2. We apply this method to determine {\eta} in a sample of 80 PG quasars with well determined L_bol, where Mdot is set by thin accretion disk model fits to the optical luminosity density, and the M determination based on the bulge stellar velocity dispersion (13 objects) or the broad line region (BLR). For the BLR-based masses, we derive a mean log {\eta} = -1.05 +/- 0.52 consistent with the Soltan argument based estimates. We find a strong correlation of {\eta} with M, rising from {\eta} ~ 0.03 at M = 10^7 M{\odot} and L/L_Edd ~ 1 to {\eta} ~ 0.4 at M = 10^9 M{\odot} and L/L_Edd ~ 0.3. This trend is related to the overall uniformity of L_opt/L_bol in our sample, particularly the lack of the expected increase in L_opt/L_bol with increasing M (and decreasing L/L_Edd), which is a generic property of thermal disk emission at fixed {\eta}. The significant uncertainty in the M determination is not large enough to remove the correlation. The rising {\eta} with M may imply a rise in the black hole spin with M, as proposed based on other indirect arguments.