Accretion rates and radiative efficiencies of Sagittarius A* and nearby supermassive black holes estimated using empirical relations: Implications for accretion models

TL;DR

This study estimates accretion rates and radiative efficiencies of supermassive black holes using empirical relations, challenging existing models and revealing higher efficiencies and an inverse relation with accretion rate.

Contribution

It introduces a method to estimate black hole accretion and efficiency using empirical relations, providing new insights into accretion physics and challenging some theoretical models.

Findings

Accretion rates range from 0.00002 to 0.09 solar masses per year.

Radiative efficiencies are higher than expected, around 0.9 for Sagittarius A*.

Efficiency increases as accretion rate decreases and black hole ages.

Abstract

The Bondi accretion rate of black holes in our and nearby galaxies Messier 87, NGC 3115, NGC 1600, and Cygnus A have been determined or constrained using Chandra or other observations. It, however, remains unknown how much mass from the Bondi radius reaches each black hole and how much is accreted. We determine the accretion rate and radiative efficiency for each black hole using two well-tested empirical relations: one relates a black hole's accretion rate to its mass and redshift, and the other relates the radiative efficiency to the Eddington ratio and redshift. We get an accretion rate of ~0.00002 solar mass per year and radiative efficiency of ~0.9 for Sagittarius A* and an accretion rate of ~0.09 solar masses per year and radiative efficiency of ~0.68 for NGC 1600; and values in between these extremes for the rest. The derived mass inflow rate onto each black hole (not the accretion rate) essentially matches the reported Bondi accretion rate. Thus, the results do not support the ADIOS and CDAF models, but whether the dissipated energy not reflected in a black hole's observed luminosity is advected as in the ADAF model remains uncertain. Furthermore, contrary to current model expectations, the derived radiative efficiencies are orders of magnitude higher and radiative efficiency increases as the accretion rate decreases and a BH ages. A physical basis is found relating the empirical formulation of accretion rate to Bondi accretion.