Kavli Affiliate: Feng Yuan
| First 5 Authors: Fu-Guo Xie, Ramesh Narayan, Feng Yuan, ,
| Summary:
An important parameter in the theory of hot accretion flows around black
holes is $delta$, which describes the fraction of “viscously” dissipated
energy in the accretion flow that directly heats the electrons. The radiative
efficiency of a hot accretion flow is determined by its value if other
parameters, such as the accretion rate, are determined. Unfortunately, the
value of $delta$ is hard to determine from first principles. The recent Event
Horizon Telescope Collaboration (EHTC) results on M87* and Sgr A* provide us
with a different way of constraining $delta$. By combining the mass accretion
rates in M87* and Sgr A* estimated by the EHTC with the measured bolometric
luminosities of the two sources, we derive good constraints on the radiative
efficiencies of the respective accretion flows. In parallel, we use a
theoretical model of hot magnetically arrested disks (MAD) to calculate the
expected radiative efficiency as a function of $delta$ (and accretion rate).
By comparing the EHTC-derived radiative efficiencies with the theoretical MAD
model, we find that Sgr A* requires $delta > 0.3$, with the most likely value
being $delta sim 0.5$. A similar comparison in the case of M87* gives
inconclusive results, because there is still a large uncertainty in the
accretion rate in this source.
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