http://arxiv.org/abs/1406.7420
The mechanisms that produce and power relativistic jets are fundamental open questions in black hole (BH) astrophysics. In order to constrain these mechanisms, we analyze the energy efficiency of jet production ‘eta’ based on archival Chandra observations of a sample of 27 nearby, low-luminosity active galactic nuclei. We obtain ‘eta’ as the ratio of the jet power ‘Pjet’, inferred from the energetics of jet powered X-ray emitting cavities, to the BH mass accretion rate ‘MdotBH’. The standard assumption in estimating ‘MdotBH’ is that all the gas from the Bondi radius ‘r_B’ makes it down to the BH. It is now clear, however, that in reality only a small fraction of the gas reaches the hole. To account for this effect, we first compute ‘MdotB’ from the gas temperature and density profiles estimated around a few tens of ‘r_B’. Then, we account for the gas lost on the way to the BH via the standard disk mass-loss scaling, Mdot(r) \propto (r/r_B)^s MdotB. This leads to much lower values of MdotBH and higher values of eta than in previous studies. If hot accretion flows are characterized by 0.5 <= s <= 0.6 — on the lower end of recent theoretical and observational studies — then dynamically-important magnetic fields near rapidly spinning BHs are necessary to account for the high eta~100-300 per cent in the sample. Moreover, values of s>0.6 are essentially ruled out, or there would be insufficient energy to power the jets. We discuss the impact of a significant extra cold gas supply besides X-ray bright gas at ~r_B on our estimates and the implications of our results for the distribution of black hole spins.
R. Nemmen and A. Tchekhovskoy
Tue, 1 Jul 14
21/70
Comments: Submitted to MNRAS. 13 pages, 7 figures
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