2015MNRAS.449..316N

The mechanisms that produce and power relativistic jets are fundamental open questions in black hole (BH) astrophysics. In order to constrain these mechanisms, we analyse the energy efficiency of jet production η based on archival Chandra observations of 27 nearby, low-luminosity active galactic nuclei. We obtain η as the ratio of the jet power, inferred from the energetics of jet powered X-ray emitting cavities, to the BH mass accretion rate 4{dot}M_BH. The standard assumption in estimating 4{dot}M_BH is that all the gas from the Bondi radius rB makes it down to the BH. It is now clear, however, that only a small fraction of the gas reaches the hole. To account for this effect, we use the standard disc mass-loss scaling, 4{dot}M(r) ∝(r/r_B)^s 4{dot}M_Bondi. This leads to much lower values of 4{dot}M_BH and higher values of η 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 η ~ 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 implications of our results for the distribution of massive BH spins and the possible impact of a significant extra cold gas supply on our estimates.