paper

Black Hole Spin and the Radio Loud/Quiet Dichotomy of Active Galactic Nuclei

arXiv:0911.2228 · doi:10.1088/0004-637X/711/1/50

Abstract

Radio loud active galactic nuclei (AGN) are on average 1000 times brighter in the radio band compared to radio quiet AGN. We investigate whether this radio loud/quiet dichotomy can be due to differences in the spin of the central black holes that power the radio-emitting jets. Using general relativistic magnetohydrodynamic simulations, we construct steady state axisymmetric numerical models for a wide range of black hole spins (dimensionless spin parameter 0.1 <= a <= 0.9999 and a variety of jet geometries. We assume that the total magnetic flux through the black hole horizon at radius r_H(a) is held constant. If the black hole is surrounded by a thin accretion disk, we find that the total black hole power output depends approximately quadratically on the angular frequency of the hole, P \propto Ω_H^2 \propto (a/r_H)^2. We conclude that, in this scenario, differences in the black hole spin can produce power variations of only a few tens at most. However, if the disk is thick such that the jet subtends a narrow solid angle around the polar axis, then the power dependence becomes much steeper, P \propto Ω_H^4 or even \propto Ω_H^6. Power variations of 1000 are then possible for realistic black hole spin distributions. We derive an analytic solution that accurately reproduces the steeper scaling of jet power with Ω_H, and we provide a numerical fitting formula that reproduces all our simulation results. We discuss other physical effects that might contribute to the observed radio loud/quiet dichotomy of AGN.

15 pages, 7 figures, accepted to ApJ

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