%0 Journal Article
%T Retrograde versus Prograde Models of Accreting Black Holes
%A David Garofalo
%J Advances in Astronomy
%D 2013
%I Hindawi Publishing Corporation
%R 10.1155/2013/213105
%X There is a general consensus that magnetic fields, accretion disks, and rotating black holes are instrumental in the generation of the most powerful sources of energy in the known universe. Nonetheless, because magnetized accretion onto rotating black holes involves both the complications of nonlinear magnetohydrodynamics that currently cannot fully be treated numerically, and uncertainties about the origin of magnetic fields that at present are part of the input, the space of possible solutions remains less constrained. Consequently, the literature still bears witness to the proliferation of rather different black hole engine models. But the accumulated wealth of observational data is now sufficient to meaningfully distinguish between them. It is in this light that this critical paper compares the recent retrograde framework with standard ¡°spin paradigm¡± prograde models. 1. Introduction When Roy Kerr presented his solution at the Texas Symposium almost five decades ago, the astronomical community, ironically, was too busy with the recent discovery of quasars to pay attention. But the importance of black holes is now grounded in observations pointing to a breadth and depth of a black hole impact that is likely still underestimated. In galaxies, black holes appear to participate in triggering and in quenching star formation, in heating and expanding gas, and in altering the mode of accretion [1¨C7]. They are connected to bulges and stellar dispersions at the spatial extremes of galaxies [8, 9], and in many cases the impact of black holes appears on cluster environments as well [1, 10¨C13]. But black hole influence is also observed on smaller scales, with stellar-mass black holes producing a rich panoply of observational signatures [14]. Black holes are both spatially and gravitationally irrelevant to galaxies as a whole, so the influence they exert is thought to occur during active phases when large amounts of energy spew from their centers in both kinetic form and radiation spanning the entire electromagnetic spectrum. Because the black hole scaling relations are ubiquitous, perhaps all galaxies experience an active phase during which black holes reveal their presence to the larger galaxy. And this active phase would involve accretion onto supermassive black holes [15, 16], the formation of powerful winds [17], and in some cases jet formation [18, 19], but the details of how jets are related to accretion remain elusive. The earliest analytic models involve accretion in either thin-disk or advection-dominated form [15, 16, 20, 21], and/or spin energy
%U http://www.hindawi.com/journals/aa/2013/213105/