EFFECTIVE INNER RADIUS OF TILTED BLACK HOLE ACCRETION DISKS

One of the primary means of determining the spin a of an astrophysical black hole is by actually measuring the inner radius rin of a surrounding accretion disk and using that to infer a. By comparing a number of different estimates of rin from simulations of tilted accretion disks with differing black hole spins, we show that such a procedure can give quite wrong answers. Over the range 0 ⩽ a/M ⩽ 0.9, we find that, for moderately thick disks (H/r ∼ 0.2) with modest tilt (15°), rin is nearly independent of spin. This result is likely dependent on tilt, such that for larger tilts, it may even be that rin would increase with increasing spin. In the opposite limit, we confirm through numerical simulations of untilted disks that, in the limit of zero tilt, rin recovers approximately the expected dependence on a.