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 r in of a surrounding accretion disk and using that to infer a. By comparing a number of different estimates of r in 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 deg.), r in is nearly independent of spin. This result is likely dependent on tilt, such that for larger tilts, it may even be that r in would increase with increasing spin. In the opposite limit, we confirm through numerical simulations of untilted disks that, in the limit of zero tilt, r in recovers approximately the expected dependence on a.