Effective Inner Radius of Tilted Black Hole Accretion Disks

TL;DR

This paper investigates how the inner radius of tilted black hole accretion disks affects spin measurements, revealing that tilt can cause significant inaccuracies in traditional methods.

Contribution

It demonstrates through simulations that tilt influences the inner radius, leading to potential errors in black hole spin estimation methods based on this measurement.

Findings

Inner radius nearly independent of spin for tilted disks with moderate tilt

Tilt can cause the inner radius to increase with spin at larger tilts

In untilted disks, the inner radius aligns with expected spin dependence

Abstract

One of the primary means of determining the spin of an astrophysical black hole is by actually measuring the inner radius of a surrounding accretion disk and using that to infer the spin. By comparing a number of different estimates of the inner radius 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 degrees), the inner radius is nearly independent of spin. This result is likely dependent on tilt, such that for larger tilts, it may even be that the inner radius would increase with increasing spin. In the opposite limit, we confirm through numerical simulations of untilted disks that, in the limit of zero tilt, the inner radius recovers approximately the expected dependence on spin.