radius of the accretion disc?

[tiny-tim]

What is the inner radius of an accretion disc?

Is it simply the radius of the smallest possible orbit of a massive particle (the marginally stable radius), 6M ?

[stevebd1]

Hi tiny-tim

In most cases, the inner edge of the accretion disk is considered to coincide with the marginally stable which is 6M for a static black hole, reducing to 1M for a maximal Kerr black hole where a/M=1, though there seems to be some debate that the properties of the accretion disk itself can have an effect on this-

A couple of links-

'Where is a Marginally Stable Orbit in Luminous Accretion Disk' by Ken-ya Watarai Watarai, Shin Mineshige
http://quasar.cc.osaka-kyoiku.ac.jp/~watarai/presen/KoreaPoster2004.pdf

'The radiation properties of an accretion disk with a non-zero torque on its inner edge' by Xinwu Cao, Y.D. Xu
http://arxiv.org/abs/astro-ph/0211220

'Black Hole Accretion Disks On The Edge' by Kristen Menou
http://arxiv.org/abs/astro-ph/0304421