# Innermost stable circular orbit

The innermost stable circular orbit (often called the ISCO) is the smallest marginally stable circular orbit in which a test particle can stably orbit a massive object in general relativity.[1] The location of the ISCO, the ISCO-radius (${\displaystyle r_{\mathrm {isco} }}$), depends on the angular momentum (spin) of the central object.

The ISCO plays an important role in black hole accretion disks since it marks the inner edge of the disk.

## Non-rotating black holes

For a non-spinning massive object, where the gravitational field can be expressed with the Schwarzschild metric, the ISCO is located at

${\displaystyle r_{\mathrm {ms} }=6{\frac {GM}{c^{2}}}=3R_{S},}$

where ${\displaystyle R_{S}}$ is the Schwarzschild radius of the massive object with mass ${\displaystyle M}$. Thus, even for a non-spinning object, the ISCO radius is only three times the Schwarzschild radius, ${\displaystyle R_{S}}$, suggesting that only black holes and neutron stars have innermost stable circular orbits outside of their surfaces. As the angular momentum of the central object increases, ${\displaystyle r_{\mathrm {isco} }}$ decreases.

Circular orbits are still possible between the ISCO and the so called marginally bound orbit, which has a radius of

${\displaystyle r_{\mathrm {mb} }=4{\frac {GM}{c^{2}}}={2R_{S}},}$

but they are unstable.

For a massless test particle like a photon, the only possible but unstable circular orbit is exactly at the photon sphere.[2] Inside the photon sphere, no circular orbits exist. Its radius is

${\displaystyle r_{\mathrm {ph} }=3{\frac {GM}{c^{2}}}={1.5R_{S}}.}$

The lack of stability inside the ISCO is explained by the fact that lowering the orbit does not free enough potential energy for the orbital speed necessary: the acceleration gained is too little. This is usually shown by a graph of the orbital effective potential which is lowest at the ISCO.

## Rotating black holes

The case for rotating black holes is somewhat more complicated. The equatorial ISCO in the Kerr metric depends on whether the orbit is prograde (negative sign below) or retrograde (positive sign):

${\displaystyle r_{\mathrm {ms} }={\frac {GM}{c^{2}}}\left(3+Z_{2}\pm {\sqrt {(3-Z_{1})(3+Z_{1}+2Z_{2})}}\right)\leq 9{\frac {GM}{c^{2}}}=4.5R_{S}}$

where

${\displaystyle Z_{1}=1+{\sqrt[{3}]{1-\chi ^{2}}}\left({\sqrt[{3}]{1+\chi }}+{\sqrt[{3}]{1-\chi }}\right)}$
${\displaystyle Z_{2}={\sqrt {3\chi ^{2}+Z_{1}^{2}}}}$

with the rotation parameter ${\displaystyle \chi =2a/R_{S}=cJ/(M^{2}G)}$.[3] As the rotation rate of the black hole increases ${\displaystyle \chi \to 1}$, the retrograde ISCO increases towards ${\displaystyle r_{\mathrm {ms} }\to 9GM/c^{2}}$ (4.5 times the a=0 horizon radius) while the prograde ISCO decreases. [4]

The marginally bound radius and photon sphere radius in the equatorial plane are respectively

${\displaystyle r_{\mathrm {mb} }={\frac {GM}{c^{2}}}(1+{\sqrt {1\pm \chi }})^{2}\leq {\frac {3+{\sqrt {8}}}{2}}R_{S}\approx 5.828427{\frac {GM}{c^{2}}}\approx 2.9142R_{S}}$,
${\displaystyle r_{\mathrm {ph} }=2{\frac {GM}{c^{2}}}(1+\cos({\tfrac {2}{3}}\cos ^{-1}(\pm \chi )))\leq 4{\frac {GM}{c^{2}}}=2R_{S}}$.

All these three radii drop to ${\displaystyle r_{\mathrm {ms} }\to r_{\mathrm {mb} }\to r_{\mathrm {ph} }\to R_{S}/2}$ for prograde rotation at an extremal black hole with ${\displaystyle -\chi \to -1}$, still logically and locally distinguishable though.

If the particle is also spinning there is a further split in ISCO radius depending on whether the spin is aligned with or against the black hole rotation.[5]

## References

1. ^ Misner, Charles; Thorne, Kip S.; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0.
2. ^ Carroll, Sean M. (December 1997). "Lecture Notes on General Relativity: The Schwarzschild Solution and Black Holes". arXiv:gr-qc/9712019. Bibcode:1997gr.qc....12019C. Retrieved 2017-04-11.
3. ^ Bardeen, James M.; Press, William H.; Teukolsky, Saul A. (1972). "Rotating black holes: locally nonrotating frames, energy extraction, and scalar synchrotron radiation". The Astrophysical Journal. 178: 347–370. Bibcode:1972ApJ...178..347B. doi:10.1086/151796.
4. ^ Hirata, Christopher M. (December 2011). "Lecture XXVII: Kerr black holes: II. Precession, circular orbits, and stability" (PDF). Caltech. Retrieved 5 March 2018.
5. ^ Jefremov, Paul I; Tsupko, Oleg Yu; Bisnovatyi-Kogan, Gennady S (15 June 2015). "Innermost stable circular orbits of spinning test particles in Schwarzschild and Kerr space-times". Physical Review D. 91 (12): 124030. arXiv:1503.07060. Bibcode:2015PhRvD..91l4030J. doi:10.1103/PhysRevD.91.124030. S2CID 119233768.