- 945
- 155
In the ESO's press conference, someone mentioned that it is hard to get a black hole's angular momentum from the shape of its shadow. So I tracked down how one calculates a BH's shadow's boundary, and I found [1801.00860] Shadows and strong gravitational lensing: a brief review. The math is a bit involved, but I implemented it in Mathematica, and I found that the shadow is approximately circular but offset toward the receding part of the limb. The radius is close to the radius in the nonrotating (Schwarzschild) limit: ##3 \sqrt{3} M##, and the offset is ##- (2a) ( n_{obs} \times n_{AM})## with the observation and the angular-momentum directions. M = mass, a = (ang mom)/M, multiplied by G/c^2 to get lengths.