On 5 Mar 2020, at 15:42, Erik Schnetter <schnetter@gmail.commailto:schnetter@gmail.com> wrote:
Beyhan
The QuasiLocalMeasures thorn can examine not only horizons, but also other 2-surfaces. You can set up a surface that is large and which encloses both the remnant and surrounding matter, but which is still inside the emitted gravitational wave train. QuasiLocalMeasures can then calculate the angular momentum contained inside that sphere.
I'm not familiar with the method as applied to neutron stars, but for a black hole system, I would probably try to do this by computing the "ADM angular momentum" of the spacetime, as well as the "Bondi angular momentum loss", their difference being the "remaining" angular momentum in the system. I think this is fairly rigorous when done with masses, but I put the quotes around the angular momenta as I don't think these quantities are on as firm a footing.
In practice, one *should* be able to compute the "ADM angular momentum" on the initial data slice by evaluating the formula on a set of finite-radius spheres using QuasiLocalMeasures, similar to what Erik mentioned, and then extrapolating to spatial infinity. I don't know if there are reasons why this won't work for neutron star initial data. The "Bondi angular momentum loss" could be calculated by measuring the angular momentum flux in the emitted gravitational waves. This is technically very challenging to get accurate. You need quite a lot of resolution, and wave extraction far enough out that you can cleanly extrapolate it to future null infinity. There are also severe complications due to junk radiation.
So this approach is quite hard to implement.
-- Ian Hinder Research Software Engineer University of Manchester, UK