Dear Ian, Thank you very much for your reply. I will certainly benefit from your comment as well.
Best regards, Beyhan. On Sat, Mar 7, 2020 at 12:41 AM Ian Hinder <[email protected]> wrote: > > > On 5 Mar 2020, at 15:42, Erik Schnetter <[email protected]> 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 > >
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