[Users] Determination of the final spin of the remnant of a binary neutron star merger

Beyhan Karakaş beyhannkarakas at gmail.com
Sat Mar 7 04:17:37 CST 2020

Dear Ian,

Thank you very much for your reply. I will certainly benefit from your
comment as well.

Best regards,


On Sat, Mar 7, 2020 at 12:41 AM Ian Hinder <ian.hinder at manchester.ac.uk>

> On 5 Mar 2020, at 15:42, Erik Schnetter <schnetter at 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
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