QuasiLocalMeasures qlm_spin doesn't work for spins not aligned along the z axis

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Issue #2160 wontfix
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Einstein Toolkit version: ET_2018_02.

Parameter file: qc0-mclachlan.par in einsteinexamples with the additional parameter TwoPunctures::par_S_plus [0] = +0.3

Problem: The diagnostic qlm_spin[0] returns a spin of 0, whereas the expected one should be 0.3.

If instead of TwoPunctures::par_S_plus [1] = +0.3 one sets TwoPunctures::par_S_plus [2] = +0.3, qlm_spin[0] is correctly outputting 0.3

This happens because QuasiLocalMeasures finds in both cases the rotational Killing vector d/dphi (as seen setting QuasiLocalMeasures::veryverbose = yes), which is correct in the second case (when the axis aligned with the z-direction), but doesn't sound right when the rotation is with respect to the x-axis.

Changing QuasiLocalMeasures::killing_vector_method to axial or gradient does not change the outcome.

Best regards, Gabriele

Keyword: QuasiLocalMeasures

Comments (2)

  1. Roland Haas

    A similar question was asked on the mailing list and responded to in http://lists.einsteintoolkit.org/pipermail/users/2021-April/007931.html with:

    QuasiLocalMeasures regular spin measurement assumes that the black hole
    spin is roughly aligned with the z direction. A spin near the equatorial
    plane does not work well. In this case you'll have to use the coordinate
    spins (qlm_coordspinx, qlm_coordspiny, qlm_coordspinz); these are less
    accurate for distorted black holes, but will still work very well under
    these circumstances.

    so the Killing vector method was only ever designed to work with spins more or less along the z-direction and changing the method to compute the Killing vector will not help.

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