Usage of SimInspiralFD
Currently Bilby uses the function SimInspiralFD
(which internally calls XLALSimInspiralTDFromTD
) whenever a time-domain waveform model is being used. However, this can lead to unexpected results for precessing waveforms that do not support f_{\rm ref}
, e.g. SEOBNRv4P
. This is because SimInspiralFD
internally changes the starting frequency of the waveform such that at the requested starting frequency the waveform is properly conditioned. This is problematic because the spins for such waveforms have to be defined at the starting frequency. If the transformation from PE sampling parameters (e.g. tilt angles) is done at one frequency but the waveform is actually generated at another, this changes the waveform that is being produced. If the shift in frequency was constant, this would be inconvenient but would just amount to have a different reference frequency. However, the amount that the SimInspiralFD
steps back is parameter dependent (see here). Thus every waveform during sampling would in effect be evaluated at a different reference frequency.
Possible ways to proceed that come to mind are
- modify
SimInspiralFD
such that this change in starting frequency is not done for such waveforms as is already the case for one approximant here. Note, however, that the tapering done in this case has not been tested extensively. - use
ChooseTDWaveform
directly and do all the conditioning and FFTing internally within bilby