Reference-Comparison Notes¤

This unlisted page records implementation details behind reference-specific limits. It intentionally excludes run outputs and historical calibration data. For the enforcing thresholds, see Reference Comparisons and Limits.

Frequency-domain comparisons¤

The frequency-domain overlap is a raw same-grid comparison at identical inputs. It is not maximised over time or phase.

IMRPhenomPv2¤

LAL estimates the coalescence-time correction from the derivative of a natural-cubic spline through a small phase grid around ringdown. ripple computes the corresponding derivative directly with JAX autodiff. The resulting time correction can differ near the merger-ringdown feature, appearing as a linear phase ramp rather than an amplitude error. At zero in-plane spin, both implementations remain continuous. IMRPhenomPv2 is nevertheless not required to reduce to IMRPhenomD: its LAL convention swaps the component masses and spins, changing their assignment in asymmetric phase terms.

IMRPhenomXHM¤

The phase of the (3, 2) mode is sensitive to spheroidal-to-spherical mixing near ringdown. The intermediate phase fit is constrained by the phase derivative at the transition, so small differences there can shift the fitted phase. Investigate a discrepancy in this region as a mode-mixing or phase-derivative issue rather than an amplitude discrepancy.

IMRPhenomXP and IMRPhenomXPHM¤

The MSA precession correction is ill-conditioned near angular-momentum resonances, where terms involving d0 + d2 + d4 suffer cancellation. Small float64 differences in spin-evolution coefficients can then be amplified in the precession angles, particularly at high inclination. IMRPhenomXPHM also inherits the (3, 2) mixing sensitivity through its XHM co-precessing seed. The LAL reference explicitly requests PhenomXPrecVersion=222, so an MSA-initialization failure is surfaced rather than silently selecting a different precession approximation. The BBH prior lands near this ill-conditioning rarely, so both models agree with LAL at the float64 floor.

IMRPhenomXP_NRTidalv3¤

LAL builds this approximant through its XLALSimIMRPhenomXPHM code path, restricted to the (2, ±2) modes. Multibanding is on by default there and must be disabled for the comparison (PhenomXPHMThresholdMband = PhenomXHMThresholdMband = 0, see tests/cross_validation/reference/lal.py); otherwise LAL disagrees with its own multibanding-off output for edge-on samples. The twist cutoff matches LAL bin-for-bin: Mf <= (fCutDef/M_sec)*M_sec, inclusive, with fCutDef in {0.3, 0.33}. The one corner case is chiEff > 0.99, where ripple's co-precessing amplitude is separately zeroed at Mf = 0.3 (fM_CUT in IMRPhenomXAS.py) so Mf in (0.3, 0.33] stays zero while LAL keeps it; it is unreachable within the BNS test prior. The dominant source of overlap loss is the MSA spin-evolution cubic's near-alignment ill-conditioning, which the BNS prior's low spins hit far more often than the BBH prior does. See tests/cross_validation/msa_precession_instability.md for the mechanism and why it shows up as an edge-on-amplified hc-only error that the SNR-weighted combined metric absorbs.

Continuous-wave comparisons¤

ExactPulsarSignal uses direct LALPulsar building blocks because CWMakeFakeData always includes the Einstein and Shapiro delays that the model intentionally omits. CWMakeFakeData instead provides the end-to-end comparison for the full pulsar models, including detector response, barycentering, and orbital modulation where applicable. That reference path interpolates propagation delays and detector response. A delay error produces a phase error that grows with frequency, so its normalized mismatch has a frequency-squared bound. Detector-response interpolation has a separate amplitude effect, which is covered by the relative-norm bound. The bounds account for those reference approximations; ripple should not reproduce them.