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title WaveformModes Guide
nav_order 6

WaveformModes — Conceptual Guide

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Table of contents

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  1. TOC {:toc}

Class: nrcats.WaveformModes
Inherits from: sxs.WaveformModes (ndarray subclass)

Auto-generated API reference: For a complete method listing with signatures and docstrings, see API Reference → waveform.

WaveformModes is the central data object returned by all catalog get() calls. It stores complex gravitational-wave strain multipole modes $$h_{\ell m}(t)$$ in dimensionless NR units and provides methods to convert to physical units, extract individual modes, sum to polarizations, apply frame rotations, and compute mismatches.

Construction

Three construction paths are available. In normal usage you will not call these directly — use the catalog get() method instead.

From HDF5 (RIT / MAYA waveform files)

WaveformModes.load_from_h5(file_path, metadata=metadata_dict, verbosity=0)

Reads amp_l{ell}_m{em}/X,Y and phase_l{ell}_m{em}/X,Y datasets. Interpolates all modes onto a common uniform time grid. Returns a WaveformModes with shape (n_times, n_modes).

From tar.gz (RIT psi4 files)

WaveformModes.load_from_targz(file_path, metadata=metadata_dict, verbosity=0)

Reads ASCII .asc / .dat / .txt files (columns: time, real, imag) from inside the archive. Missing modes are filled with zeros. Interpolates onto a uniform grid.

Wrapping an sxs.WaveformModes (SXS catalog)

WaveformModes(raw_sxs_obj.data, raw_sxs_obj.time, sim_metadata=metadata_dict, **meta)

Used internally by SXSCatalog.get(). The sxs.WaveformModes.data property may return a memoryview; all arithmetic wraps it with np.array(..., dtype=complex).

Core usage patterns

Getting a physically-scaled mode

get_mode() is the primary method for retrieving a scaled individual mode:

mode22 = wfm.get_mode(
    2, 2,
    total_mass=60.0,          # M_sun
    distance=100.0,           # Mpc
    delta_t_seconds=1./4096,  # physical seconds (4096 Hz sampling)
)
# mode22 is a complex pycbc.types.TimeSeries

The amplitude is scaled by $$G M_\text{tot} / (c^2 \cdot d_\text{Mpc} \cdot \text{Mpc})$$ and the time epoch is set so that $$t=0$$ coincides with the peak of the $$(2,2)$$ mode.

Getting polarizations

get_td_waveform() sums over all modes with spin-weight $$-2$$ spherical harmonics:

$$H = h_+ + i h_\times = \sum_{\ell,m} {}^{-2}Y_{\ell m}(\iota, \phi_c) , h_{\ell m}(t)$$

pols = wfm.get_td_waveform(
    total_mass=40.,           # M_sun
    distance=100.,            # Mpc
    inclination=0.2,          # radians
    coa_phase=0.3,            # radians
    delta_t_seconds=1./4096,  # physical seconds
)
hp = pols.real()
hc = -1 * pols.imag()

Polarization convention: Returns conjugate(h) so that .real() gives $$h_+$$ and .imag() gives $$+h_\times$$. This differs from LAL convention where imag() = -h_\times. Pass lal_convention=True to get LAL-compatible output.

Starting-frequency helpers

# GW frequency at waveform start, normalized to 1 M_sun — divide by total_mass to get Hz
f_start_1msun = wfm.f_lower_at_1Msun()
f_start_hz = f_start_1msun / total_mass_msun

# GW frequency at the relaxation epoch
f_relax_hz = wfm.f_lower_at_relaxation(total_mass=60.0)

Trimming to relaxation time

wfm_trimmed = wfm.trim_to_relaxation_time(total_mass=60.0)

Reads the relaxation time from metadata (tries keys 'relaxed-time', 'relaxation_time', 'reference_time').

Sampling interval parameters

get_mode() and get_td_waveform() accept two explicit sampling parameters:

Parameter Interpretation
delta_t_seconds Physical seconds (e.g. 1/4096 for 4096 Hz sampling)
delta_t_Msun Dimensionless M units (NR native, e.g. 0.5 means $$0.5,GM/c^3$$)

The returned TimeSeries.delta_t is always in physical seconds.

Deprecated: The old delta_t keyword argument (which inferred units from the magnitude) is deprecated. Use delta_t_seconds or delta_t_Msun explicitly.

Frame rotation

Rotating a WaveformModes object

import quaternionic
R = quaternionic.array.from_euler_angles(alpha, beta, gamma)
wfm_rotated = wfm.rotated(R)

Applies Wigner D-matrix rotation: $$h^{\text{rot}}{\ell m}(t) = \sum{m'} h_{\ell m'}(t) , D^\ell_{m'm}(R)$$

Uses the spherical package for Wigner D-matrix computation.

Rotating a mode dict (surrogate / PyCBC format)

from nrcats import apply_wigner_rotation_to_mode_dict

rotated_modes = apply_wigner_rotation_to_mode_dict(
    mode_dict,      # {(ell, m): pycbc.TimeSeries, ...}
    R,              # quaternionic rotation
    ell_max=4,
)

Useful for rotating surrogate model outputs into the NR frame for direct comparison.

Mismatch methods

match_sphere_averaged(other, psd, f_lower, delta_t, ...)

This method calculates the match (noise-weighted overlap) between two waveforms, integrated over the entire sphere of possible observer directions (sky-averaged) and maximized over coordinate time shift $$t_c$$, coalescence phase $$\phi_c$$, and active/passive $$SO(3)$$ rotation $$R$$.

Spherical Integration & Mode-by-Mode Equivalence

The overlap between the full multidimensional strain fields $$h_1(t, \Omega)$$ and $$h_2(t, \Omega)$$ over the sphere $$S^2$$ is defined as: $$\mathcal{O}{\text{sphere}}(h_1, h_2) = \frac{\int{S^2} \langle h_1(t, \Omega) \mid h_2(t, \Omega) \rangle_t , d\Omega}{\sqrt{\left[ \int_{S^2} \langle h_1(t, \Omega) \mid h_1(t, \Omega) \rangle_t , d\Omega \right] \left[ \int_{S^2} \langle h_2(t, \Omega) \mid h_2(t, \Omega) \rangle_t , d\Omega \right]}}$$

By expanding the strain fields in spin-weighted spherical harmonics $${}^{-2}Y_{\ell m}(\theta, \phi)$$ and invoking their orthonormality: $$\int_{S^2} {}^{-2}Y_{\ell m}^*(\Omega) , {}^{-2}Y_{\ell' m'}(\Omega) , d\Omega = \delta_{\ell \ell'} , \delta_{m m'}$$ all cross-terms between different modes decouple and vanish. The integrated overlap simplifies to the sum of the mode-by-mode overlaps: $$\int_{S^2} \langle h_1(t, \Omega) \mid h_2(t, \Omega) \rangle_t , d\Omega = \sum_{\ell, m} \langle h_{1, \ell m} \mid h_{2, \ell m} \rangle_t$$

Extrinsic Optimization

To align the coordinate frames, the second waveform is transformed via: $$h_{2, \ell m}^{\text{rot, shifted}}(t) = e^{-i m \phi_c} \sum_{m'=-\ell}^{\ell} h_{2, \ell m'}(t - t_c) , D^{\ell}{m' m}(R)$$ where $$D^\ell{m'm}(R)$$ is the Wigner D-matrix. The returned mismatch $$\mathcal{M}$$ is defined as: $$\mathcal{M} = 1 - \max_{t_c,, \phi_c,, R \in SO(3)} \left[ \frac{\sum_{\ell, m} \langle h_{1, \ell m} \mid h_{2, \ell m}^{\text{rot, shifted}}(t_c, \phi_c, R) \rangle_t}{\sqrt{\left( \sum_{\ell, m} \langle h_{1, \ell m} \mid h_{1, \ell m} \rangle_t \right) \left( \sum_{\ell, m} \langle h_{2, \ell m} \mid h_{2, \ell m} \rangle_t \right)}} \right]$$

mismatch = wfm_a.match_sphere_averaged(
    wfm_b,
    psd=my_psd,
    f_lower=20.0,            # Hz
    delta_t_seconds=1./4096,
)

match_sphere_averaged_bms_maximized(other, psd, f_lower, j_max, ...)

This extended method performs the optimization over the infinite-dimensional BMS supertranslation group at null infinity $$\mathcal{I}^+$$ in addition to the standard rigid rotations and shifts.

Supertranslations correspond to direction-dependent retarded-time shifts: $$u' = u - \alpha(\theta, \phi)$$ where the supertranslation field $$\alpha(\theta, \phi)$$ is decomposed into scalar spherical harmonics: $$\alpha(\theta, \phi) = \sum_{j=0}^{j_{\text{max}}} \sum_{k=-j}^{j} \alpha_{jk} , Y_{jk}(\theta, \phi)$$ For small supertranslations, the waveform modes undergo first-order mixing: $$h'{\ell m}(u) = h{\ell m}(u) - \sum_{j=0}^{j_{\text{max}}} \sum_{k=-j}^{j} \sum_{p, q} \alpha_{jk} , \mathcal{G}^{\ell m}{jk,pq} , \dot{h}{pq}(u)$$ where $$\mathcal{G}^{\ell m}{jk,pq}$$ are the spin-weighted Gaunt coefficients: $$\mathcal{G}^{\ell m}{jk,pq} = \int_{S^2} {}^{-2}Y_{\ell m}^*(\Omega) , Y_{jk}(\Omega) , {}^{-2}Y_{pq}(\Omega) , d\Omega$$

This method optimizes $$t_c$$, $$\phi_c$$, $$R \in SO(3)$$, and the supertranslation coefficients $$\alpha_{jk}$$ (where $$j=1$$ shifts the coordinate origin to correct for center-of-mass drift, and $$j \ge 2$$ modes correct for proper supertranslations) using a Nelder-Mead simplex solver to find the global minimum mismatch.

Note

The scri package is required to compute the spin-weighted Gaunt coupling coefficients $$\mathcal{G}^{\ell m}_{jk,pq}$$.

Unit conventions

All data stored internally in WaveformModes uses geometrized, mass-scaled dimensionless units:

Quantity Dimensionless unit
Time $$GM_\text{tot}/c^3$$
Amplitude ($$r , h_{\ell m}$$) $$GM_\text{tot}/c^2$$

Physical conversion factors (from nrcats.utils):

import lal

# Time: 1 NR M unit → seconds
m_secs = total_mass_msun * lal.MTSUN_SI

# Amplitude: dimensionless → strain at distance d_Mpc
amp_scale = (lal.G_SI * total_mass_msun * lal.MSUN_SI
             / (lal.C_SI**2 * d_Mpc * 1e6 * lal.PC_SI))

Design notes

Per-instance _filepath attribute

_filepath is extracted and stored as a per-instance attribute before the parent sxs.WaveformModes.__new__() call. This prevents class-level sharing where loading a second simulation would overwrite the first simulation's cached file path.

Non-writable memoryview wrapping

sxs.WaveformModes.data may return a non-writable memoryview. All arithmetic in WaveformModes wraps the data with np.array(..., dtype=complex) before any in-place operations to avoid ValueError: assignment destination is read-only.