postmerger

postmerger provides surrogate fits for binary black-hole remnants.

Available models

• 3dq8_20M: Ringdown amplitudes from non-precessing, quasi-circular black-hole binaries. Calibrated at $20M$ after the peak of the $(2,2)$ strain. See the example notebook 3dq8_20M for usage details. Described in the following paper: 2408.05276 .

Installation

From source

git clone https://github.com/cpacilio/postmerger.git
cd postmerger
pip install .

Basic usage

import postmerger

List of available fits

postmerger.allowed_fits
>>> ['3dq8_20M']

Load fit

fitname = '3dq8_20M'
fit = postmerger.load_fit(fitname)

Predict amplitudes and phases

Predicting amplitudes and phases is performed through the predict_amp and predict_phase methods of fit. The arguments are model-specific. See the notebooks below for examples of specific models:

• 3dq8_20M

Evaluate quasi-normal modes of Kerr black holes

We provide a QNM evaluator. See the notebook qnm_Kerr for advanced usage.

## mass and spin
mass = 67
spin = 0.67

## mode
mode = (2,2,0)

## evaluate in SI units
f, tau = postmerger.qnm_Kerr(mass,spin,mode,SI_units=True)

## results
print('frequency (Hz): ',f)
print('damping time (s): ',tau)
>>> frequency (Hz): 250.71404280475124
>>> damping time (s): 0.004032098030215414

Evaluate spherical-spheroidal mixing coefficients

We provide a mixing-coefficient evaluator. See the notebook spherical_spheroidal_mixing for further details.

## evaluate mu_{2320}

## spherical-harmonic indices
lm = (3,2)

## spheroidal-harmonic indices
mode = (2,2,0)

## final spin
spin = 0.68

mu_re, mu_im = postmerger.spherical_spheroidal_mixing(lm,mode,spin)

## results
print(mu_re+1j*mu_im)
>>> (0.0665939069543019+0.011046238081249502j)

Compute final mass and final spin

We provide functions to compute final mass and final spin from binary parameters. See the notebook final_mass_spin for further details.

## (we set a binary with anti-aligned spins ending into a black-hole with final spin pointing downward)

## initial masses (mass1>=mass2 is required)
mass1 = 25
mass2 = 5

## initial spins (magnitudes)
spin1 = 0.9
spin2 = 0.1

## angle between spins and z-direction
## [0,pi/2] is positive-z direction
## [pi/2,pi] is negative-z direction
beta = np.pi
gamma = 0.

## relative orientation between spin1 and spin2
## here, since the spins are (anti-)aligned, alpha is forced to be arccos(cos(beta)*cos(gamma))
alpha = np.arccos(np.cos(beta)*np.cos(gamma))

## compute final mass and final spin
massf = postmerger.final_mass(mass1,mass2,spin1,spin2,alpha,beta,gamma)
spinf, thetaf = postmerger.final_spin(mass1,mass2,spin1,spin2,,alpha,beta,gamma,return_angle=True)

## results
print('final mass: ',massf)
print('final spin: ',spinf)
print('final orientation: ',np.cos(thetaf))
>>> final mass: 29.62197225289648
>>> final spin: 0.12753062487767092
>>> final orientation: -1.0

Reproducibility

To help reproducibility, we provide data on which our model were trained:

• data_for_3dq8_20M

Citation

If you use postmerger in your work, please cite the following entries:

@article{Pacilio:2024tdl,
    author = {Pacilio, Costantino and Bhagwat, Swetha and Nobili, Francesco and Gerosa, Davide},
    title = {{postmerger: A flexible mapping of ringdown amplitudes for non-precessing binary black holes}},
    eprint = {2408.05276},
    archivePrefix = {arXiv},
    primaryClass = {gr-qc},
    month = {8},
    year = {2024}
}

@software{pacilio_2024_13220424,
  author       = {Pacilio, Costantino and
                  Swetha, Bhagwat and
                  Francesco, Nobili and
                  Gerosa, Davide},
  title        = {postmerger},
  month        = aug,
  year         = 2024,
  publisher    = {Zenodo},
  version      = {v0.0.2},
  doi          = {10.5281/zenodo.13220424},
  url          = {https://doi.org/10.5281/zenodo.13220424}
}

Coming soon

• Surrogate fits for the final mass and final spin of the remnant
• Full API