Population Reweighting

POSYDON allows you to reweight your populations after they have been evolved, based on the initial conditions of the binaries and single stars. This is a form of importance sampling: the weights are obtained from the ratio of the PDF of the target population to the PDF of the simulated population, evaluated at the initial conditions of each model.

The reweighting step is performed on a TransientPopulation object with the get_model_weights method. The implementation expects the following initial properties to be available in the population data:

S1_mass_i: initial primary mass
S2_mass_i: initial secondary mass
orbital_period_i: initial orbital period
state_i: initial state, which is used to distinguish binaries from initially single stars

Supported initial-condition distributions

The reweighting implementation in posydon.popsyn.norm_pop currently supports several distribution families. The table below summarizes the main options.

Supported reweighting distributions
Initial condition	Supported schemes	Required parameters	Notes
Primary mass	Any IMF class available in `posydon.popsyn.IMFs`	`primary_mass_scheme`, `primary_mass_min`, `primary_mass_max`	The scheme name is resolved dynamically from the IMF module. If it is not recognized, a flat distribution is used and a warning is emitted.
Mass ratio	`flat_mass_ratio`, `power_law_mass_ratio`	For `flat_mass_ratio`: `secondary_mass_scheme` plus `secondary_mass_min` and `secondary_mass_max` (or explicit `q_min`/`q_max`). For `power_law_mass_ratio`: `mass_ratio_slope` and optional `q_min`/`q_max`.	The flat mass-ratio distribution can be defined either from the allowed secondary-mass range or from explicit mass-ratio bounds.
Orbital period	`period` with `Sana+12_period_extended` or `power_law`; or `separation` with `log_uniform`	For `period`: `orbital_period_min` and `orbital_period_max`. For `Sana+12_period_extended` no additional parameters are needed. For `power_law`: `power_law_slope`. For `separation`: `orbital_separation_min` and `orbital_separation_max`.	Periods are interpreted in days. Separation-based inputs are converted internally to periods using the orbital-separation relation.
Binary fraction	`const`	`binary_fraction_scheme`, `binary_fraction_const`	This is currently implemented as a constant binary fraction and is used to distinguish single-star and binary PDFs.

How the weights are computed

The method evaluates the target PDF and the simulation PDF at the initial conditions of each system and computes

weights = (PDF_target / PDF_sim) * (mean_mass_sim / mean_mass_pop) * (1 / M_sim)

where M_sim is the total simulated mass, mean_mass_sim is the mean mass of the simulated population, and mean_mass_pop is the mean mass of the requested target population.

In practice, this means that the reweighted population is only meaningful when the target and simulation populations share the same support for the sampled initial-condition distributions. If the target distribution extends beyond the range covered by the simulation, you might miss systems!