stroopwafel: simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources

Abstract

Gravitational-wave observations of double compact object (DCO) mergers are providing new insights into the physics of massive stars and the evolution of binary systems. Making the most of expected near-future observations for understanding stellar physics will rely on comparisons with binary population synthesis models. However, the vast majority of simulated binaries never produce DCOs, which makes calculating such populations computationally inefficient. We present an importance sampling algorithm, STROOPWAFEL, that improves the computational efficiency of population studies of rare events, by focusing the simulation around regions of the initial parameter space found to produce outputs of interest. We implement the algorithm in the binary population synthesis code COMPAS, and compare the efficiency of our implementation to the standard method of Monte Carlo sampling from the birth probability distributions. STROOPWAFEL finds ∼25-200 times more DCO mergers than the standard sampling method with the same simulation size, and so speeds up simulations by up to two orders of magnitude. Finding more DCO mergers automatically maps the parameter space with far higher resolution than when using the traditional sampling. This increase in efficiency also leads to a decrease of a factor ∼3-10 in statistical sampling uncertainty for the predictions from the simulations. This is particularly notable for the distribution functions of observable quantities such as the black hole and neutron star chirp mass distribution, including in the tails of the distribution functions where predictions using standard sampling can be dominated by sampling noise.

Modelling double neutron stars: radio and gravitational waves

“…Stevenson et al 2017;Barrett et al 2018;Stevenson et al 2019;Bavera et al 2019) or a NS (e.g. Vigna-Gómez et al 2018;Neijssel et al 2019;Broekgaarden et al 2019), but did not follow any subsequent evolution of these objects. To analyse whether a NS is also a pulsar, other properties such as the magnetic field and spin period need to assigned to the NS and calculated over time, e.g.…”

The origin of spin in binary black holes

“…When we perform binary population synthesis simulations, we only model binary systems, neglecting the population of single stars. Furthermore, to save on computational costs, we restrict the mass of the primary star to a suitable range m A < m primary < m B so that we only consider initial binaries that can be progenitors of the systems we want to study (this is a basic version of the importance sampling approach described by Broekgaarden et al 2019). This means that we model only a fraction of the underlying stellar population.…”

Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave Observations II: Double Compact Object Rates and Properties

“…We simulate populations of GW sources with the rapid binary population synthesis code from the COMPAS 1 suite (Stevenson et al 2017;Barrett et al 2018;Vigna-Gómez et al 2018;Broekgaarden et al 2019;Neijssel et al 2019;Team COMPAS: J. Riley et al 2021). We model the formation of BH and NS mergers from the isolated binary evolution channel where the merging DCOs form from massive stars born in a binary system (Smarr & Blandford 1976;Srinivasan 1989).…”