COSMIC Variance in Binary Population Synthesis

TL;DR

This paper introduces COSMIC, a Python-based, community-driven binary population synthesis suite adapted from BSE to model compact-object binaries and their progenitors. COSMIC provides an end-to-end workflow—from initializing binary populations through fixed-population convergence to generating astrophysical realizations and synthetic catalogs—while incorporating updated wind, mass-transfer, SN, natal-kick, spin, pulsar, and merger physics. As a proof of concept, the authors simulate the Milky Way's stellar remnants and predict LISA-detectable populations, finding roughly 10^8 compact binaries in the Galaxy and about 10^4 resolvable by LISA over a 4-year observation. The work establishes COSMIC as a flexible tool to study uncertainties across binary-evolution physics, star-formation histories, and Galactic structure, enabling self-consistent comparisons with future GW and electromagnetic observations.

Abstract

The formation and evolution of binary stars is a critical component of several fields in astronomy. The most numerous sources for gravitational wave observatories are inspiraling and/or merging compact binaries, while binary stars are present in nearly every electromagnetic survey regardless of the target population. Simulations of large binary populations serve to both predict and inform observations of electromagnetic and gravitational wave sources. Binary population synthesis is a tool that balances physical modeling with simulation speed to produce large binary populations on timescales of days. We present a community-developed binary population synthesis suite: COSMIC which is designed to simulate compact-object binary populations and their progenitors. As a proof of concept, we simulate the Galactic population of compact binaries and their gravitational wave signal observable by the Laser Interferometer Space Antenna (LISA). We find that $\sim10^8$ compact binaries reside in the Milky Way today, while $\sim10^4$ of them may be resolvable by LISA.

Figures (5)

• Figure 1: Schematic for the process COSMIC uses to generate a fixed population. Generally, the process moves from left to right. All quantities in the boxes are produced and available to the user, while all arrows represent modules within COSMIC that facilitate the evolution process. For a discussion of the $match$ and convergence, see \ref{['sec: convergence']}. See also the list of steps outlined below in Section \ref{['sec:cosmic']}.
• Figure 2: The first three columns show normalized histograms of the semimajor axis at formation for simulated NS+NS populations where each column includes the population from the previous ones. The fifth column shows the evolution of the $match$ as the size of the simulated population grows, where we show Log$_{10}$(1-$match$) to illustrate how the $match$ tends to unity.
• Figure 3: Normalized histograms of the primary mass, secondary mass, semilatus rectum, and formation time of binaries at formation with different combinations of WDs, NSs, and BHs. The WDs are split into separate populations for helium (He), carbon/oxygen (CO), and oxygen/neon (ONe) sources. The solid lines show the formation properties of the solar metallicity population while the dashed lines show the $15\%$ solar metallicity population.
• Figure 4: Amplitude spectral density (ASD) as a function of frequency for each population in our Milky Way realization. The grey lines are the same in each plot and show the total ASD from the full population. The black line shows the LISA amplitude spectral density noise floor, without a Galactic foreground contribution.
• Figure 5: Scatter points of the ASD vs GW frequency of the systems resolved with $S/N > 7$ for each population in our Milky Way realization. The simulated irreducible foreground and LISA sensitivity is shown in black.