[Paper Review] The effect of supernova natal kicks on compact object merger rate

Krzysztof Belczyński, T. Bulik|ArXiv.org|Jan 14, 1999

Gamma-ray bursts and supernovae6 citations

TL;DR

This paper investigates how supernova natal kicks—impulsive velocities imparted to neutron stars at birth—affect the merger rate of compact objects like neutron stars and black holes. Using binary population synthesis models, the authors show that the merger rate varies by a factor of 30 across the currently observed range of kick velocities, highlighting a major uncertainty in predicting gravitational wave and gamma-ray burst rates.

ABSTRACT

Mergers of compact objects may lead to different astrophysical phenomena: they may provide sources of observable gravitational radiation, and also may be connected with gamma-ray bursts. Estimate of the rate with which such mergers take place are based on assumptions about various parameters describing the binary evolution. The distribution of one of these parameters - the kick velocity a neutron star receives at birth will strongly influence the number and orbital parameters of compact objects binaries. We calculate these effect using population synthesis of binary stars and show that the expected compact object merger rate changes by a factor of 30 when the kick velocity varies within its current observational bounds.

Motivation & Objective

• To assess the impact of neutron star natal kicks on the formation rate of compact object binaries.
• To quantify how uncertainties in kick velocity distributions affect merger rate predictions.
• To improve estimates of gravitational wave and gamma-ray burst progenitor rates using binary evolution models.
• To evaluate the sensitivity of merger rates to the range of observed kick velocities.
• To provide a robust framework for interpreting future gravitational wave detections.

Proposed method

• Employed binary population synthesis simulations to model the evolution of massive binary star systems.
• Incorporated variable natal kick velocities for neutron stars at core-collapse supernovae.
• Tracked the formation and evolution of compact object binaries through multiple evolutionary phases.
• Calculated merger rates by integrating over the full distribution of initial binary parameters and kick velocities.
• Used observational constraints on kick velocity distributions to define input parameter ranges.
• Compared merger rates across different assumed kick velocity distributions to assess sensitivity.

Experimental results

Research questions

• RQ1 How does the distribution of supernova natal kicks influence the expected merger rate of compact objects?
• RQ2 What is the range of merger rates predicted when varying kick velocities within current observational bounds?
• RQ3 How sensitive are compact object merger rates to uncertainties in natal kick parameters?
• RQ4 What fraction of compact object binaries survive to merge given different kick velocity assumptions?
• RQ5 How do these variations affect predictions for gravitational wave and gamma-ray burst event rates?

Key findings

• The compact object merger rate varies by a factor of 30 when the natal kick velocity for neutron stars is varied across its currently observed range.
• Higher kick velocities reduce the number of surviving compact object binaries due to ejection from the system.
• Lower or zero kick velocities lead to a significantly higher merger rate, as binaries are more likely to remain bound.
• The observed range of kick velocities introduces a large uncertainty in merger rate predictions.
• The results imply that natal kick velocity is a dominant source of uncertainty in estimating gravitational wave event rates.
• The study underscores the importance of constraining natal kick distributions for accurate astrophysical rate predictions.