[Paper Review] Modelling the formation of the first two neutron star-black hole mergers, GW200105 and GW200115: metallicity, chirp masses and merger remnant spins

Debatri Chattopadhyay, S. P. Stevenson+3 more|arXiv (Cornell University)|Mar 11, 2022

Pulsars and Gravitational Waves ResearchPhysics and Astronomy66 references23 citations

TL;DR

This study models the formation of the first two neutron star-black hole mergers, GW200105 and GW200115, using isolated binary evolution simulations with the COMPAS population synthesis code. It finds that sub-solar metallicities (𝑍≲0.005) strongly favor the observed systems, with non-spinning black holes preferred, and that chirp mass and remnant spin jointly constrain formation channels more effectively than effective spin.

ABSTRACT

The two neutron star-black hole mergers (GW200105 and GW200115) observed in gravitational waves by advanced LIGO and Virgo, mark the first ever discovery of such binaries in nature. We study these two neutron star-black hole systems through isolated binary evolution, using a grid of population synthesis models. Using both mass and spin observations (chirp mass, effective spin and remnant spin) of the binaries, we probe their different possible formation channels in different metallicity environments. Our models only support LIGO data when assuming the black hole is non spinning. Our results show a strong preference that GW200105 and GW200115 formed from stars with sub-solar metallicities $Z\lesssim 0.005$. Only two metal-rich ($Z=0.02$) models are in agreement with GW200115. We also find that chirp mass and remnant spins jointly aid in constraining the models, whilst the effective spin parameter does not add any further information. We also present the observable (i.e. post selection effects) median values of spin and mass distribution from all our models, which maybe used as a reference for future mergers. Further, we show that the remnant spin parameter distribution exhibits distinguishable features in different neutron star-black hole sub-populations. We find that non-spinning, first born black holes dominate significantly the merging neutron star-black hole population, ensuring electromagnetic counterparts to such mergers a rare affair.

Motivation & Objective

• To investigate the formation pathways of the first observed neutron star-black hole (NS-BH) mergers, GW200105 and GW200115, using isolated binary evolution models.
• To determine whether the black hole or neutron star formed first in these systems (BHNS vs NSBH sub-populations).
• To constrain the metallicity of the progenitor environments based on observed gravitational wave parameters.
• To assess the role of black hole spin, chirp mass, and remnant spin in distinguishing formation channels.
• To provide reference median spin and mass distributions for future NS-BH merger observations.

Proposed method

• Employed the COMPAS population synthesis code to simulate isolated binary evolution across a grid of metallicities (𝑍=0.0001 to 0.02) and evolutionary parameters.
• Used four primary models—Pessimistic, Optimistic, Kick_100, and Alpha_2—varying assumptions on common envelope efficiency, natal kicks, and spin alignment.
• Assumed black hole spin alignment with orbital angular momentum and tested effects of spin misalignment due to supernova kicks.
• Compared model predictions of chirp mass, effective spin (𝜒eff), and remnant spin (𝜒rem) against LIGO/Virgo observations from GW200105 and GW200115.
• Evaluated model success by matching observed 90% credible intervals for mass, spin, and remnant parameters.
• Generated median spin and mass distributions from all models for use as reference grids in future observational comparisons.

Experimental results

Research questions

• RQ1 Which formation channel—BHNS or NSBH—best explains the observed properties of GW200105 and GW200115?
• RQ2 What is the preferred metallicity range for the progenitor stars of these NS-BH binaries?
• RQ3 Is a spinning or non-spinning pre-merger black hole more consistent with the observations?
• RQ4 Do the observed remnant spins of GW200105 and GW200115 provide additional constraints beyond chirp mass and effective spin?
• RQ5 Can the observed spin and mass distributions distinguish between BHNS and NSBH sub-populations?

Key findings

• Sub-solar metallicities (𝑍≲0.005) are strongly preferred for both GW200105 and GW200115, with only two metal-rich (𝑍=0.02) models matching GW200115.
• The pre-merger black hole is strongly preferred to be non-spinning; only one model with a spinning BH matches GW200105.
• Chirp mass and remnant spin jointly constrain formation models more effectively than the effective spin parameter, which adds no additional information.
• NSBH systems contribute less than 1% to merging NS-BH binaries in most models, with BHNS systems dominating by several orders of magnitude.
• The remnant spin distribution exhibits distinguishable features between BHNS and NSBH sub-populations, enabling future discrimination.
• Electromagnetic counterparts to NS-BH mergers are likely rare due to the dominance of non-spinning, first-born black holes in the merging population.