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The maximum offsets of binary neutron star mergers from host galaxies

Ilya Mandel, Om Sharan Salafia, Andrew Levan, Paul Disberg

TL;DR

The paper derives a simple analytic upper bound on how far binary neutron star mergers can occur from their birth galaxies, showing a maximum offset $D_ ext{max}$ that scales as $D_ ext{max} \lesssim 300\,\mathrm{kpc}\left(\dfrac{v_ ext{esc}}{500\,\mathrm{km\,s}^{-1}}\right)^{-7}$. This bound follows from the interplay between the pre-second-SN orbital velocity constraining the systemic kick and the gravitational-wave delay time $\tau_ ext{GW} \propto a^4 \propto v^{-8}$. The authors illustrate the results with COMPAS population-synthesis simulations and discuss how the host mass, via $v_ ext{esc}\approx 750\, (M_*/10^{11}\,M_\odot)^{1/4}$, further shapes the possible offsets. They argue that the offset is a probabilistic quantity requiring revised host-association methods, and explore possible links between DNS masses, kicks, offsets, and EM signatures of mergers, highlighting future observational and modeling directions.

Abstract

We analytically derive, and illustrate with a population synthesis model, the maximum offset of binary neutron star mergers ejected from their host galaxies. This approximate maximum offset is 300 kpc $\times\ (v_\mathrm{esc} / 500\ \mathrm{km}\ \mathrm{s}^{-1})^{-7}$, where $v_\mathrm{esc}$ is the escape velocity from the host galaxy. Massive hosts with high escape velocities are unlikely to yield very large offsets. This maximum offset should inform the host associations of mergers that are not coincident with galaxies. We also discuss potential correlations between offsets and system masses, and possibly the duration of the gamma-ray burst accompanying the merger.

The maximum offsets of binary neutron star mergers from host galaxies

TL;DR

The paper derives a simple analytic upper bound on how far binary neutron star mergers can occur from their birth galaxies, showing a maximum offset that scales as . This bound follows from the interplay between the pre-second-SN orbital velocity constraining the systemic kick and the gravitational-wave delay time . The authors illustrate the results with COMPAS population-synthesis simulations and discuss how the host mass, via , further shapes the possible offsets. They argue that the offset is a probabilistic quantity requiring revised host-association methods, and explore possible links between DNS masses, kicks, offsets, and EM signatures of mergers, highlighting future observational and modeling directions.

Abstract

We analytically derive, and illustrate with a population synthesis model, the maximum offset of binary neutron star mergers ejected from their host galaxies. This approximate maximum offset is 300 kpc , where is the escape velocity from the host galaxy. Massive hosts with high escape velocities are unlikely to yield very large offsets. This maximum offset should inform the host associations of mergers that are not coincident with galaxies. We also discuss potential correlations between offsets and system masses, and possibly the duration of the gamma-ray burst accompanying the merger.

Paper Structure

This paper contains 3 sections, 4 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Maximum offsets
  3. Discussion

Figures (4)

  • Figure 1: The final systemic kick velocity $v_\mathrm{sys}$ of merging DNS is limited by the orbital velocity prior to the second supernova (blue circles), with only a small subset of binaries receiving significant contributions from the first supernova (red stars); contributions from Blaauw:1961 (yellow diamonds) and natal (purple squares) kicks associated with the second supernova are also shown for this COMPAS simulation.
  • Figure 2: The distance travelled by merging DNS from the COMPAS simulation as a function of the binary systemic kick, ignoring the impact of the galactic potential. More than 95% merge within the maximum distance estimate of Equation \ref{['eq:Dmax']}, shown with a red dashed line. Here, we show all DNS, even those that merge after more than 14 Gyr.
  • Figure 3: The claimed host offsets of short GRB from the samples of Fong:2022Nugent:2022 as a function of putative host stellar mass, divided into Gold (large red), Silver (medium blue), or Bronze (small green) based on the chance alignment probability following current host association protocols. Equation \ref{['eq:Dmaxmass']} is shown in black, with the region of disallowed offsets above it shaded in grey.
  • Figure 4: The pre-supernova mass of the progenitor of the second-born neutron star vs. the distance travelled prior to merger (ignoring the impact of the galactic potential), for DNS merging within 14 Gyr. The color denotes the systemic kick of the binary.