Forward Modeling of Double Neutron Stars: Insights from Highly-Offset Short Gamma-Ray Bursts

TL;DR

This study models the kinematic evolution of double neutron star progenitors for highly-offset short gamma-ray bursts, revealing they likely received significant natal kicks, which helps understand their origins and offsets.

Contribution

It introduces a semi-analytic galactic model combined with binary population synthesis to infer progenitor velocities and natal kicks for offset short gamma-ray bursts.

Findings

Progenitors require post-supernova velocities of hundreds of km/s.

Second-born neutron stars likely received natal kicks >200 km/s.

Method constrains progenitor properties using host galaxy models and burst offsets.

Abstract

We present a detailed analysis of two well-localized, highly offset short gamma-ray bursts---GRB~070809 and GRB~090515---investigating the kinematic evolution of their progenitors from compact object formation until merger. Calibrating to observations of their most probable host galaxies, we construct semi-analytic galactic models that account for star formation history and galaxy growth over time. We pair detailed kinematic evolution with compact binary population modeling to infer viable post-supernova velocities and inspiral times. By populating binary tracers according to the star formation history of the host and kinematically evolving their post-supernova trajectories through the time-dependent galactic potential, we find that systems matching the observed offsets of the bursts require post-supernova systemic velocities of hundreds of kilometers per second. Marginalizing over uncertainties in the stellar mass--halo mass relation, we find that the second-born neutron star in the GRB~070809 and GRB~090515 progenitor systems received a natal kick of $\gtrsim 200~\mathrm{km\,s}^{-1}$ at the 78\% and 91\% credible levels, respectively. Applying our analysis to the full catalog of localized short gamma-ray bursts will provide unique constraints on their progenitors and help unravel the selection effects inherent to observing transients that are highly offset with respect to their hosts.