On the Progenitor of Binary Neutron Star Merger GW170817

TL;DR

This paper constrains the properties of the progenitor system of GW170817 by modeling its binary evolution and supernova kicks, providing insights into neutron star merger origins.

Contribution

It introduces a method to derive progenitor constraints for GW170817 using simulated populations and kinematic evolution, considering various initial conditions.

Findings

Constraints are comparable to Galactic BNS progenitors.

Merger proximity to galaxy center is a key constraint.

Progenitor constraints are insensitive to star formation history for populations older than 1 Gyr.

Abstract

On 2017 August 17 the merger of two compact objects with masses consistent with two neutron stars was discovered through gravitational-wave (GW170817), gamma-ray (GRB 170817A), and optical (SSS17a/AT 2017gfo) observations. The optical source was associated with the early-type galaxy NGC 4993 at a distance of just $\sim$40 Mpc, consistent with the gravitational-wave measurement, and the merger was localized to be at a projected distance of $\sim$2 kpc away from the galaxy's center. We use this minimal set of facts and the mass posteriors of the two neutron stars to derive the first constraints on the progenitor of GW170817 at the time of the second supernova (SN). We generate simulated progenitor populations and follow the three-dimensional kinematic evolution from the binary neutron star (BNS) birth to the merger time, accounting for pre-SN galactic motion, for considerably different input distributions of the progenitor mass, pre-SN semimajor axis, and SN-kick velocity. Though not considerably tight, we find these constraints to be comparable to those for Galactic BNS progenitors. The derived constraints are very strongly influenced by the requirement of keeping the binary bound after the second SN and having the merger occur relatively close to the center of the galaxy. These constraints are insensitive to the galaxy's star formation history, provided the stellar populations are older than 1 Gyr.