Probing the origin of the kilonova candidate GRB 230307A: analysis of host galaxy and offset

TL;DR

This study analyzes the host galaxy and offset of GRB 230307A to determine if its kilonova originated from a binary neutron-star merger within the galaxy or from a globular cluster, using JWST and MUSE data.

Contribution

It provides a detailed analysis combining observational data and simulations to evaluate the origin scenarios of the kilonova associated with GRB 230307A.

Findings

A globular-cluster origin is unlikely based on JWST data.

A disk-origin for the BNS merger is possible but requires fine-tuned conditions.

Only 0.1% of simulated systems match the observed offset and galaxy mass model.

Abstract

We investigate the host galaxy of the long gamma-ray burst GRB 230307A, which is associated with a kilonova candidate likely produced by a binary neutron-star (BNS) merger. The transient occurred at a projected offset of ~40 kpc from its host. We consider two explanations for this large distance: (i) NSs that merge inside a remote globular cluster, or (ii) a BNS that formed in the disk whose orbit was strongly modified by the NS natal kicks. Using JWST data and comparisons with known globular clusters, we show that a globular-cluster origin is unlikely. Using JWST and MUSE data, we derive the host galaxy morphology, stellar mass, estimate the atomic gas (HI+He) contribution, and the host rotation curve. Assuming an NFW halo and applying Bayesian inference, we obtain a mass model for the host. From this model, we compute the time required for a disk-formed BNS, with a given natal kick, to reach the observed offset while marginalizing over uncertainties and over the initial position in the disk. We compare these results with BNS-merger simulations from a population-synthesis code combined with stellar evolutionary tracks, which provide the coalescence time and kick velocity for each realization. The two approaches have an overlap in the kick-time diagram, but only 0.1% of the simulated systems is compatible with the galaxy-mass model. This indicates that a disk origin is possible, but requires fine-tuned conditions for the kilonova to occur at such a large distance from the host galaxy.