Probing the Origin of the Kilonova Candidate GRB 230307A: Analysis of Host Galaxy and Offset

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 (GC), 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 GCs, we show that a GC origin is unlikely. Using JWST and MUSE data, we derive the host galaxy morphology, the stellar mass, estimate the atomic gas (H I+He) contribution, and find the host rotation curve. Assuming a Navarro–Frenk–White 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 are 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.