Analysis of the JWST spectra of the kilonova AT 2023vfi accompanying GRB 230307A

TL;DR

This paper analyzes JWST spectra of the kilonova AT 2023vfi, identifying potential r-process elements responsible for observed emission features and comparing findings with the previous kilonova AT 2017gfo.

Contribution

It presents the first detailed spectral analysis of AT 2023vfi using JWST data, proposing new candidate ions for observed emission features and highlighting the need for expanded atomic data.

Findings

Identified emission line centroids at specific wavelengths with velocity widths of 0.057-0.110c.

Ruled out Ba II and Ra II as sources of observed features.

Proposed Te I-III, Er I-III, and W III as promising ions for further investigation.

Abstract

Kilonovae are key to advancing our understanding of r-process nucleosynthesis. To date, only two kilonovae have been spectroscopically observed, AT 2017gfo and AT 2023vfi. Here, we present an analysis of the James Webb Space Telescope (JWST) spectra obtained +29 and +61 days post-merger for AT 2023vfi (the kilonova associated with GRB 230307A). After re-reducing and photometrically flux-calibrating the data, we empirically model the observed X-ray to mid-infrared continua with a power law and a blackbody, to replicate the non-thermal afterglow and apparent thermal continuum $\gtrsim 2 \, \mu$m. We fit Gaussians to the apparent emission features, obtaining line centroids of $20218_{-38}^{+37}$, $21874 \pm 89$ and $44168_{-152}^{+153}$\,\AA, and velocity widths spanning $0.057 - 0.110$\,c. These line centroid constraints facilitated a detailed forbidden line identification search, from which we shortlist a number of r-process species spanning all three r-process peaks. We rule out Ba II and Ra II as candidates and propose Te I-III, Er I-III and W III as the most promising ions for further investigation, as they plausibly produce multiple emission features from one (W III) or multiple (Te I-III, Er I-III) ion stages. We compare to the spectra of AT 2017gfo, which also exhibit prominent emission at $\sim 2.1 \, \mu$m, and conclude that [Te III] $\lambda$21050 remains the most plausible cause of the observed $\sim 2.1 \, \mu$m emission in both kilonovae. However, the observed line centroids are not consistent between both objects, and they are significantly offset from [Te III] $\lambda$21050. The next strongest [Te III] transition at 29290\,\AA\ is not observed, and we quantify its detectability. Further study is required, with particular emphasis on expanding the available atomic data to enable quantitative non-LTE spectral modelling.