A lanthanide-rich kilonova in the aftermath of a long gamma-ray burst

Observationally, kilonovae are astrophysical transients powered by the radioactive decay of nuclei heavier than iron, thought to be synthesized in the merger of two compact objects^1-4. Over the first few days, the kilonova evolution is dominated by a large number of radioactive isotopes contributing to the heating rate^2,5. On timescales of weeks to months, its behaviour is predicted to differ depending on the ejecta composition and the merger remnant^6-8. Previous work has shown that the kilonova associated with gamma-ray burst 230307A is similar to kilonova AT2017gfo (ref. ⁹), and mid-infrared spectra revealed an emission line at 2.15 micrometres that was attributed to tellurium. Here we report a multi-wavelength analysis, including publicly available James Webb Space Telescope data⁹ and our own Hubble Space Telescope data, for the same gamma-ray burst. We model its evolution up to two months after the burst and show that, at these late times, the recession of the photospheric radius and the rapidly decaying bolometric luminosity (L_bol ∝ t^-2.7±0.4, where t is time) support the recombination of lanthanide-rich ejecta as they cool.