Abstract
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 objects1,2,3,4. Over the first few days, the kilonova evolution is dominated by a large number of radioactive isotopes contributing to the heating rate2,5. On timescales of weeks to months, its behaviour is predicted to differ depending on the ejecta composition and the merger remnant6,7,8. 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 (Lbol ∝ t−2.7±0.4, where t is time) support the recombination of lanthanide-rich ejecta as they cool.
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Data availability
Swift/XRT products are available from the online GRB repository (https://www.swift.ac.uk/xrt_products). Swift/UVOT data are available from Swift Data Access (https://www.swift.ac.uk/archive). X-shooter data are available from ESO Science Archive Facility (https://archive.eso.org). HST and JWST data are available from Mikulski Archive for Space Telescopes (https://mast.stsci.edu). Chandra data are available from Chandra Data Archive (https://cda.harvard.edu/chaser). The TESS lightcurve is available from TessTransients archive (https://tess.mit.edu/public/tesstransients). Gemini data are available from Gemini Observatory Archive (https://archive.gemini.edu). XMM-Newton data are available from XMM-Newton Science Archive (https://www.cosmos.esa.int/web/xmm-newton/xsa). Fermi/GBM data are available from Fermi Science Support Center (FSSC) FTP archive https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm. All the processed data are available upon request to the corresponding authors. Source data are provided with this paper.
Code availability
Results can be reproduced using standard free analysis packages. Methods are fully described. Codes used to produce figures can be made available upon request.
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Acknowledgements
This work was supported by the European Research Council through the Consolidator grant BHianca (grant agreement ID 101002761) and, in part, by the National Science Foundation (under award number 2108950). This work was in part carried out at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-2210452. The development of afterglow models used in this work was partially supported by the European Union Horizon 2020 programme under the AHEAD2020 project (grant agreement number 871158). B.O. acknowledges useful discussions with J. Pierel and O. Fox regarding JWST analysis. M.I., G.S.H.P., S.-W.C., H.C. and M.J. acknowledge support from the National Research Foundation of Korea (NRF) grants, no. 2020R1A2C3011091 and no. 2021M3F7A1084525, funded by the Korea government (MSIT). C.R.B. acknowledges the financial support from CNPq (316072/2021-4) and from FAPERJ (grants 201.456/2022 and 210.330/2022) and the FINEP contract 01.22.0505.00 (ref.1891/22). C.R.B. made use of HPC Sci-Mind servers machines developed and supported by the CBPF AI LAB team. This research has made use of the KMTNet system operated by the Korea Astronomy and Space Science Institute (KASI) at three host sites of CTIO in Chile, SAAO in South Africa, and SSO in Australia. Data transfer from the host site to KASI and SNU was supported by the Korea Research Environment Open NETwork (KREONET). A.J.C.-T. acknowledges funding of the Spanish Ministry project PID2020-118491GB-I00/AEI/10.13039/501100011033. The observations included data obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia e Inovações (MCTI/LNA) do Brasil, the US National Science Foundation’s NOIRLab, the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory. SkyMapper is owned and operated by The Australian National University’s Research School of Astronomy and Astrophysics. The survey data were processed and provided by the SkyMapper Team at ANU. The SkyMapper node of the All-Sky Virtual Observatory (ASVO) is hosted at the National Computational Infrastructure (NCI). Development and support of the SkyMapper node of the ASVO has been funded in part by Astronomy Australia Limited (AAL) and the Australian Government through the Commonwealth’s Education Investment Fund (EIF) and National Collaborative Research Infrastructure Strategy (NCRIS), particularly the National eResearch Collaboration Tools and Resources (NeCTAR) and the Australian National Data Service Projects (ANDS).
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Y.-H.Y. led the analysis of the prompt emission, SEDs and multi-wavelength lightcurves. E.T. initiated the project, coordinated the observations and their interpretation. B.O. led the study of the host galaxy. C.R.B., A.J.C.-T., Y.H., C.D.K., M.M., F.N., I.P.-G. and J.H.G. acquired and reduced the data of the SOAR telescope. R.R. led the analysis of the radio data. B.O. reduced the JWST data. E.T. and B.O. acquired and reduced the HST data. E.T., B.O. and Y.-H.Y. acquired and reduced the XMM-Newton data. E.T. and Y.-H.Y. reduced the Swift data. Y.-H.Y. reduced the TESS data. M.I., G.S.H.P., M.J., S.-W.C., H.C. and C.-U.L. acquired and reduced the data of the KMTNet and RASA36 telescopes. E.T., J.D., K.D. and A.K. acquired and reduced the data of the PRIME telescope. B.O., S.D. and J.H.G. acquired and reduced the data of the Gemini telescope. J.H.G. and E.T. reduced the data of the X-shooter telescope. Y.-H.Y., G.R., H.v.E. and Z.-G.D. contributed to afterglow modelling and their physical interpretation. Z.-K.P. contributed to possible progenitors. C.L.F. contributed to the interpretation of the data. Y.-H.Y., E.T., B.O. and C.L.F. wrote the paper, with contributions from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Empirical model for the nIR, optical, and X-ray lightcurves.
The lightcurves are modeled using PL segments, Fν ∝ t−αν−0.8. The gray lines represent the best-fit models. Different symbols indicate observations with different filters. Error bars and upper limits are 1σ c.l. and 3σ c.l., respectively.
Extended Data Fig. 2 Properties of the potential host galaxies.
a, Probability of chance coincidence for galaxies in the field of GRB 230307A. Likely unrelated galaxies are displayed as gray circles. The candidate host galaxies G*, LMC (purple crosses) and G1 (red star) are highlighted. b, Optical spectrum of the bright galaxy G1. The observed spectrum is shown in blue and the error spectrum in black. Line identifications are made at z = 0.0647 ± 0.0003. The spectrum is smoothed with a Savitzky-Golay filter of two pixels for display purposes. c,d, Spectral energy distribution of the bright galaxy G1. The model SED (blue line) and model photometry (blue squares) derived using Prospector are compared to the observed photometry (red circles). Filter bandpasses are shown at the bottom of panel c in gray. Fit residuals are shown in d. Error bars represent 1σ uncertainties.
Extended Data Fig. 3 Prompt emission properties of GRB 230307A.
a,b, Gamma-ray lightcurves of GRB 230307A (red) and GRB 211211A (dark) from Fermi/GBM in the energy range of 10−25 keV and 0.8−10 MeV with 0.2 s binsize. The purple shaded area roughly represents the time range of the initial pulse of the lightcurve, as depicted in the zoomed-in panel (c) with 5 ms binsize in the energy range 10–350 keV. d, The Amati-relation diagram. The plum/gray/green circles represent Type I (short) GRBs/Type II (long) GRBs/magnetar giant flares, and the corresponding color solid line and the area between dashed lines are the best-fit model and 95% c.l., respectively. GRB 230307A (whole burst) shifts following the red line when located at different redshifts. The red stars represent it at the three most probable host galaxies (G1, LMC and G*), while the GF is only reasonable when we treat the initial pulse as the main burst (zoom-in panel c). Hybrid GRB 211211A is shown in the blue circle. The purple shaded (z > 0.23)/hatched (z > 0.43) area is ruled out by the expansion velocity of the photosphere radius at T0 + 1.2 d/28.9 d being limited to less than the speed of light. The orange hatched area is ruled out by the SED (z ≲ 3.3). The red dashed line indicates the redshift where it departs from the 95% c.l. for the distribution of Type I GRBs. Error bars represent 1σ uncertainties.
Extended Data Fig. 5 Results for a forward shock plus two-component kilonova model for Dataset 2.
a. Posterior probability distributions of parameters. b. The prior bounds and posterior medians for parameters. The values corresponding to the two kilonova components are denoted by the subscript 1 or 2. Uniform priors are employed for all parameters except for the electron index p, which is a truncated-Gaussian prior (2.46 ± 0.20) derived from the spectral index βX = 0.73 ± 0.10 (ref. 71) according to standard closure relations72. Error bars represent 1σ uncertainties.
Extended Data Fig. 6 Neodymium opacities in the 1−5 μm range at 3 temperatures: 0.24 eV, 0.17 eV and 0.07 eV.
In local thermodynamic equilibrium, these correspond to ionization fractions of 1.0 (T = 0.24 eV), 0.886 (T = 0.17 eV) and 10−6 (T = 0.07 eV). The material begins to recombine between 0.24 and 0.17 eV (2,000–2,500 K). As it recombines, the number of bound-bound lines in the 1–5 μm range decreases significantly, causing a drop in the opacity.
Supplementary information
Supplementary Information
The supplementary information includes the data reduction for multi-wavelength observations, empirical modelling of multi-wavelength lightcurves and discussions about kilonova bolometric luminosity. Supplementary tables are also provided in this file.
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Yang, YH., Troja, E., O’Connor, B. et al. A lanthanide-rich kilonova in the aftermath of a long gamma-ray burst. Nature 626, 742–745 (2024). https://doi.org/10.1038/s41586-023-06979-5
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DOI: https://doi.org/10.1038/s41586-023-06979-5
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