[Paper Review] Heavy element nucleosynthesis associated with a gamma-ray burst

J. H. Gillanders, E. Troja+10 more|arXiv (Cornell University)|Aug 1, 2023

Gamma-ray bursts and supernovae15 citations

TL;DR

The paper presents spectroscopic evidence linking GRB 230307A to a compact-object merger and shows that the late-time, infrared-dominated emission requires high-opacity ejecta from r-process nucleosynthesis, identifying possible heavy element signatures.

ABSTRACT

Kilonovae are a novel class of astrophysical transients, and the only observationally-confirmed site of rapid neutron capture nucleosynthesis (the r-process) in the Universe. To date, only a handful of kilonovae have been detected, with just a single spectroscopically-observed event (AT 2017gfo). Spectra of AT 2017gfo provided evidence for the formation of elements heavier than iron; however, these spectra were collected during the first ~ 10 days, when emission from light r-process elements dominates the observations. Heavier elements, if synthesised, are expected to shape the late-time evolution of the kilonova, beyond the phases for which we have spectral observations. Here we present spectroscopic observations of a rapidly-reddening thermal transient, following the gamma-ray burst, GRB 230307A. Early (2.4 day) optical spectroscopy identifies the presence of a hot (T ~ 6700 K) thermal continuum. By 29 days, this component has expanded and cooled significantly (T ~ 640 K), yet it remains optically thick, indicating the presence of high-opacity ejecta. We show that these properties can only be explained by the merger of compact objects, and further, leads us to infer the production of the heavy lanthanide elements. We identify several spectral features (in both absorption and emission), whose cause can be explained by newly-synthesised heavy elements. This event marks only the second recorded spectroscopic evidence for the synthesis of r-process elements, and the first to be observed at such late times.

Motivation & Objective

• Demonstrate that the thermal emission following GRB 230307A originates from a compact-object merger ejecta.
• Constrain the ejecta properties (mass, velocity, and opacity) through optical/NIR/MIR spectroscopy and empirical modelling.
• Assess whether the observed spectra require lanthanide/actinide train of heavy elements indicative of r-process nucleosynthesis.
• Evaluate whether a neutron-star–neutron-star/black hole merger best explains the data compared to a white dwarf–neutron star/black hole merger.
• Identify spectral features that can be linked to specific r-process elements.
• Provide spectroscopic evidence for late-time kilonova evolution in a GRB context.

Proposed method

• Obtain and analyze optical to MIR spectra of GRB 230307A across multiple epochs (Gemini/GMOS at 2.4 days, JWST at 29 and 61 days).
• Decompose the spectra into a non-thermal afterglow power-law component and a thermal blackbody component to isolate the kilonova signal.
• Fit the thermal component with a one-zone, grey-opacity kilonova model to estimate ejecta mass, velocity, and opacity.
• Compare observations with kilonova radiative-transfer models (two-component LANL SuperNu models) and semi-analytic kilonova modelling (M19) to assess opacity requirements.
• Perform line identifications to match potential r-process transitions (lanthanides and specific elements like Te, Nd, Er) to observed emission/absorption features.
• Discuss WD–NS/BH versus NS–NS/BH merger scenarios and assess which better fits the full dataset.

Experimental results

Research questions

• RQ1 Can the late-time, infrared-dominated thermal emission following GRB 230307A be explained by a kilonova powered by r-process nucleosynthesis?
• RQ2 What ejecta properties (mass, velocity, opacity) are required to reproduce the observed spectra and continuum evolution?
• RQ3 Do spectral features point to specific r-process elements (e.g., lanthanides like Nd, Er, Te) in the ejecta?
• RQ4 Which progenitor scenario (NS–NS/BH merger vs WD–NS/BH merger) best explains the full spectroscopic and photometric dataset?
• RQ5 Is this GRB the second spectroscopically-confirmed kilonova and the first with late-time MIR spectroscopy?

Key findings

• The transient’s thermal component evolves from Tph ~ 6700 K at 2.4 days to ~640 K at 29 days, with high ejecta velocity and sustained optical depth.
• A high-opacity, optically thick ejecta is required to explain the late-time infrared continuum, consistent with heavy-element (r-process) nucleosynthesis.
• Spectral features at ~2.1 μm and ~4.4 μm (emission) and ~3.4–4.0 μm (absorption) can be interpreted as transitions from r-process elements such as Te, Nd, and Er, indicating lanthanide-rich ejecta.
• Two-component kilonova models (lanthanide-rich dynamical ejecta and lanthanide-poor wind) broadly match expectations but struggle to reproduce the exact continuum shape, suggesting photospheric emission or missing opacity data.
• Semi-analytic modelling (M19) yields best-fit ejecta mass m ≈ 0.059 solar masses, velocity v ≈ 0.088 c, and opacity κ ≈ 10 cm^2 g^-1, indicating high opacities are essential to reproduce the infrared spectrum.
• Analysis favors a NS–NS/BH merger as the progenitor, aligning with the observed rapid reddening and consistency with GRB-associated kilonova rates.