JWST detection of heavy neutron capture elements in a compact object merger

Abstract: The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, biological and cultural importance, such as thorium, iodine and gold. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.

• The paper demonstrates that JWST spectroscopy of GRB 230307A reveals a prominent 2.15‑micron tellurium line, confirming heavy neutron‐capture nucleosynthesis.
• It details imaging and spectroscopic observations at 29 and 61 days post-burst to compare kilonova signatures with those from events like AT2017gfo.
• The study discusses implications for nucleosynthesis models and cosmic chemical evolution, highlighting the role of compact object mergers in heavy element creation.

JWST Detection of Heavy Neutron Capture Elements in a Compact Object Merger

The paper presented by Levan et al. examines observations of the gamma-ray burst GRB 230307A, particularly focusing on nucleosynthesis resulting from compact object mergers. The researchers employed the James Webb Space Telescope (JWST) to analyze the afterglow of GRB 230307A, a long-duration gamma-ray burst associated with a binary compact object merger. The study provides new insights into the synthesis of heavy neutron-capture elements within such energetic astrophysical events.

Key Observations and Methodology

The detection and analysis in this study centered around the use of both imaging and spectroscopy from JWST, captured at 29 and 61 days post-burst. This data facilitated the identification of emission lines, particularly focusing on a prominent line at 2.15 microns. This feature was interpreted as arising from tellurium, a heavy element in the second peak of the r-process abundance patterns. The element's detection is notable as it offers direct evidence of heavy neutron-capture processes occurring in GRB-induced kilonovae. In addition to tellurium, the spectral observations suggest significant production of lanthanides, contributing to the red color of the kilonova emission predominantly localized in the mid-infrared spectrum.

Numerical and Comparative Results

The spectroscopic analysis conducted revealed characteristics of the kilonova associated with GRB 230307A that draw significant parallels to the well-documented kilonova AT2017gfo, known from the gravitational wave-detected GW170817 merger event. Notably, both GRBs showed similar rapid spectral evolution and decline, confirming the presence of similar processes in the aftermath of the mergers.

The paper reports that GRB 230307A exhibited an isotropic equivalent energy release of over $10^{51}$ erg, placing it among the more energetic events observed. Moreover, correlations between the measured spectral line features and surviving elements like tellurium provide not only evidence of heavy element production but a useful benchmark for modeling kilonova emissions across varied astrophysical conditions.

Interpretation and Framework

The results elucidate the role compact object mergers play in the distribution and production of heavy elements across the universe. Previously, it was established that such events contribute to the presence of elements like strontium. The clear identification of tellurium extends this understanding to heavier atomic mass numbers, underscoring the importance of neutron-rich environments in kilonova physics.

The identification of a kilonova signature associated with a long-duration gamma-ray burst also presents a noteworthy expansion of the framework through which these cosmic explosions are understood. Typically, long-duration bursts are associated with the collapse of massive stars, while this view supports a more amalgamated landscape of progenitors leading to such phenomena.

Implications and Future Prospects

The findings have substantial implications for nucleosynthesis models, enhancing predictions of resultant chemical abundances following neutron star mergers. Furthermore, observationally, the work demonstrates the vital role JWST and its mid-infrared capabilities play in unpacking the complexities of post-merger ejecta, particularly when ground-based telescopes face limitations. The sensitive detection of mid-infrared signals opens avenues for exploring faint or distant kilonova exploders potentially invisible at shorter wavelengths.

As a prospective outlook, these results are likely to drive a reevaluation of not just the rates of r-process production from GRBs but also the spatial distribution of such processes within galaxies. Additionally, the connection between kilonovae and gravitational wave detections offers an exciting frontier, potentially enriching our understanding of both the local and cosmological environments conducive to r-process nucleosynthesis.

In conclusion, the work by Levan et al. significantly amplifies our granularity in understanding the astrophysical sites of heavy element production, leveraging both observational data and comparative analysis to position GRB 230307A within a broader paradigm of cosmic chemical evolution.