Heavy-element production in a compact object merger observed by JWST

The merger of binary compact objects (neutron stars and black holes) is a significant area of astrophysical research. These events are believed to be the source of heavy-element nucleosynthesis via the rapid neutron capture process (r-process), and they are also associated with long-duration gamma-ray bursts (GRBs) and high-frequency gravitational waves (GWs). Understanding these mergers is crucial for unraveling the origin of heavy elements and the processes that power GRBs. This study focuses on GRB 230307A, an exceptionally bright GRB, to investigate the production of heavy elements in such events. The extreme brightness of GRB 230307A provides a unique opportunity to study the kilonova associated with the merger in unprecedented detail using the JWST's high sensitivity and spectral resolution in the mid-infrared. The paper aims to characterize the kilonova's properties, identify the specific heavy elements produced, and assess the contribution of GRBs to the overall heavy-element budget of the universe. The importance of this research stems from the need to test theoretical models of kilonova emission and r-process nucleosynthesis. Observing a bright event like GRB 230307A allows for more robust constraints on these models and a clearer picture of the processes at play.

Previous research has established a link between long-duration GRBs and compact object mergers, particularly neutron star-neutron star mergers. The discovery of GW170817 and its associated kilonova, AT2017gfo, provided strong evidence for this connection. Several other GRBs have been proposed as potential merger candidates. However, the detailed characterization of heavy element production in these events has been limited by observational challenges. Studies of kilonova emission have been crucial in constraining r-process models, showing that the abundances of lanthanides and other heavy elements vary depending on the merger parameters. The detection of specific elements in kilonova spectra can provide invaluable insights into the nucleosynthesis process and test different theoretical predictions. This study builds upon this existing literature by exploiting the unique capabilities of the JWST to obtain detailed spectroscopic information of GRB 230307A's kilonova, offering a more comprehensive understanding of the heavy element production in this specific merger event, extending the information from GW170817, which was less luminous.

The research involved a multi-wavelength observational campaign following the detection of GRB 230307A. The burst was detected by the Fermi Gamma-ray Burst Monitor (GBM) and GECAM. Its high energy properties were characterized using data from several high-energy instruments, which allowed for source triangulation via the InterPlanetary Network (IPN). Optical and near-infrared observations were obtained using the ULTRACAM, Gemini South telescope, and Very Large Telescope (VLT). X-ray observations were made using the Swift X-ray Telescope (XRT) and the Chandra X-ray Observatory. Radio observations used the Australia Telescope Compact Array. The redshift of the host galaxy was determined using MUSE integral field spectrograph observations. Crucially, the James Webb Space Telescope (JWST) played a pivotal role in the study, providing mid-infrared imaging and spectroscopy 29 and 61 days after the burst. The JWST's NIRCam provided images, while NIRSpec provided spectra. Data analysis involved characterizing the GRB's light curves, identifying the kilonova emission, and analyzing the spectroscopic data to identify emission lines corresponding to specific heavy elements. Spectral modeling was used to compare the observed data with theoretical models of kilonova emission. The observed data were carefully analyzed, and stringent statistical tests were performed to assess the significance of various detections. The uncertainties in measurements were taken into account in order to arrive at robust scientific conclusions.

The authors observed GRB 230307A, the second brightest GRB ever detected, which had an unusually weak afterglow given its bright prompt emission. JWST observations, 29 and 61 days post-burst, revealed a very red source in the mid-infrared. The NIRSpec spectroscopy revealed an emission line at 21.15 microns, which was interpreted as tellurium (A = 130). The observed red colors indicate significant production of lanthanides. The combination of the tellurium emission line and the very red continuum strongly suggests that the kilonova is rich in heavy r-process elements across a broad atomic mass range. The rapid decay rate of the object (2.4 mag in F444W) in a second JWST observation further confirms the kilonova interpretation. The properties of GRB 230307A show striking similarities to GRB 211211A, which was also accompanied by a kilonova. The early-time data revealed a blue component transitioning to a red component, consistent with the kilonova. The NIRSpec spectrum showed a broad absorption feature at approximately 11.25 microns and a rising continuum up to 4.5 microns, consistent with kilonova models. The later-time emission was comparable with that seen in AT2017gfo, which is consistent with kilonova emission. The findings suggest that mergers contribute substantially to the bright GRB population. The similarity with the kilonova associated with GW170817 provides further support for the merger interpretation. Furthermore, the extreme brightness of the GRB and its kilonova allowed the team to detect heavy elements using the JWST.

The detection of tellurium and the red color of the kilonova in GRB 230307A provides strong evidence for r-process nucleosynthesis across a broad atomic mass range in compact object mergers. The similarities between GRB 230307A and AT2017gfo strengthen the interpretation that the observed properties are characteristic of kilonovae associated with mergers. The unusually weak afterglow in relation to its bright prompt emission could be a distinguishing feature of long GRBs from mergers compared to other GRB types. The ability of the JWST to detect kilonova emission at high redshifts opens up new possibilities for studying mergers throughout the universe and for extending the effective reach of GW detectors, which currently have limited reach. However, the long duration of the prompt emission in GRB 230307A presents a challenge to current models of compact object mergers, requiring more detailed general-relativistic hydrodynamic simulations to fully understand the dynamics and thermal processes.

This study demonstrates the power of combining multi-wavelength observations, particularly with the JWST, in investigating compact object mergers and heavy element production. The detection of tellurium and other heavy elements in the kilonova of GRB 230307A provides strong evidence for the contribution of such mergers to r-process nucleosynthesis. Future studies could focus on characterizing the properties of a larger sample of GRBs to better understand the diversity of kilonova emission and its relationship to the properties of the merger. Further simulations combining GRB and kilonova formation processes will be crucial to reconcile observations and theoretical predictions.

The study is limited by the relatively small sample size of extremely bright GRBs with detailed kilonova observations. The interpretation of the spectroscopic features relies on kilonova models, and the accuracy of these models affects the conclusions about the specific elements produced. The duration of the prompt gamma-ray emission in GRB 230307A remains a challenge to current models of compact object mergers. Further research is needed to understand the underlying physics of such long-duration bursts.