James Webb Space Telescope observes massive kilonova explosion for first time

In perhaps the biggest plot twist: the GRB - the second brightness of all time - lasted half a minute, i.e. a second "long" burst accompanied by r-process production. Likely a neutron star merger, but one which challenges our ideas about how long the central engine should "jet".

— Brian Metzger (@bluekilonova) July 6, 2023

Second brightest GRB recorded to date

A team of astronomers led by Andrew Levan from Radboud University in the Netherlands investigated GRB 230307A, which is sourced to the merging of neutron stars. 

This is the second brightest GRB recorded to date, and it was discovered on March 7, 2023, by NASA's Fermi Gamma-ray Space Telescope. 

The GRB lasted only for 34 seconds and was detected by many telescopes at the same time, allowing researchers to pinpoint its origin.

The follow-up observation of the kilonova was conducted twice using Webb, while its mid-infrared imaging and spectroscopy instrument was used to gather data. 

The first Webb observation occurred 29 days after the GRB discovery, and then again at 61 days. During these observations, the JWST detected a transition from a blue hue to a red hue, which serves as a distinctive indication of a kilonova explosion.

Using a powerful vision of the Webb, the team spotted several galaxies around the kilonova. The galaxy identified as the brightest, and thus the most probable location of this explosion, is situated approximately 8.3 million light-years away from Earth.

Kilonova explosions also produce metals

These colossal explosions have also been found to produce the most massive elements in the universe, confirming a long-standing speculation that was finally substantiated in 2017.

According to Space.com, these heavy metals are thought to develop via a mechanism known as neutron capture or the r-process. As the name implies, the process allows atomic nuclei to capture neutrons, resulting in the formation of new and heavier elements such as iron, thorium, gold, platinum, uranium, and many more.

And r-processes occur only in extreme and violent settings, such as those seen surrounding neutron star mergers. 

"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 team wrote in a paper uploaded on the pre-print server. 

These heavier elements are important to understand the evolution and formation of stars found in the universe.

There are other potential methods for studying neutron star collisions as well, most notably gravitational waves.

But, at the time when the signal from the Laser Interferometer Gravitational-Wave Observatory (LIGO), responsible for detecting GRBs, reached Earth, the observatory was not operational. The facility was temporarily closed for three years for upgrades before reopening in May 2023. 

These new findings are currently in the review process, while the pre-print version has been uploaded on arXiv.

Study 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.