Second brightest gamma-ray burst: revealing the mysteries of heavy element formation


Scientists observed the second brightest gamma-ray burst ever recorded on 7th March 2023, using ground and space-based telescopes including the Fermi Gamma-ray Monitor, NASA’s James Webb Space Telescope and the Neil Gehrels Swift Observatory.

Second only in brightness to the gamma-ray burst affectionately dubbed the Brightest Of All Time (BOAT), GRB-230307A promises to help uncover the mysteries of how the heaviest elements form. An international study involving multiple UK universities was published in Nature late last month, examining the origin of this extraordinarily bright burst of radiation.

Gamma-ray bursts (GRBs) are usually caused by cosmological explosions or events and can emit more energy in two seconds than our Sun will over its whole lifetime. Gamma rays are more difficult to observe due to their very short wavelength, as they are not in the visible part of the electromagnetic spectrum.

GRBs were first detected in the late 1960s by the US Airforce Vela satellites on the lookout for Soviet Nuclear Testing that would violate the nuclear treaty ban. It would be five years until the results would be declassified and analysed by Los Alamos scientists before the GRBs were confirmed to be of cosmic origin.

There are two distinct categories that GRBs are classified under, known as long and short duration. Short-duration GRBs last less than two seconds, 0.3 seconds on average, and are the result of neutron stars (the incredibly dense ‘star corpses’ left after the supernovae of red supergiants that weren’t quite big enough to become black holes) merging with each other or with black holes.

Long-duration bursts are usually triggered by the collapse of a massive star and have an average duration of about 30 seconds. Both types of GRB are caused by explosive cosmic events that can also create black holes. However, despite its extra-long duration of 200 seconds, GRB-230307A seems to have been caused by the merging of two neutron stars.

GRB-230307A seems to have been caused by the merging of two neutron stars

The stars that resulted in GRB-230307A were once gravitationally bound to each other in a binary pair. As one of the stars exploded as a supernova at the end of its life, it was blasted 120,000 light-years out of its home galaxy, with its partner following suit, still being gravitationally bound as neutron stars. It was many hundred million years later that the two neutron stars merged together.

This neutron star merger also resulted in a large explosion known as a ‘kilonova’. A kilonova produces a large amount of heavy elements which then undergo radioactive decay, resulting in electromagnetic radiation being detected by telescopes on Earth and in space. Individual elements can be detected in the aftermath of the kilonova, as the wavelength of light they emit and absorb is unique to each element. Due to the sensitivity of
the James Webb Space Telescope, some individual heavy elements were identified in aftermath of GRB-230307A, some for the very first time.

We have known that the lightest elements, hydrogen and helium, were formed in the Big Bang since the 1950s. Light elements including nitrogen and carbon which are essential to life, form from the fusion of hydrogen and helium in the cores of stars, and the remaining elements up to iron in the periodic table, at position 26 of 94 of the naturally occurring elements, form during supernova explosions. However, it wasn’t until 2019 that we found a large array of the heavy elements (formed by rapid neutron capture) are formed in kilonova explosions as a result of neutron star mergers.

A large array of the heavy elements are formed in kilonova explosions as a result of neutron star mergers

This observation is the second time that spectroscopic techniques have detected individual heavy elements after a neutron star merger. Tellurium is one of the heavy elements detected in the aftermath, which is rarer than platinum is on Earth. Iodine, essential for controlling metabolism in the body, and thorium which are both needed to help sustain life on Earth have also been identified in the material ejected by the kilonova. Now that it has been seen that neutron star mergers can power long-duration GRBs, the next steps involve finding and understanding what drives the long-lived mergers as well as seeing if heavier elements are created within their associated kilonovae.

Though GRB-230307A occurred a billion light-years away, it has helped researchers understand the conditions in which precious and heavy chemical elements are formed and created within our Universe. Contributions from instruments such as the James Webb Space Telescope promise to help as the journey to uncover how the heavier elements form continues.

Image: University of Warwick/Mark Garlick via Wikimedia Commons

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