A gamma-ray burst that astronomers had long associated with the merger of neutron stars could actually have occurred during the collapse of massive stars to form black holes. The new analysis shows that this scenario explains the observations more accurately than the neutron star merger version.
A Return to the Collapsar Theory
A team of physicists from Los Alamos National Laboratory analyzed two long-duration gamma-ray bursts, GRB 211211A and GRB 230307A, detected by the Fermi Gamma-ray Burst Monitor aboard NASA’s spacecraft in 2021 and 2023, respectively. Earlier, some scientists believed that both events were caused by the collision of two neutron stars, because the accompanying flash of light resembled a kilonova, which is usually associated with the formation of heavy elements.
The new modeling has shifted the interpretation back toward a collapsar scenario. A collapsar is a rapidly rotating massive star that collapses into a black hole, releasing powerful gamma radiation in the process. The results were published in the peer-reviewed journal The Astrophysical Journal Letters. According to Universe Today, they force astrophysicists to reconsider the assumptions built into models of such events.
A Trace of Elements
The team recreated both events on the Chicoma supercomputer to determine which heavy elements could have formed during the explosions through rapid neutron capture. Last year, the same group proposed a new mechanism for the synthesis of elements heavier than iron specifically in collapsars.
According to Los Alamos National Laboratory, the modeled chemical composition, without significant admixture of the heaviest elements – lanthanides, gold and lead, agreed almost exactly with the observed data of both bursts.
This became the main argument in favor of the collapsar interpretation. Reddening of the spectrum usually points to the formation of lanthanides, a group of rare-earth metals that efficiently absorb and scatter light, causing the radiation to shift into the infrared region. In this case, however, the model explained the signal without invoking the heaviest elements.
A More Complex Picture of Kilonovae
Theoretical physicist Matthew Mumpower, a co-author of the study, explained that the type of kilonova observed in these long-duration gamma-ray bursts does not necessarily imply the synthesis of gold, even if the signal contains a red component typically associated with lanthanide production. A simple single-component model explains the data without additional assumptions, while kilonovae themselves appear to be far more diverse and difficult to interpret than previously believed.
In 2023, the James Webb Space Telescope detected the chemical signature of tellurium in the spectrum of the kilonova following GRB 230307A, which was considered one of the strongest pieces of evidence for the formation of heavy elements during the merger of neutron stars. The new study calls that interpretation into question and shows that a similar spectral signature can also arise in a collapsar scenario.
Future observations, including the detection of gravitational waves, will help astrophysicists determine the origin of kilonovae and their associated gamma-ray bursts more precisely. In late 2026, the LIGO detectors are expected to begin a new observing cycle, IR1, while the next full-scale cycle, O5, is projected to increase the number of detected kilonovae by an order of magnitude compared with the previous run.
