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Rewriting the book on gamma-ray bursts

An unusual cosmic explosion has thrown an astronomical spanner in the works, causing scientists to rethink what they know about short- and long-duration gamma-ray bursts (GRBs).

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Scientists will need to reconsider their long-held beliefs about GRBs after a recent discovery made by an international team of astronomers. Following an unusual GRB, their research led to evidence of a previously unobserved hybrid neutron star merger event. The astronomers’ findings were published in the journal ‘Nature’. GRBs are the high-energy radiation arising from immense cosmic blasts of gamma rays and serve as a signature for different types of dying stars. For decades, astronomers believed they had a good grasp of the basic nature of GRBs: bursts of long duration – lasting more than 2 seconds – happen when a massive star reaches the end of its life and its core collapses to form a black hole, resulting in the giant explosion of a supernova. On the other hand, short-duration bursts of less than 2 seconds occur when two hyper-dense objects, such as two neutron stars, collide, blasting immense energy in an explosion known as a kilonova.

The picture is not so simple

“Astronomers have long believed that gamma-ray bursts fell into two categories: long-duration bursts from imploding stars and short-duration bursts from merging compact stellar objects,” states astrophysicist and study co-author Dr Chris Fryer of Los Alamos National Laboratory (LANL), United States, in a news release posted on ‘EurekAlert!’. “But in a recently observed event, we’ve found a kilonova along with a long-duration gamma-ray burst, and that has thrown a wrench into this simple picture.” The research leading to the identification of the first long-duration GRB originating from a neutron star merger was supported by the EU-funded BHianca and AHEAD2020 projects. On 11 December 2021, a number of observatories and satellites detected a GRB of almost unprecedented brightness from the outskirts of a galaxy about 1 billion light years away. Named GRB 211211A, the burst lasted 50 seconds, but its emission characteristics did not fit the profile of long-duration bursts. Follow-up observations led to the identification of a kilonova, which up to now had been associated with the short-duration bursts occurring with the merger of compact stellar objects. “Our modeling team at Los Alamos compared the observation to a suite of supernova and kilonova simulations, and we were unable to convincingly match the signal to a supernova model, whereas several kilonova models give a good match of the optical and infrared data points,” explains co-author Dr Ryan Wollaeger, also from LANL, in the news item. “There is still more theoretical modeling to do to fully understand this transient, however.” This unusual GRB is the first evidence astronomers have found of a hybrid event that overturns their commonly held views on GRBs. Study co-author and LANL team member Dr Eve Chase concludes: “We can no longer assume that all short-duration bursts come from neutron-star mergers, while long-duration bursts come from supernovae. We now realize that gamma-ray bursts are much harder to classify. This detection pushes our understanding of gamma-ray bursts to the limits.” BHianca (Black Hole Interactions and Neutron star Collisions Across the universe) ends in 2026. AHEAD2020 (Integrated Activities for the High Energy Astrophysics Domain) ends in 2024. Both projects are coordinated by Tor Vergata University of Rome. For more information, please see: BHianca project AHEAD2020 project website

Keywords

BHianca, AHEAD2020, gamma-ray burst, star, neutron star, kilonova, radiation, cosmic blast