The universe doesn’t always cooperate. Science has puzzled over gamma-ray bursts since they were first detected in the 1960s, and just when we think we have the basics solidified, something happens to upend conventional wisdom. Astronomers have spent the last year studying a gamma-ray burst dubbed GRB 211211A. Based on the power of the burst, scientists expected to find evidence of a supernova but have identified the source as a merger between two neutron stars, known as a kilonova.
It’s impossible to predict gamma-ray bursts, so a new detection usually prompts astronomers to turn their scopes toward the origin. That’s what happened when GRB 211211A lit up detectors in 2021. Researchers from NASA, Los Alamos National Laboratory, and numerous universities went looking for the source of the two-minute blast, expecting to find a recently exploded star. Imagine the surprise when they found a neutron star merger instead. This finding has now been confirmed and published by two independent teams.
“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,” said lead author Chris Fryer, a researcher at Los Alamos National Laboratory. The discovery of a long-duration GRB associated with a neutron star merger paints a more complicated picture of these transient stellar events.
This event was detected simultaneously by the Fermi and Swift telescopes at the end of 2021. Astronomers pinpointed its origin about 1 billion light-years away in the constellation Boötes. While nowhere near Earth, GRB 211211A was much closer than the average gamma-ray burst. Its nearness made the supernova origin unlikely, and the optical signal wasn’t typical for a kilonova. Neutron stars spiraling toward collision produce a distinctive gravitational wave signature, but unfortunately, the Laser Interferometer Gravitational-Wave Observatory (LIGO) was not online at the time to confirm.
“It was something we had never seen before,” said study co-author Simone Dichiara, an astrophysicist at Penn State. Scientists created models to explain the hybrid event, and scanned the afterglow of GRB 211211A. Kilonovas produce a distinctive signal, brighter in infrared than visible light because of the dense neutron-rich matter that is ejected from the collision. The teams used this signal to confirm a collision as the cause of GRB 211211A rather than a supernova. This discovery will force a reevaluation of the mechanism behind GRBs and could affect how they are studied.