Rare cosmic explosion blasts hole in established science

On Dec. 11, 2021, NASA’s Neil Gehrels Swift Observatory detected a  blast of high-energy light from a galaxy roughly 1 billion light-years  away. The event, which was simultaneously detected by Fermi Gamma-ray  Space Telescope, brings into question what was thought to be settled  science concerning gamma-ray bursts (GRBs), the most energetic  explosions in the universe.

”We were able to observe this event only because it was so close to  us,” said Eleonora Troja, an associate professor at the University of  Rome Tor Vergata and lead author on the paper. “It is very rare that  we observe such powerful explosions in our cosmic backyard, and every  time we do we learn about the most extreme objects in the Universe.”

The Swift team was able to rapidly identify the explosion’s location,  in the constellation Boötes, enabling other facilities to quickly  respond with follow-up observations. Their observations have provided  the earliest look yet at the first stages of a kilonova, according to  a NASA release. Their findings were published today (Dec. 7) in the  journal Nature (https://www.nature.com/articles/s41586-022-05327-3).

A. Simonnet (Sonoma State University) and NASA’s Goddard Space Flight

“It was something we had never seen before,” said Simone Dichiara,  assistant research professor of astronomy and astrophysics at Penn  State and member of the Swift team. “We knew it wasn’t associated with  a supernova, the death of a massive star. It was a completely  different kind of optical signal, one that we associate with a  kilonova, the explosion triggered by colliding neutron stars.”

Gamma-ray bursts come in two varieties: long and short. Scientists  previously understood long GRBs, which last a couple of seconds to one  minute, as forming when a super massive star explodes as a supernova.  Short GRBs, which last less than two seconds, were previously thought  to only occur when two compact objects – like two neutron stars or a  neutron star and a black hole – collide to form a kilonova.

NASA/ESA/HST/Troja et al. (2022)

The revelation that a kilonova could trigger a long gamma ray burst  rewrites the decades-long paradigm of cosmic explosions – that long  GRBs are the strictly the signature of the death of massive stars. The  discovery means not all long GRBs are made by supernovae, some are  produced by the merger of neutron stars.

“This event showed to us how our well-established knowledge of the  universe was in fact only a partial and incomplete view,” said Hendrik  van Eerten, a senior lecturer at University of Bath and co-author of  the study.  “We spent months trying to figure out alternative  explanations, but in the end this is the only one that works well.”

The development of models used in this work was partially supported by  the European Union Horizon 2020 Programme under the AHEAD2020 project  (grant agreement number 871158). The work was also supported by the  European Research Council through the Consolidator grant BHianca and  by the National Science Foundation.