Most known supernova-powered gamma-ray bursts are ‘long’ (lasting more than two seconds), but a gamma-ray burst event called GRB 200826A lasted just 0.6 seconds.
This illustration depicts a collapsing star that is producing two short gamma-ray jets. Image credit: Gemini Observatory / NOIRLab / NSF / AURA / J. da Silva / M. Zamani, NOIRLab.
Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the Universe. In just a few seconds, a typical GRB will release more energy than the Sun will over its 10-billion-year lifetime.
Astronomers are still figuring out exactly what causes these events.
They divide GRBs into two broad categories based on their duration: short and long GRBs.
Short GRBs blaze into life in less than two seconds and are thought to be caused by the merging of binary neutron stars.
Long GRBs are associated with supernova explosions caused by the implosions of massive stars.
However, the recent discovery of GRB 200826A — the shortest-ever event produced during a supernova — shows that GRBs don’t fit neatly into the boxes astronomers have created for them.
“This discovery represents the shortest gamma-ray emission caused by a supernova during the collapse of a massive star,” said Tomás Ahumada, a Ph.D. candidate at the University of Maryland and NASA’s Goddard Space Flight Center.
“It lasted for only 0.6 second, and it sits on the brink between a successful and a failed gamma-ray burst.”
Ahumada and his colleagues believe that GRB 200826A and some other supernova-related GRBs are appearing short because the jets of gamma rays that emerge from the collapsing star’s poles aren’t strong enough to completely escape the star and that other collapsing stars have such weak jets that they don’t produce GRBs at all.
The discovery also suggests that some of supernova-caused GRBs are masquerading as short GRBs thought to be created by neutron-star mergers, and are therefore not getting counted as the supernova kind.
“Our discovery suggests that, since we observe many more of these supernovae than long gamma-ray bursts, most collapsing stars fail to produce a GRB jet that breaks through the outer envelope of the collapsing star,” Ahumada said.
“We think this event was effectively a fizzle, one that was close to not happening at all.”
The astronomers were able to determine that GRB 200826A originated from a supernova explosion, designated ZTF20abwysqy (AT2020scz), thanks to the imaging capabilities of the Gemini Multi-Object Spectrograph on the 8-m Frederick C. Gillett Gemini North telescope at the Gemini Observatory.
They captured images of the event’s host galaxy 28, 45, and 80 days after the GRB was first detected on August 26, 2020, by a network of observatories that included NASA’s Fermi Gamma-ray Space Telescope.
The Gemini observations allowed the team to spot the tell-tale rise in energy that signifies a supernova, despite the blast’s location in a galaxy 6.6 billion light-years away.
“This was a complicated endeavor as we needed to separate the light of an already faint galaxy from the light of a supernova,” Ahumada said.
“Gemini is the only ground-based telescope that can do follow-up observations like this with a flexible-enough schedule to let us squeeze in our observations.”
The discovery is described in a paper in the journal Nature Astronomy.
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T. Ahumada et al. Discovery and confirmation of the shortest gamma-ray burst from a collapsar. Nat Astron, published online July 26, 2021; doi: 10.1038/s41550-021-01428-7
