Artist’s impression of a gamma-ray burst that may have come from dead stars colliding
International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlic/M. Zamani
A strange flash of gamma rays from space turns our ideas about stellar collisions upside down. This gamma-ray burst (GRB) appears to be from two stars colliding near the center of an ancient galaxy, a very different origin from other similar events.
There are two types of GRBs: short ones, lasting 2 seconds or less, and long ones. Long GRBs are generally believed to occur when a massive star explodes in a supernova, while most short GRBs seem to come from binary neutron stars — incredibly dense stellar corpses — colliding into each other.
The specimen in question, called GRB191019A, was a long GRB, but nevertheless appears to have come from two dead stars, or possibly a star and a black hole, colliding.
Anya Nugent at Northwestern University in Illinois and her colleagues used data from six observatories to dig into the details of the powerful blast, which occurred in 2019 and lasted just over 1 minute. They found that the outburst came close to the center of a galaxy about 3.3 billion light-years away, but saw no trace of the supernova expected to be needed for a long GRB.
Those supernovas are more common in young, active galaxies, but this galaxy is extreme old. Most of its massive stars have already passed the main phase of their lives and have evolved into neutron stars, white dwarfs and black holes. Because GRB191019A came from so close to the center of its galaxy, where those stellar corpses roam in abundance, the researchers found that it’s most likely two of them that collided to create this blast of radiation.
We’ve never seen such concrete evidence of two stars colliding in such an environment, says Nugent. “With binary neutron stars, we think they are born together, die together and eventually merge,” she says. “This is our first observational evidence that these stars were not born together: they were born, they died, and finally they found each other in their deaths.”
But it’s still a mystery how one of these stellar collisions could produce a full minute of radiation instead of the fast flash typical of short GRBs.
“The idea that long GRBs could come from mergers really confuses a lot of astronomers — we still have to figure out how to get even that much emission,” says Nugent. The team hopes that spotting more GRBs like this can help unravel the mystery.
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