A strange flash of gamma rays from space is upending our ideas on stellar collisions. This gamma ray burst (GRB) seems to have come from two stars smashing together near the centre of an old galaxy, a vastly different origin from other events like it.
There are two types of GRBs: short ones, which last two seconds or less, and long ones. Long GRBs are generally thought to occur when a massive star explodes in a supernova, whereas most short GRBs seem to come from binary neutron stars – incredibly dense stellar corpses – smashing together.
The one in question, called GRB191019A, was a long GRB, but nevertheless seems 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 powerful blast, which occurred in 2019 and lasted a little over one minute. They found that the burst came from close to the centre of a galaxy about 3.3 billion light years away, but saw no hint of the supernova expected to be required for a long GRB.
Those supernovae tend to be more common in young, active galaxies, but this galaxy is extremely old. Most of its massive stars have already gone through the main phase of their lives and evolved into neutron stars, white dwarfs and black holes. Because GRB191019A came from so close to the centre of its galaxy, where those stellar corpses whiz around in abundance, the researchers found that it’s most likely two of them collided to create this blast of radiation.
We’ve never seen such concrete evidence of two stars colliding in this kind of environment before, says Nugent. “With binary neutron stars, we think they’re born together, they die together, and eventually merge together,” she says. “This is our first observational evidence that these stars weren’t born together: they were born, they died, and eventually in their death they found each other.”
But it’s still puzzling how one of these stellar collisions could produce a full minute of radiation instead of the quick flare typical of short GRBs.
“The idea that long GRBs could come from mergers is really throwing a lot of astronomers for a loop – we still need to figure out how we could even be getting this much emission,” says Nugent. The team hopes that spotting more GRBs like this could help unravel the mystery.