A strange gamma-ray flash from space is upsetting our ideas about stellar collisions. This gamma-ray burst (GRB) appears to come from two stars that collided near the center of an old galaxy, a very different origin from other similar events.
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, while most short GRBs appear to come from binary neutron stars, incredibly dense stellar corpses, colliding with each other.
The one in question, named GRB191019A, was a long GRB, but nevertheless appears to have come from two dead stars, or possibly a star and black hole, colliding.
Anya Nugent at Northwestern University in Illinois and colleagues used data from six observatories to delve into the details of the powerful explosion, which occurred in 2019 and lasted just over a minute. They found that the outburst came from near the center of a galaxy some 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 Most of its massive stars have already passed 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 center of its galaxy, where such stellar corpses buzz in abundance, the researchers found that two of them most likely collided to create this burst of radiation.
We have never seen such concrete evidence of two stars colliding in this type of environment before, Nugent says. “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 in their death they met.”
But it remains puzzling how one of these stellar collisions could produce a full minute of radiation instead of the typical rapid flare of short GRBs.
“The idea that long GRBs could come from mergers is really confusing a lot of astronomers; we have yet to figure out how we could get that much emission,” says Nugent. The team hopes that detecting more GRBs like this could help unravel the mystery.
