First long gamma-ray burst ever observed at centre of ancient galaxy

For the first time, an international team of astronomers has observed a long gamma-ray burst near the centre of an ancient galaxy. This is special because these kinds of gamma-ray bursts typically occur when massive stars collapse or when neutron stars circle each other for a long time, and there are no such stars at the centre of ancient galaxies. The team, led by Andrew Levan (Radboud University), are publishing their findings in Nature Astronomy.

The general consensus used to be that long gamma ray bursts of at least a few seconds can only occur when a very heavy star collapses into a supernova at the end of its life. In 2022, a second potential trigger of long gamma ray bursts was uncovered when two large stars, which had been orbiting each other all their lives, turned into neutron stars at the end and collided into a kilonova. Now in 2023, it seems that long gamma ray bursts can occur in a third way.

"Our data indicate that this is a case of two separate neutron stars merging. So not neutron stars that have orbited each other for a longer period," says lead researcher Andrew Levan (Radboud University). "We suspect that the neutron stars were pushed together by the gravity of the many surrounding stars at the centre of the galaxy."

The team of researchers studied the aftermath of a gamma ray burst observed by the Neil Gehrels Swift Observatory on 19 October 2019. They did so using the Gemini South telescope in Chile, the Nordic Optical Telescope on the Canary Island of La Palma, and space telescope Hubble.

Their observations show that the burst was caused near the centre of an ancient galaxy. This immediately provides two arguments pointing to the merging of two sources.

The first argument is that there are almost no heavy stars in ancient galaxies that could collapse into supernovas, because heavy stars typically occur in young galaxies. In addition, supernovas emit bright light, which was not observed in this case.

A second argument is that the centre of galaxies is a busy place. There are hundreds of thousands of normal stars, white dwarfs, neutron stars, black holes, and dust clouds all orbiting a supermassive black hole. Altogether, this represents over 10 million stars and objects crammed about four light years away. "That is an area comparable to the distance between our sun and the next star," Levan explains. "So the probability of a collision in the centre of a galaxy is much higher than, say, at the outskirts, where we are."

The researchers are still leaving room for alternative explanations. The prolonged gamma ray burst could also result from the collision of compact objects other than neutron stars, for example black holes or white dwarfs. In the future, the researchers hope to be able to observe long gamma-ray bursts at the same time as gravitational waves. This would help them to make more definitive statements about the origin of the radiation.

Literature reference A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy. Door: Andrew J. Levan et al. In: Nature Astronomy. [ preprint (pdf)]