An international team of astronomers has detected a long gamma-ray burst in an ancient galaxy, likely caused by the merger of two separate neutron stars, challenging traditional understanding of the causes of such explosions. The team used multiple telescopes to analyze the 2019 outburst, and although other possible causes were considered, they hope future observations will clarify the origins of this phenomenon.
For the first time, an international team of astronomers has observed a long gamma-ray burst near the center of an ancient galaxy. This is special because these types of gamma-ray bursts usually occur when massive stars collapse or neutron stars orbit each other for a long time, and there are no such stars at the center of ancient galaxies. The team, led by Andrew Levan (Radboud University), published their findings in natural astronomy.
The general consensus was that long gamma-ray bursts of at least a few seconds could only occur when a very heavy star collapsed into a supernova at the end of its life. In 2022, a possible second trigger for long gamma-ray bursts was revealed when two massive stars, which had orbited each other their entire lives, eventually became neutron stars and collided with kilonovas. Now in the year 2023, it looks like long gamma ray bursts could happen in a third way.
“Our data indicate that this is a case of the merger of two separate neutron stars. We suspect that the neutron stars were pushed together by the gravity of many surrounding stars in the center of the galaxy,” says lead researcher Andrew Levan (Radboud University).
The team of researchers studied the effects of a gamma-ray burst detected by the Neil Gehrels Swift Observatory on October 19, 2019. They did this using the Gemini South telescope in Chile, the Northern Optical Telescope on Canary Island in La Palma, and the[{” attribute=””>Hubble Space Telescope.
Their observations show that the burst was caused near the center 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 supernovae, because heavy stars typically occur in young galaxies. In addition, supernovae emit bright optical light, which was not observed in this case.
A second argument is that the center of galaxies are busy places. 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 into a space of a few light-years across. “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 center 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.
For more on this discovery:
Reference: “A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy” by Andrew J. Levan, Daniele B. Malesani, Benjamin P. Gompertz, Anya E. Nugent, Matt Nicholl, Samantha R. Oates, Daniel A. Perley, Jillian Rastinejad, Brian D. Metzger, Steve Schulze, Elizabeth R. Stanway, Anne Inkenhaag, Tayyaba Zafar, J. Feliciano Agüí Fernández, Ashley A. Chrimes, Kornpob Bhirombhakdi, Antonio de Ugarte Postigo, Wen-fai Fong, Andrew S. Fruchter, Giacomo Fragione, Johan P. U. Fynbo, Nicola Gaspari, Kasper E. Heintz, Jens Hjorth, Pall Jakobsson, Peter G. Jonker, Gavin P. Lamb, Ilya Mandel, Soheb Mandhai, Maria E. Ravasio, Jesper Sollerman and Nial R. Tanvir, 22 June 2023, Nature Astronomy.
DOI: 10.1038/s41550-023-01998-8