An international team of astronomers has detected a long burst of gamma rays in an ancient galaxy, possibly caused by the merger of two separate neutron stars, challenging traditional understanding of the cause of the explosion. The team used several telescopes to analyze the 2019 explosion, and while other possible causes are considered, they hope future observations will clarify the origin of this phenomenon.
For the first time, an international team of astronomers observed a long burst of gamma rays 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 long periods of time, and there were no such stars in the centers of ancient galaxies. The team led by Andrew Levan (Radboud University) published their findings in natural astronomy.
The general consensus is that gamma-ray bursts that are at least a few seconds long can only occur when a very heavy star collapses into a supernova late in its life. In 2022, a second possible trigger for long gamma-ray bursts is revealed when two massive stars, which have been orbiting each other their entire lives, finally become neutron stars and collide with a kilonova. Now in 2023, it looks like long bursts of gamma rays could occur in a third way.
“Our data shows that this is a case of the merging of two separate neutron stars. We suspect the neutron stars are being pushed together by the gravity of the many stars around them at the center of the galaxy,” said lead researcher Andrew Levan (Radboud University).
The research team studied the effect of the gamma-ray bursts detected by the Neil Gehrels Swift Observatory on October 19, 2019. They did this using the Gemini South telescope in Chile, the North Optics Telescope on the Canary Islands in La Palma, and the North Optics Telescope on the Canary Islands in La Palma.[{”attribute=””>HubbleSpaceTelescope[{”attribute=””>HubbleSpaceTelescope
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: Andrew J. Levan, Daniel 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 Fernandez , Ashley A. Chrimes , Kornpob Bhirombhakdi , Antonio de Ugarte Postigo , Wen-fai Fong, Andrew S. Fruchter, Giacomo Fragione, Johan PU 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
2023-07-12 08:29:39
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