Tracking powerful gamma-ray bursts with the Gemini South telescope in the Elqui Valley, astronomers believe they are in the presence of monumental stellar collisions

After studying a powerful gamma-ray burst (GRB) with the Gemini South telescope, operated by NSF’s NOIRLab and AURA in the Coquimbo Region of Chile, a team of astronomers investigate whether they are in the presence of in a never-before-seen form of star destruction. Unlike most GRBs, which are caused by the explosion of massive stars or by the merger of neutron stars, the researchers conclude that this particular GRB that they observed from Chile was the result of a literal collision of stars or of Stellar remnants in the compact environment surrounding a supermassive black hole at the core of a very old galaxy.

Most of the stars in the Universe die in predictable ways based on their mass. Thus, relatively low-mass stars, such as our Sun, shed their outer layers as they age to become a white dwarf star. The most massive stars shine much brighter and die much younger in powerful supernova explosions, creating ultra-dense objects like neutron stars and black holes, also called stellar remnants. If two of these stellar remnants form a binary system, then they could eventually collide. However, new research points to a fourth hypothetical option, but one that has never been seen before. A team of astronomers were using the Gemini South telescope in Chile, among other telescopes , to search for the origins of a long-duration gamma-ray burst (GRB). Their research led them to discover evidence of a car wrecking race-like collision of stars or stellar remnants in the chaotic and densely populated region near a supermassive black hole in an ancient galaxy. Gemini South is part of the Gemini International Observatory that operates in Chile NSF’s NOIRLab.

“These new results show that stars can meet their end in some of the densest regions of the Universe where they can collide,” said Andrew Levan, an astronomer at Radboud University in the Netherlands and lead author of the research appearing in the issue. Nature Astronomy magazine. “This discovery is exciting because it allows us to understand how stars die and answers other questions, such as what unexpected sources could create gravitational waves that we could detect on Earth.” The oldest galaxies have long ceased to have intense star formation processes and also would not have many, if any, giant stars left, which are the main sources of long GRBs. However, their cores are teeming with stars and a collection of ultradense stellar remnants, such as white dwarf stars, neutron stars, and black holes. Astronomers have long suspected that in the turbulent hive of activity surrounding a supermassive black hole, it would only be a matter of time before two stellar objects collided to produce an RGB. Despite the above, the evidence for this type of merger has been quite elusive. The first indications of such an event occurred on October 19, 2019 when NASA’s Neil Gehrels Swift Observatory detected a bright gamma-ray flash that lasted just over a minute. Any RGB that lasts longer than 2 seconds is considered “long”. These outbursts usually come from the supernova death of a star at least 10 times the mass of our Sun, but this is not always the case.

Astronomers then used Gemini South to make long-term observations of the GRB’s fading afterglow, with the goal of learning more about its origins. The observations allowed astronomers to pinpoint the location of the GRB in a region less than 100 light-years from the nucleus of an ancient galaxy, very close to the supermassive black hole. At the site, the researchers found no evidence of a supernova, which would have left its mark on the light studied by Gemini South. “Our follow-up observation told us that rather than being a massive star collapsing, the outburst was likely caused by the merger of two compact objects,” Levan said. To this he added that “by pinpointing its location at the center of a previously identified ancient galaxy, we had the first tantalizing evidence of a new path to ‘kill’ a star.” In normal galactic environments the production of long GRBs from colliding stellar remnants, such as neutron stars and black holes, is thought to be extremely rare. However, the nuclei of ancient galaxies are anything but normal, and a million or more stars can be found crammed into a region a few light-years across. Such an extreme population density may be great enough for occasional stellar collisions to occur, especially under the titanic gravitational influence of a supermassive black hole, which would perturb the motions of stars and send them hurtling in random directions. Eventually these stray stars would intersect and merge, causing a titanic explosion that could be seen from great cosmic distances. It is possible that similar events could occur regularly in equally populated regions throughout the Universe, but have hitherto gone unnoticed. One possible reason for this is that the galactic centers are chock full of dust and gas, which could obscure a GRB’s initial flash and afterglow. This particular GRB, identified as GRB 191019A, may be a rare exception, allowing astronomers to detect the outburst and study its subsequent effects.

Levan highlighted that this type of study “is a great example of how this area is advancing thanks to the collaboration of many facilities that work together, from the detection of GRBs, through the discoveries of afterglows and their distances, with telescopes like Gemini, to the detailed dissection of the events with observations across the electromagnetic spectrum.”

source: Levan, A. J., Malesani, D. B., Gompertz, B. P., et al. (2023) “A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy.” Nature Astronomy. DOI: 10.1038/s41550-023-01998-8