Unlike most gamma-ray bursts, which are caused by exploding massive stars or the chance mergers of neutron stars, a long gamma-ray burst event dubbed GRB 191019A came from the collision of stars or stellar remnants in the jam-packed environment surrounding a supermassive black hole at the core of an ancient galaxy, suggests an analysis of data from the Gemini South telescope, part of the International Gemini Observatory operated by NSF’s NOIRLab.
GRB 191019A came from the collision of stars or stellar remnants in the jam-packed environment surrounding a supermassive black hole at the core of an ancient galaxy. Image credit: International Gemini Observatory / NOIRLab / NSF / AURA / M. Garlick / M. Zamani.
Most stars in the Universe die in predictable ways, depending on their mass.
Relatively low-mass stars like our Sun slough off their outer layers in old age and eventually fade to become white dwarf stars.
More massive stars burn brighter and die sooner in cataclysmic supernova explosions, creating ultradense objects like neutron stars and black holes.
If two such stellar remnants form a binary system, they also can eventually collide.
New research, however, points to a long-hypothesized, but never-before-seen, fourth option.
“Our new results show that stars can meet their demise in some of the densest regions of the Universe where they can be driven to collide,” said Radboud University astronomer Andrew Levan.
“This is exciting for understanding how stars die and for answering other questions, such as what unexpected sources might create gravitational waves that we could detect on Earth.”
Ancient galaxies are long past their star-forming prime and would have few, if any, remaining giant stars, the principal source of long gamma-ray bursts (GRBs).
Their cores, however, are teeming with stars and a menagerie of ultra-dense stellar remnants, such as white dwarf stars, neutron stars, and black holes.
Astronomers have long suspected that in the turbulent beehive of activity surrounding a supermassive black hole, it would only be a matter of time until two stellar objects collide to produce a GRB. Evidence for that type of merger, however, has been elusive.
The first hints that such an event had occurred were seen on October 19, 2019, when NASA’s Neil Gehrels Swift Observatory detected a bright flash of gamma rays that lasted for a little more than one minute — GRB 191019A.
The astronomers then used Gemini South to make long-term observations of the GRB’s fading afterglow to learn more about its origins.
The observations allowed the astronomers to pinpoint the location of GRB 191019A to a region less than 100 light-years from the nucleus of an ancient galaxy, which placed it very near the galaxy’s supermassive black hole.
The researchers also found no evidence of a corresponding supernova, which would leave its imprint on the light studied by Gemini South.
“Our follow-up observation told us that rather than being a massive star collapsing, the burst was most likely caused by the merger of two compact objects,” Dr. Levan said.
“By pinpointing its location to the center of a previously identified ancient galaxy, we had the first tantalizing evidence of a new pathway for stars to meet their demise.”
In normal galactic environments, the production of long GRBs from colliding stellar remnants such as neutron stars and black holes is thought to be vanishingly rare.
The cores of ancient galaxies, however, are anything but normal and there may be a million or more stars crammed into a region just a few light-years across.
Such extreme population density may be great enough that occasional stellar collisions can occur, especially under the titanic gravitational influence of a supermassive black hole, which would perturb the motions of stars and send them careening in random directions.
Eventually, these wayward stars would intersect and merge, triggering a titanic explosion that could be observed from vast cosmic distances.
It is possible that such events occur routinely in similarly crowded regions across the Universe but have gone unnoticed until this point.
A possible reason for their obscurity is that galactic centers are brimming with dust and gas, which could obscure both the initial flash of the GRB and the resulting afterglow.
GRB 191019A may be a rare exception, allowing astronomers to detect the burst and study its after effects.
“Studying gamma-ray bursts like these is a great example of how the field is really advanced by many facilities working together, from the detection of the GRB, to the discoveries of afterglows and distances with telescopes like Gemini, through to detailed dissection of events with observations across the electromagnetic spectrum,” Dr. Levan said.
A paper describing the findings was published today in the journal Nature Astronomy.
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A.J. Levan et al. A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy. Nat Astron, published online June 22, 2023; doi: 10.1038/s41550-023-01998-8
