When astronomers detected a powerful gamma-ray burst (GRB) in October 2019, the most likely explanation was that a massive, dying star in a distant galaxy exploded in a supernova. But data from later observations showed that the explosion was triggered by the collision of stars (or their remnants) in a densely populated region near the supermassive black hole of an ancient galaxy, according to new leaf Published in Nature Astronomy. Such a rare event has been postulated, but this is the first observational evidence.
As mentioned earlier, gamma ray bursts are extremely high-energy explosions in distant galaxies that last between milliseconds to several hours. There are two categories of gamma ray bursts. Most (70 percent) are long bursts lasting more than two seconds, often with a bright glow. These are usually associated with galaxies with rapid star formation. Astronomers believe that the long outbursts are related to the death of massive stars collapsing to form a neutron star or black hole (or, alternatively, a newly formed magnetar). The mini black hole will produce jets of high-energy particles moving near the speed of light, powerful enough to pierce the remnants of the progenitor star, and emit X-rays and gamma rays.
GRBs that last less than 2 seconds (about 30 percent) are considered short bursts, usually emanating from regions with very little star formation. Astronomers believe that these gamma-ray bursts are caused by the merger of two neutron stars or the merger of a neutron star with a black hole forming a “kilonova”.
A gamma-ray burst detected by NASA’s Neil Gehrels Swift Observatory in 2019 falls into the long range. But astronomers were baffled because they couldn’t find evidence of a similar supernova. “For every 100 events that fit into the traditional classification scheme for gamma-ray bursts, there is at least one strange ball that throws us into a loop,” said co-author Wen Fei Fung, an astrophysicist at Northwestern University. “However, it is these strange balls that tell us the most about the amazing variety of explosions that the universe is capable of.”
Curiously, Fong and his colleagues followed the fading of the afterglow’s afterglow using the Gemini International Observatory, augmented by data collected by the Scandinavian Optical Telescopes and the Hubble Space Telescope. The afterglow enabled them to locate the GRB in a region just 100 light-years from the core of an ancient galaxy – that is, very close to the supermassive black hole at its centre. They concluded that the explosion was caused by the collision of two stars or stellar remnants.
This is important because there are three known processes for the death of a star, depending on its mass. Massive stars explode in a supernova, while a star with the mass of our sun discards its outer layers and eventually fades into a white dwarf. The stellar remnants formed from supernovae – neutron stars or black holes – can form binary systems and eventually collide.
Now we have a fourth alternative: stars can collide in the densely packed regions of ancient galaxies — an event that’s very rare in active galaxies, which aren’t as dense. An ancient galaxy could contain a million stars in a region only a few light years across. In this case, the gravitational effects of being so close to a supermassive black hole would alter the motions of those stars so that they move in random directions. Eventually a collision is bound to happen.
In fact, the authors point out that these types of collisions may not be that rare; We just don’t detect signals of GRBs and afterglows because of all the dust and gas blocking our view of the ancient galactic centers. If astronomers can capture a gravitational wave signature in conjunction with this type of GRB in the future, it could tell them more about this type of stellar death.
“These new results show that stars can meet their demise in some of the denser regions of the universe where they can be pushed into collisions,” said co-author Andrew Levan, an astronomer at Radboud University in the Netherlands. “This is exciting for understanding how stars die and for answering other questions, such as the unexpected sources that might create the gravitational waves that we can detect on Earth.”
DOI: Nature Astronomy, 2023. 10.1038 / s41550-023-01998-8 (about DOIs).
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