In a groundbreaking discovery, an international team of astrophysicists may have stumbled upon a previously unseen phenomenon that could lead to the destruction of stars. While investigating the origins of a powerful gamma-ray burst (GRB), the researchers found evidence suggesting that GRB 191019A resulted from the collision of stars or stellar remnants in the dense environment surrounding a supermassive black hole at the core of an ancient galaxy. This unexpected revelation sheds light on a long-theorized but never-before-observed method of demolishing a star and generating a GRB.
Published in the journal Nature Astronomy, the study was led by Radboud University in the Netherlands, with the participation of astronomers from Northwestern University. The team’s findings challenge the conventional understanding that most GRBs originate from the explosions of massive stars or neutron-star mergers. Instead, this extraordinary GRB appears to have emerged from the chaotic collisions occurring within the crowded vicinity of a supermassive black hole in an ancient galaxy.
Wen-fai Fong, an astrophysicist from Northwestern University and a co-author of the study, emphasized the significance of such exceptional events, stating, “For every hundred events that fit into the traditional classification scheme of gamma-ray bursts, there is at least one oddball that throws us for a loop. However, it is these oddballs that tell us the most about the spectacular diversity of explosions that the universe is capable of.”
Giacomo Fragione, another co-author from Northwestern University, expressed enthusiasm for the discovery, noting, “This remarkable discovery grants us a tantalizing glimpse into the intricate dynamics at work within these cosmic environments, establishing them as factories of events that would otherwise be deemed impossible.” Fragione also highlighted the potential of these findings in unraveling the mystery of other phenomena, such as the generation of gravitational waves detectable on Earth.
Traditionally, stars meet their demise through one of three expected scenarios based on their mass. Low-mass stars, like our sun, shed their outer layers as they age and eventually become white dwarf stars. More massive stars undergo cataclysmic supernovae explosions, resulting in the formation of ultra-dense objects such as neutron stars and black holes. The third scenario involves the collision of two stellar remnants within a binary system.
However, the recent study introduces a fourth possibility. It reveals that stars can be destroyed through collisions in some of the densest regions of the universe, particularly in the proximity of supermassive black holes at the cores of galaxies. This finding has significant implications for understanding stellar deaths and investigating potential sources of gravitational waves detectable on Earth.
Ancient galaxies, although past their star-forming prime, still contain numerous stars and dense remnants, such as white dwarfs, neutron stars, and black holes, concentrated around their cores. Astronomers have long theorized that within the tumultuous environment surrounding a supermassive black hole, the collision of two stellar objects could produce a GRB. However, direct evidence of such mergers has remained elusive.
On October 19, 2019, astronomers detected the first indications of such an event when NASA’s Neil Gehrels Swift Observatory observed a brief burst of gamma rays. Typically, a GRB lasting longer than two seconds is classified as “long” and is associated with the collapse of stars at least ten times the mass of our sun.
Subsequently, the researchers employed the Gemini South telescope in Chile, part of the International Gemini Observatory, to conduct extensive observations of the fading afterglow of the GRB. These observations enabled them to pinpoint the location of GRB 191019A to a region less than 100 light-years from the core of an ancient galaxy, near the supermassive black hole. Interestingly, the researchers found no evidence of a supernova accompanying the GRB, which would typically be expected in a burst resulting from the collapse of a massive star. This absence of a supernova led the researchers to conclude that GRB 191019A is not a typical stellar collapse event.
Jillian Rastinejad, a Ph.D. student in astronomy at Northwestern University and a member of the research group, explained that the location of GRB 191019A within the nucleus of the host galaxy suggests a new, yet unproven theory for the formation of gravitational-wave-emitting sources.
In typical galactic environments, the occurrence of long GRBs from the collision of stellar remnants like neutron stars and black holes is extremely rare. However, the dense cores of ancient galaxies defy the norm, hosting a million or more stars packed into a region just a few light-years across. This high population density increases the likelihood of stellar collisions, especially under the immense gravitational influence of a supermassive black hole. The perturbations caused by the black hole can send stars on erratic paths, eventually leading to their intersection and merger. Such a merger would trigger a colossal explosion observable from great cosmic distances.
Anya Nugent, another Ph.D. student in astronomy at Northwestern University and a crucial contributor to the study, highlighted the exceptional nature of this event. She explained that while long GRBs are not typically found in old and inactive galaxies like the host of GRB 191019A, short GRBs, originating from mergers, have not been observed to have such a close connection to the nuclei of their host galaxies. This discovery opens up new possibilities for understanding the formation of binary systems, a phenomenon that has rarely been observed before.
It is conceivable that similar events occur frequently in densely populated regions throughout the universe but have remained unnoticed until now due to the presence of dust and gas, which can obscure the initial burst of a GRB and its subsequent afterglow. GRB 191019A might be a rare exception that allows astronomers to detect the burst and study its aftermath.
Wen-fai Fong emphasized the need for further exploration of these events to match GRB detections with corresponding gravitational-wave detections. Such a combination would provide deeper insights into the true nature of these phenomena and validate their origins, even in the most challenging and obscured environments. The forthcoming Vera C. Rubin Observatory, set to begin operations in 2025, will play a crucial role in advancing this research.
The published study, titled “A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy,” presents this groundbreaking discovery and its implications for our understanding of stellar deaths and the formation of gravitational-wave-emitting sources.
Source: Northwestern University