An international team of astrophysicists have traced a gamma-ray burst to the nucleus of an ancient galaxy, revealing a new possible type of star death.
While searching for the origins of a powerful gamma-ray burst, the team may have found a new possible death of a star.
Typically, a gamma-ray burst originates from exploding massive stars or neutron-star mergers.
However, the team found that GRB 191019A came from the collision of stars or stellar remnants in the environment surrounding a supermassive black hole at the core of an ancient galaxy.
The environment, akin to a demolition derby, points to a long-hypothesised but never-before-seen possible star death that can generate a gamma-ray burst.
The study, ‘A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy’, is published in Nature Astronomy.
Gamma-ray bursts can reveal mysteries of the Universe
“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,” said Northwestern astrophysicist and study co-author Wen-fai Fong,
“However, it is these oddballs that tell us the most about the spectacular diversity of explosions that the Universe is capable of.”
“The discovery of these extraordinary phenomena within dense stellar systems, especially those encircling supermassive black holes at the cores of galaxies, is undeniably exciting,” said Northwestern astrophysicist and study co-author Giacomo Fragione.
“This remarkable discovery grants us a tantalising glimpse into the intricate dynamics at work within these cosmic environments, establishing them as factories of events that would otherwise be deemed impossible.”
How do stars die?
There are three possible types of star death.
First, when relatively low-mass stars like the Sun reach old age, they shed their outer layers – fading to become white dwarf stars.
On the other hand, more massive stars burn brighter and explode faster in cataclysmic supernovae explosions. This creates ultra-dense objects like neutron stars and black holes.
The third possible type of star death occurs when two such stellar remnants form a binary system and eventually collide.
However, the new study has found that there is potentially a fourth type of star death.
The fourth type of star death
“Our 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 lead author Andrew Levan, an astronomer with Radboud University.
“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 well past their star-forming prime have few remaining massive stars.
However, their cores teem with stars and a menagerie of ultra-dense stellar remnants, such as white dwarfs, neutron stars, and black holes.
It has long been suspected that in this area of great activity, it would only be a matter of time before two stellar objects collided to produce a gamma-ray burst.
However, evidence suggesting a merger of this type has remained elusive.
Detecting a gamma-ray burst in an ancient galaxy
On 19 October 2019, astronomers glimpsed hints of such a merger when NASA’s Neil Gehrels Swift Observatory detected a bright flash of gamma rays that lasted a little over one minute.
A gamma-ray burst lasting longer than two seconds is considered long. These bursts come from a star’s death that is at least ten times the mass of the Sun.
The team then used the Gemini South telescope in Chile to make long term observations of the gamma-ray burst’s afterglow.
The observations allowed the team to pinpoint the location of the gamma-ray burst to a region less than 100 light-years from the nucleus of an ancient galaxy, near the galaxy’s supermassive black hole.
The researchers also found no evidence of a supernova that would leave its imprint on the light captured by Gemini South.
“The lack of a supernova accompanying the long GRB 191019A tells us that this burst is not a typical massive star death,” said Rastinejad, who performed calculations to ensure a supernova was not hiding within the data.
“The location of GRB 191019A, embedded in the nucleus of the host galaxy, teases a predicted but not yet evidenced theory for how gravitational-wave emitting sources might form.”
The event confounds every expectation about the environments of gamma-ray bursts
In typical galactic environments, the production of long gamma-ray bursts from colliding star remnants is incredibly rare.
The cores of ancient galaxies, however, are anything but typical. And there may be a million of more stars crammed into a region just a few light-years across.
Such extreme population density may be great enough to cause occasional stellar collisions and the death of a star. This is especially true under the gravitational influence of a supermassive black hole that would perturb the motions of stars, sending them in random directions.
The stars would eventually merge, the death triggering a gigantic explosion that could be observed from vast cosmic distances.
“This event confounds almost every expectation we have for the environments of short and long GRBs,” said Nugent, who performed crucial modelling of the host galaxy.
“While long GRBs are never found in galaxies as old and dead as GRB 191019A’s host, short GRBs, with their merger origins, have not been observed to be so connected to their hosts’ nuclei. The discovery of this event in the core of its old, quiescent galaxy opens the door to promising new avenues for the formation of binary systems that have rarely been observed before.”
This type of star death may occur in other parts of the Universe
The researchers believe that it is possible that this type of star death may have occurred in other crowded regions of the Universe but has gone unnoticed until now.
A possible reason for this is because galactic centres are comprising of dust and gas. These could obscure the initial flash of the gamma-ray burst and the resulting afterglow.
GRB 191019A may be a rare exception, allowing astronomers to detect the burst and study its aftereffects.
“While this event is the first of its kind to be discovered, it’s possible there are more out there that are hidden by the large amounts of dust close to their galaxies,” Fong said.
“Indeed, if this long-duration event came from merging compact objects, it contributes to the growing population of GRBs that defies our traditional classifications.”
By working to discover more of this type of star death, the researchers hope to match a GRB detection with a corresponding gravitational-wave detection.
This would reveal more about their true nature, even in the murkiest of environments.
