A burst of intense light from a galaxy that is just over a billion years old is increasing our understanding of the most powerful explosions in the Universe.
The gamma-ray burst appears to be the result of the merger of two neutron stars. This in itself is not surprising; Neutron stars can emit short, intense bursts of high-energy radiation when they are smashed together.
What is puzzling is the timing of the eruption. The burst of gamma radiation lasted 50 seconds – the length previously thought to be associated with supernova explosions.
Astronomer Chris Fryer of Los Alamos National Laboratory explained: “Scientists have long believed that gamma ray bursts fall into two groups.
“But in recent events, we’ve found a kilonova along with a long-term gamma-ray burst, and this has thrown a wrench into this simple picture.”
Gamma radiation is the most powerful light in the Universe, produced by the decay of atomic nuclei. And gamma-ray bursts are massive, releasing as much energy in a few seconds as the Sun would release in 10 billion years. It is only violent events that can cause this violent explosion.
Light from the collision of a neutron star reached Earth in 2017, we saw, for the first time, how this could happen. It described a kilonova explosion – between a classic nova and a supernova in energy – accompanied by a short burst of gamma-rays. Taken together, all the light gave us a blueprint for interpreting a similar short burst of gamma rays.
Researchers have also observed long-lasting gamma-ray bursts from supernovae. When a giant star reaches the end of its life, it becomes unstable and explodes.
When a long-duration gamma-ray burst occurred in December last year (named GRB211211A), astronomers turned their telescopes to look for the light that followed the burst. Surprisingly, they found an object that faded very quickly to become very bright, as well as infrared light.
“There are many objects in our night sky that cool quickly,” says astronomer Wen-fai Fong of Northwestern University.
“We image the source in different filters to get more color information, which helps us determine the source. In this case, the red color was visible, and the green color quickly disappeared. This color change is a typical sign of a kilonova, and kilonovae end. They are only from merging neutron stars.”
An analysis of the incident revealed some interesting facts. For example, tracing the event to its own galaxy, which is 1.1 billion light-years away, revealed a young galaxy that is still in the process of forming stars. This is a stark contrast to the old, dead, star-forming galaxy in which the 2017 collision was discovered. This means that the search for kilonova events may need to be extended to more types of galaxies.
And, as we saw with the 2017 merger, combining neutron stars is what creates the heavy elements, such as gold and platinum. A team of scientists sampled the smoke from GRB211211A and found that the explosion produced about 1,000 times the mass of Earth in heavy matter.
As for why the event was so different from its time, we still don’t know. Everything about this, except for gamma-ray bursts, is consistent with the history of neutron star mergers, which raises, scientists say, some very interesting possibilities.
“This was a spectacular burst of gamma-rays. We do not expect that the merger could last about two seconds. In a way, this one flew for about a full minute. It is possible that this behavior can be explained by a long-lived neutron star, but we cannot say that what we saw it was a neutron star being torn apart by a black hole,” says physicist Benjamin Gompertz of the University of Birmingham in the UK.
“Learning more about these events will help us determine which is the correct answer and the information we obtained from GRB 211211A will be very useful in this interpretation.”
The phenomenon has been analyzed in five papers published in Nature. They can be found here, here, here, here, and here.
