Gamma-ray bursts (GRBs), which are bright flashes of the most energetic gamma radiation lasting from milliseconds to several seconds, have been discovered by satellites orbiting the Earth. These devastating explosions occur in galaxies billions of light-years from Earth.
When two neutron stars collide, a short-lived GRB, a subtype of GRB, is born. Mass from the Sun is compressed into these incredibly dense stars, which are half the size of London, and they produce gravitational waves in the last moments of their lives, just before triggering a GRB.
Until now, the majority of space scientists agreed that the “engine” driving such powerful but brief bursts must always come from a newly formed black hole, a region of spacetime where gravity is so intense that not even light can escape. this.
This scientific consensus is being challenged by new research by an international team of astrophysicists led by Dr. Nuria Jordana-Mitjans at University of Bath.
The study results suggest that some short-lived GRBs are not caused by black holes but rather by the birth of supermassive stars, also known as neutron star remnants.
Such findings are important because they confirm that nascent neutron stars can power some short-lived GRBs and the light emissions across the electromagnetic spectrum that have been detected accompanying them. This discovery may offer a new way to locate neutron star mergers, and therefore gravitational wave emitters, when we search for signals in the sky.
Dr Nuria Jordana-Mitjans, Research Associate, Department of Physics, University of Bath
Short-lived GRBs are well understood. When two neutron stars that have accelerated and spiraled closer together finally collide, they begin to function. A jet explosion at the crash site also emits the gamma radiation that creates a GRB, which is then followed by a more persistent afterglow.
Radioactive material that was ejected in all directions during the explosion created what is called a kilonova a day later.
However, there has long been controversy over what exactly remains when two neutron stars collide. This is called the “product” of the crash, and it is this product that provides a GRB with extraordinary energy. The results of the Bath-led study may well have brought this debate closer to the end for scientists.
Two theories are debated by space scientists. According to the first theory, neutron stars briefly merge to form an incredibly massive neutron star before it instantly decays into a black hole. The second claims that merging the two neutron stars would produce a less dense neutron star with a longer lifespan.
The age-old puzzle that has plagued astrophysics for decades is whether the origin of short-lived GRBs lies in the birth of a long-lived neutron star or a black hole.
Most astrophysicists so far have favored the black hole theory, agreeing that a GRB can only be created if the massive neutron star collapses almost instantly.
Astrophysicists study the electromagnetic signals from the resulting GRBs to gain insight into neutron star collisions. One would expect the signal from a black hole to be different from the signal from a neutron star remnant.
Dr. Jordana-Mitjans and colleagues concluded that a neutron star remnant, rather than a black hole, must have generated GRB 180618A based on the electromagnetic signal from the burst.
“For the first time, our observations highlight multiple signals from a surviving neutron star that lived at least a day after the death of the original binary neutron star.said Dr. Jordana-Mitjans.
We were thrilled to capture the first-ever optical light from this short burst of gamma rays – something that is still largely impossible to do without the aid of a robotic telescope. But when we analyzed our exquisite data, we were surprised to find that we couldn’t explain it with the standard fast-collapse black hole model of GRBs.
Carole Mundell, Study Co-Author and Professor, Extragalactic Astronomy, University of Bath
Mundell added: “Our discovery opens new hope for future studies of the sky with telescopes such as the Rubin LSST Observatory with which we could find signals from hundreds of thousands of long-lived neutron stars, before they scatter. collapse to become black holes.”
Disappearance of afterglow
The optical light from the afterglow that followed GRB 180618A disappeared after just 35 minutes, which initially baffled the researchers. Further investigation revealed that a continuous energy source was pushing the material responsible for such a brief emission, causing it to expand almost as quickly as light.
More surprisingly, this emission bore the signature of a millisecond magnetar, a young, rapidly rotating and strongly magnetized neutron star. The team discovered that the magnetar following GRB 180618A was heating up the debris from the crash as it slowed down.
The optical emission from GRB 180618A, powered by a magnetar, was 1,000 times brighter than that predicted by a conventional kilonova.
A Hiroko and Jim Sherwin Graduate Scholarship funds Nuria Jordana-Mitjans.
Jordana-Mitjans, N., et al. (2022) A short burst of gamma rays from a protomagnetar remnant. The Journal of Astrophysics. doi:10.3847/1538-4357/ac972b