Gamma-ray bursts (GRBs), which are bright flashes of the most powerful gamma-ray radiation lasting a few milliseconds to several seconds, have been discovered by satellites circling the Earth. These devastating explosions take place in galaxies billions of light years away from Earth.
When two neutron stars collide, a short-duration GRB, a subtype of GRB, is born. The mass of our sun is compressed into these incredibly massive stars, which are around the size of London. In the closing moments of their lives, just before triggering a GRB, these stars produce gravitational waves, which are recognized by scientists.
Up until now, the majority of space scientists had concurred that the “engine” driving such powerful but brief bursts must invariably originate from a newly formed black hole (a region of space-time where gravity is so strong that nothing, not even light, can escape from it). This scientific consensus is being questioned by a fresh study conducted by an international team of astrophysicists under the direction of Dr. Nuria Jordana-Mitjans at the University of Bath in the UK.
The results of the study suggest that some short-duration GRBs are not caused by black holes but rather by the creation of supramassive stars, sometimes known as neutron star remnants.
“Such discoveries are crucial as they demonstrate that young neutron stars can fuel some short-duration GRBs and the intense electromagnetic emissions that have been seen accompanying them,” said Dr. Jordana-Mitjans. This finding might provide a new method for pinpointing neutron star mergers and, consequently, gravitational wave emitters while we’re looking for signals in the night sky.
Competing theories
About short-duration GRBs, a lot is known. When two neutron stars that have been spiraling closer and faster eventually collide, life begins for them. The gamma-ray radiation that creates a GRB is released from the impact site by a brief explosion, which is followed by a longer-lasting afterglow. The radioactive material that was ejected during the explosion creates a kilonova, which is what scientists name it, a day later.
The exact nature of the “product” of the collision—what gives a GRB its incredible energy—after two neutron stars collide, however, has long been a source of controversy. Thanks to the results of the study led by Bath, scientists may now be closer to resolving this controversy.
There are two competing theories among space scientists. According to the first theory, neutron stars merge to create a momentarily incredibly massive neutron star, which then instantly collapses into a black hole. The second claim is that the merger of the two neutron stars would produce a lighter, longer-lived neutron star.
Therefore, the age-old conundrum that has plagued astrophysics for decades is whether the origin of short-duration GRBs lies in the formation of a long-lived neutron star or a black hole.
Most astrophysicists up to this point have favored the black hole theory, concurring that a GRB can only be created if the huge neutron star collapses almost instantly.
Electromagnetic signals
By analyzing the electromagnetic signals of the resulting Gamma-ray bursts, astronomers gain knowledge about neutron star collisions. A black hole’s signal should be distinct from the emission of a neutron star remnant.
Dr. Jordana-Mitjans and her colleagues concluded that a neutron star remnant, not a black hole, must have produced the burst based on the electromagnetic signal from the GRB investigated for this study (designated GRB 180618A).
Dr. Jordana-Mitjans went into further detail, stating: “For the first time, our measurements reveal several signals from a surviving neutron star that existed for at least one day following the destruction of the initial neutron star pair.”
Professor Carole Mundell, a study co-author and professor of extragalactic astronomy at Bath, where she holds the Hiroko Sherwin Chair in extragalactic astronomy, said: “We were excited to catch the very early optical light from this short gamma-ray burst—ssomething that is still largely impossible to do without using a robotic telescope. But when we examined our exquisite data, we were shocked to see that the traditional fast-collapse black hole model of GRBs was unable to account for it.
We may be able to detect signals from hundreds of thousands of these long-lived neutron stars using telescopes like the Rubin Observatory LSST thanks to our discovery before they collapse into black holes, according to the researchers.
Disappearing afterglow
The fact that the optical light from the afterglow that accompanied Gamma-ray bursts 180618A vanished after only 35 minutes confounded experts at first. Further investigation revealed that the substance responsible for the brief emission was expanding nearly as quickly as light due to some continuous energy source pushing it from behind.
The fact that this emission bore the signature of a millisecond magnetar—a young, quickly spinning, strongly magnetized neutron star—was even more unexpected. The team discovered that as the magnetar slowed down following GRB 180618A, it was reheating the debris from the crash.
The magnetar-powered optical emission in Gamma-ray bursts 180618A was 1,000 times brighter than what was predicted from a conventional kilonova.
