New finding on gamma-ray bursts
Gamma-ray bursts (GRBs) are detected by Earth-orbiting satellites as flashes of the highest energy gamma rays lasting milliseconds to hundreds of seconds. These cataclysmic explosions occur in galaxies billions of light years away from Earth.
A subtype of GRB, known as a short-term GRB, is born when two neutron stars collide. These ultra-dense stars compress the mass of our Sun to half the size of the city of London, and in the final moments of their lives, just before triggering GRBs, known to astronomers as gravitational waves. creates ripples in space-time that are being
Until now, space scientists have believed that the “engine” that drives such energetic, short-lived explosions is always a newly formed black hole (a region of spacetime where gravity is so strong that nothing, not even light, exists). ) has almost agreed to come from run away from it). A new study by an international team of astrophysicists led by Dr. But her Nuria Jordana-Mitjans at the University of Bath questions this academic orthodoxy.
According to the results of this study, some short-term her GRBs are caused by the birth of supermassive stars (also called neutron star remnants) rather than black holes. This paper is available in The Astrophysical Journal.
Dr. Jordana-Mitjans said: “Results like this are important because they confirm that newborn neutron stars can power several short-lived GRBs and the accompanying bright emissions across the electromagnetic spectrum. , which in turn could provide a new way to find gravitational-wave transmitters, scanning the sky for signals.”
Competing Theories
Much is known about short-term GRB. When two neutron stars that have been spiraling closer and accelerating finally collide, they come to life. And from the crash site, a jet blast emits gamma rays that produce GRBs, followed by a long afterglow. The next day, the radioactive material released in all directions during the explosion creates what is known as a kilonova.
But what exactly remains after two neutron stars collide, the ‘products’ of the collision and the resulting energy source that gives the GRB its extraordinary energy, has long been debated. Scientists may be closer to resolving this controversy, thanks to research led by Bath.
Space scientists are divided between his two theories. The first theory is that the neutron stars briefly coalesce into one very massive neutron star, which then instantly collapses into a black hole. The second is that two neutron stars result in a neutron star with a long life expectancy and low mass.
The question that has plagued astrophysicists for decades is whether his short-lived GRBs are powered by black holes or by the birth of long-lived neutron stars.
To this day, most astrophysicists support the black hole theory and agree that a massive neutron star would have to collapse almost immediately to create a GRB.
Electromagnetic signals
Astrophysicists learn about neutron star collisions by measuring the electromagnetic signals of the resultant GRBs. The signal originating from a black hole would be expected to differ from that coming from a neutron star remnant.
The electromagnetic signal from the GRB explored for this study (named GRB 180618A) made it clear to Dr. Jordana-Mitjans and her collaborators that a neutron star remnant rather than a black hole must have given rise to this burst.
Elaborating, Dr. Jordana-Mitjans said, “For the first time, our observations highlight multiple signals from a surviving neutron star that lived for at least one day after the death of the original neutron star binary.”
Professor Carole Mundell, 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—something that is still largely impossible to do without using a robotic telescope. But when we analyzed our exquisite data, we were surprised to find we couldn`t explain it with the standard fast-collapse black hole model of GRBs.
“Our discovery opens new hope for upcoming sky surveys with telescopes such as the Rubin Observatory LSST with which we may find signals from hundreds of thousands of such long-lived neutron stars, before they collapse to become black holes.”
Disappearing afterglow
What initially puzzled the researchers was that the optical light from the afterglow that followed GRB 180618A disappeared after just 35 minutes. Further analysis showed that the material responsible for such a brief emission was expanding close to the speed of light due to some source of continuous energy that was pushing it from behind. What is more surprising is that this emission has the hallmarks of a newborn, rapidly rotating, highly magnetized neutron star known as a millisecond magnet. The team found that the magnet behind GRB 180618A is heating up the material left over from the impact as it slows down.
In GRB 180618A, optical emission uses a magnetic field a thousand times brighter than would be expected from a typical kilonova.
Quotation: https://phys.org/news/2022-11-black-holes-dont-power-gamma-ray.html
