Black holes don’t always power gamma-ray bursts, new research shows

Gamma-ray bursts (GRBs) have been detected by satellites orbiting the Earth as bright flashes of the most energetic gamma radiation lasting milliseconds to hundreds of seconds. These catastrophic explosions occur in distant galaxies, billions of light-years from Earth.

A subtype of GRB known as short-lived GRB arises when two neutron stars collide. These ultra-dense stars have the mass of our sun reduced to half the size of a city like London, and in the last moments of their lives, just before triggering a GRB, they generate ripples in space- time, known to astronomers as gravitational waves.

Until now, space scientists have largely agreed that the “engine” powering these short-lived, energetic bursts must always come from a newly formed black hole (a region of space- time when gravity is so strong that nothing, not even light, can escape). However, new research by an international team of astrophysicists, led by Dr Nuria Jordana-Mitjans from the University of Bath, is challenging this scientific orthodoxy.

According to the study results, some short-lived GRBs are triggered by the birth of a supermassive star (otherwise known as a neutron star remnant) and not a black hole. The paper is available in The Astrophysical Journal.

Dr Jordana-Mitjans said: “Such findings are important because they confirm that newborn neutron stars can power some short-lived GRBs and the accompanying light emissions across the electromagnetic spectrum. This discovery may offer a new way to locate neutron star mergers, and hence gravitational wave emitters, as we search the sky for signals.”

Competing theories

Much is known about short-lived GRBs. They begin their life when two neutron stars, which are getting closer and closer, constantly accelerating, eventually crash. And from the crash site, a jet explosion releases the gamma radiation that makes a GRB, followed by a longer lasting afterglow. A day later, radioactive material that was expelled in all directions during the explosion produces what researchers call a kilonova.

However, precisely what remains after the collision of two neutron stars – the “product” of the crash – and therefore the source of energy that gives a GRB its extraordinary energy, has long been the subject of debate. Scientists may now be closer to resolving this debate, thanks to the findings of the Bath-led study.

Space scientists are torn between two theories. The first theory has it that neutron stars merged to briefly form an extremely massive neutron star, only for that star to then collapse into a black hole in a split second. The second argues that the two neutron stars would result in a lighter neutron star with a longer life expectancy.

So the question that has plagued astrophysicists for decades is: are short-lived GRBs powered by a black hole or by the birth of a long-lived neutron star?

To date, most astrophysicists have supported the black hole theory, agreeing that to produce a GRB it is necessary for the massive neutron star to collapse almost instantaneously.

Electromagnetic signals

Astrophysicists discover neutron star collisions by measuring the electromagnetic signals of the resulting GRBs. One would expect the signal from a black hole to differ from that from a neutron star remnant.

The GRB electromagnetic signal explored for this study (named GRB 180618A) made it clear to Dr. Jordana-Mitjans and his 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 a day after the binary neutron star’s death. origin.”

Professor Carole Mundell, co-author of the study and professor of extragalactic astronomy at Bath, where she holds the Hiroko Sherwin Chair of Extragalactic Astronomy, said: “We were delighted to capture the first-ever 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 that we couldn’t explain it with the standard collapsing black hole model fast GRBs.

“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 collapse to become black holes.”

Disappearance of afterglow

What first intrigued the researchers was that the optical afterglow light that followed GRB 180618A disappeared after just 35 minutes. Further analysis showed that the material responsible for such a brief emission was expanding at nearly the speed of light due to a continuous energy source pushing it from behind.

What was more surprising was that this emission had the imprint of a newborn, rapidly rotating, highly magnetized neutron star called the millisecond magnetar. The team discovered that the magnetar after GRB 180618A was heating up the material left over from the crash as it slowed down.

In GRB 180618A, the magnetar-powered optical emission was a thousand times brighter than expected from a typical kilonova.