New research shows that black holes do not always emit gamma-ray bursts

New research shows that black holes do not always emit gamma-ray bursts

Artist’s impression of a gamma ray burst by a neutron star. Credit: Noria Jordana Mitgans

Gamma ray bursts (GRBs) are detected by Earth-orbiting satellites as luminous flashes of the most energetic gamma rays lasting from milliseconds to hundreds of seconds. These cataclysmic eruptions occur in distant galaxies, billions of light-years away from Earth.

A subtype of GRB known as a short-lived GRB begins when two neutron stars collide. These super-dense stars have the mass of our sun compressed to half the size of a city like London, and in the last moments of their lives, just before the GRB is launched, they generate ripples in space-time — astronomers know them as gravitational waves.

So far, space scientists have largely agreed that the “engine” that powers such energetic, short-lived bursts must always come from a newly formed black hole (a region of Spare time where gravity is so strong that nothing, not even light, can escape it). However, new research by an international team of astrophysicists, led by Dr. Nuria Jordana-Metjans at the University of Bath, challenges this scientific dogma.

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

Dr Jordana-Mitjans said: ‘Results like this are important because they confirm that nascent neutron stars can power some of the short-range, bright emissions GRBs via Electromagnetic field that were disclosed accompanying them. This discovery may provide a new way to locate neutron star merger sites, and thus gravitational wave emitters, when we search the sky for signals.”

Competing theories

Much is known about short-range GRBs. They begin life when two neutron stars, spiraling ever more spirally, finally collapse, constantly accelerating. And from broken site, a burst burst releases gamma-ray radiation that makes up the GRB, followed by a longer-lived auroras. After a day, the radioactive material that was expelled in all directions during the explosion produces what the researchers call a kilonova.

However, what exactly remains after the collision of two neutron stars – the “product” of the collapse – thus power source Which gives the GRB its extraordinary energy, has long been the subject of debate. Scientists may now be closer to resolving this debate, thanks to the results of the study led by Bath.

Astronomers are divided between two theories. The first theory says that neutron stars briefly fuse to form an extremely massive neutron star, only for that star to collapse into a black hole in a split second. The second argues that the two neutron stars would result in a less massive neutron star with a higher average age.

So the question that astrophysicists have been asking for decades is this: Are short-lived GRBs powered by a black hole or by the birth of a long-lived neutron star?

So far, most astrophysicists have endorsed the black hole theory, and have agreed that to produce a GRB, it is necessary for the massive neutron star to collapse almost instantly.

Electromagnetic signals

Astrophysicists learn about neutron star collisions by measuring the electromagnetic signals generated by GRBs. The signal from a black hole is expected to differ from that from the remnants of a neutron star.

The electromagnetic signal from the GRB detected for this study (labeled GRB 180618A) by Dr Jordana-Mitjans and her collaborators indicated that the remnants of a neutron star rather than a black hole must have led to this explosion.

“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,” Dr. Jordana Mitjans said in detail.

“We were excited to capture the very early optical light from this short gamma-ray burst – something that is still impossible,” said study co-author Carol Mundell and Professor of Extragalactic Astronomy in Bath, where she holds the Hiroko Sherwin Chair in Extragalactic Astronomy. dispense with using a robotic telescope, but when we analyzed our remarkable data, we were surprised to be unable to explain it using the standard fast-collapse black hole model of GRBs.

“Our discovery opens new hope for upcoming sky surveys using telescopes such as the LSST Rubin Observatory in which we may find signals from hundreds of thousands of these long-lived objects. neutron starsbefore they collapse into black holes. “

fading twilight

What initially puzzled the researchers was that the optical light from the auroras that followed GRB 180618A disappeared after just 35 minutes. Further analysis showed that the material responsible for such a short emission was expanding near the speed of light due to some continuous energy source pushing it from behind.

What was even more surprising was that this emission had the imprint of a neutron star rotating at a high speed and extremely magnetic, dubbed a millisecond magnetar. The team found that the magnetar after GRB 180618A was reheating material leftover from the collision as it slowed.

In GRB 180618A, the magnetically powered optical emission was a thousand times brighter than would be expected from a classical kilonova.

more information:
N. Jordana-Mitjans et al, A short gamma-ray burst from an elementary magnetic remnant, Astrophysical Journal (2022). DOI: 10.3847 / 1538-4357 / ac972b

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University of Bath


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