An artist’s impression of gamma rays powered by a neutron star. Credit: Nuria Jordana-Mijans
Newborn supermassive stars, not black holes, may be responsible for gamma-ray bursts, according to recent research from the University of Bath in the UK.
Satellites orbiting Earth have detected gamma-ray bursts (GRBs) as luminous flashes of extremely energetic gamma-ray radiation that last from milliseconds to hundreds of seconds. These catastrophic explosions occur in distant galaxies billions of light years from Earth.
A type of GRB called a short-duration GRB occurs when two neutron stars collide. These superdense stars, which have the mass of our Sun compressed into a size smaller than a city, generate ripples in space-time called gravitational waves just before they trigger a GRB in its final moments.
Until now, space scientists have largely agreed that the “engine” driving such energetic and short-lived bursts must always come from a newly formed black hole (a region of space-time where gravity is so strong that nothing, not even light, can maybe run away from it). But new research by an international team of astrophysicists, led by Dr Nuria Jordana-Mityans of the University of Bath in the UK, challenges this scientific orthodoxy.
According to the study’s findings, some short-lived GRBs are triggered by the birth of a supermassive star (otherwise known as a neutron star remnant) rather than a black hole.
Dr Jordana-Mitians said: “Such findings are important because they confirm that newborn neutron stars can power some short-lived GRBs and the bright emissions in the electromagnetic spectrum that have been found to accompany them. This finding could offer a new way to locate neutron star mergers, and thus gravitational wave emitters, when we look for signals in the sky.
Competing theories
Much is known about short-lived GRBs. They begin life when two neutron stars that are getting closer and closer, constantly accelerating, finally crash. And from the crash site, a fired explosion releases the gamma ray that makes a GRB, followed by a longer-lasting afterglow. A day later, the radioactive material that was thrown in all directions during the explosion created what researchers call a kilonova.
However, exactly what is left after two neutron stars collide – the “product” of the crash – and therefore the energy source that gives a GRB its extraordinary energy, has long been a matter of debate. Scientists may now be closer to resolving this debate, thanks to the results of a study led by Bath.
Space scientists are divided between two theories. The first theory states that neutron stars merge to briefly form an extremely massive neutron star, only for that star to collapse into a black hole in a fraction of a second. The second argument claims that the two neutron stars would result in a less massive neutron star with a longer lifespan.
So the question that has haunted astrophysicists for decades is this: Are short-lived GRBs powered by a black hole or by the birth of a long-lived neutron star?
To date, most astrophysicists support the black hole theory, agreeing that to create a GRB, a massive neutron star must collapse almost instantaneously.
Electromagnetic signals
Astrophysicists learn about neutron star collisions by measuring the electromagnetic signals of the resulting GRBs. The signal coming from a black hole is expected to differ from that coming from a neutron star remnant.
The electromagnetic signal from the GRB examined for this study (called GRB 180618A) made it clear to Dr Jordana-Mityans and her colleagues that a neutron star remnant, rather than a black hole, must have produced this burst.
Elaborating, Dr Jordana-Mityans said: “For the first time, our observations highlight multiple signals from a surviving neutron star that lived for at least a day after the death of the original binary neutron star.”
Professor Carol Mandel, co-author of the study 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 almost impossible to do without the use 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-collapsing black hole GRB model.
“Our discovery opens up new hope for upcoming surveys of the sky with telescopes like the Rubin Observatory LSST, with which we can detect signals from hundreds of thousands of such long-lived neutron stars before they collapse and become black holes.”
Vanishing shine
What initially puzzled the researchers was that the optical light from the afterglow that followed GRB 180618A disappeared after only 35 minutes. Further analysis showed that the material responsible for such a brief emission is expanding near the speed of light due to some source of continuous energy pushing it behind.
More surprisingly, this emission had the imprint of a newborn, rapidly rotating and highly magnetized neutron star called a millisecond magnetar. The team found that the magnetar after GRB 180618A reheated the remnant material from the crash as it slowed down.
In GRB 180618A, the magnetar-driven optical emission was a thousand times brighter than expected from a classical kilonova.
Reference: “A brief gamma-ray burst from a protomagnetar remnant” by N. Jordana-Mitjans, CG Mundell, C. Guidorzi, RJ Smith, E. Ramírez-Ruiz, BD Metzger, S. Kobayashi, A. Gomboc, IA Steele. M. Shrestha, M. Marongiu, A. Rossi and B. Rothberg, 10 November 2022, The Astronomical Journal.DOI: 10.3847/1538-4357/ac972b
The study was funded by the Hiroko and Jim Sherwin Postgraduate Scholarship.
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