Black holes don’t always produce gamma-ray bursts
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. These extremely dense stars, which have the mass of our sun squeezed to the size of half a city like London, produce gravitational waves in the closing moments of their lives, shortly before they cause a GRB.
Up until now, most astronomers have agreed that a newly formed black hole must always be the “engine” driving such intense and brief bursts (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 fresh study conducted by an international team of astrophysicists under the direction of Dr. Nuria Jordana-Mitjans at the University of Bath.
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. The Astrophysical Journal has the published the paper.
According to Dr. Jordana-Mitjans, “These results are significant because they demonstrate that some short-duration GRBs and the strong electromagnetic emissions that have been discovered alongside them can be powered by newly formed neutron stars. This finding may provide a new method for pinpointing neutron star mergers, and consequently gravitational wave emitters, when we are looking for signals in the night sky.”
Alternate theories
About short-duration GRBs, a lot is known. When two neutron stars that have been spiralling toward each other and accelerating continuously finally 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.
However, the exact nature of the “product” of a collision between two neutron stars, which is what gives a GRB its incredible energy, has long been a source of controversy. The results of the Bath-led investigation may have brought this argument closer to an end for scientists.
Two theories are being debated by space scientists. According to the first theory, neutron stars temporarily merge to produce an incredibly huge neutron star before it instantly disintegrates into a black hole. The second claims that the merger of the two neutron stars would produce a less dense neutron star with a longer lifespan.
Therefore, the question that has perplexed astrophysics for decades is this: do short-duration GRBs result from the formation of a long-lived neutron star or a black hole?
In order to produce a GRB, the huge neutron star must collapse almost instantaneously, according to the majority of astrophysicists who have up to this point endorsed the black hole theory.
Electromagnetic signals
Astrophysicists study the electromagnetic signals of the resulting GRBs to gain knowledge of neutron star collisions. One would anticipate that the signal coming from a black hole will be different from the signal from a neutron star remnant.
Dr. Jordana-Mitjans and her colleagues concluded that the electromagnetic signature of the GRB investigated for this study, designated GRB 180618A, proved that a neutron star remnant, not a black hole, must have been the source of this burst.
For the first time, according to Dr. Jordana-Mitjans, “our observations emphasise several signals from a surviving neutron star that lasted for at least one day after the destruction of the initial neutron star binary.”
We were thrilled to catch the very early optical light from this brief gamma-ray burst—something that is still largely impossible to do without using a robotic telescope, according to Professor Carole Mundell, study co-author and professor of extragalactic astronomy at Bath, where she holds the Hiroko Sherwin Chair in Extragalactic Astronomy. 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.
Our discovery provides renewed hope for future sky surveys using telescopes like the Rubin Observatory LSST, which may enable us to detect signals from tens of thousands of such old neutron stars before they eventually collapse into black holes.
Disappearing afterglow
The optical light from the afterglow that accompanied GRB 180618A vanished after only 35 minutes, which originally baffled the researchers. Further investigation revealed that some continuous energy source was pushing the substance responsible for such a brief emission, causing it to expand almost as quickly as light.
More surprisingly, this emission bore the signature of a millisecond magnetar, a young, fast rotating neutron star that is highly magnetic. The study discovered that the magnetar that followed GRB 180618A was warming the impact debris as it slowed down.
The optical emission from GRB 180618A, generated by a magnetar, was 1,000 times brighter than predicted by a conventional kilonova.
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More information: N. Jordana-Mitjans et al, A Short Gamma-Ray Burst from a Protomagnetar Remnant, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac972b
