Not much can be achieved in a few hundred milliseconds. But for the neutron stars seen in the glow of two gamma-ray bursts, there’s more than enough time to teach us a thing or two about life, death, and the birth of black holes.
Recently, while searching through an archive of high-energy flashes in the night sky, astronomers discovered patterns in the light vibrations left by two different groups of colliding stars, indicating a pause in their journey from a superdense object to an infinite abyss of darkness.
That pause — anywhere from 10 to 300 milliseconds — is technically equivalent to two newly formed, mega-sized neutron stars, which researchers suspect are each spinning fast enough to momentarily halt their inevitable fate as black holes.
“We know that short GRBs form when orbiting neutron stars collide, and we know that they eventually collapse into one black holebut the exact sequence of events is not well understood”, says Cole Miller, astronomer at the University of Maryland, College Park (UMCP) in the USA.
“We found these gamma-ray patterns in two bursts observed by Compton in the early 1990s.”
For almost 30 years the Compton Gamma Ray Observatory orbited the Earth, collecting the glow of X-rays and gamma rays emanating from distant cataclysmic events. This archive of high-energy photons contains a wealth of data on things like colliding neutron stars releasing intense pulses of radiation known as gamma-ray bursts.
Neutron stars are true beasts of the cosmos. They pack twice the mass of our Sun into a volume of space about the size of a small city. That’s not all strange things that matterForcing electrons into protons to turn them into a heavy dust of neutrons, it can create magnetic fields unlike anything else in the universe.
Put into high rotation, these fields can accelerate particles to ridiculously high speeds and form polars Jets that appear to “pulse”. like charged lighthouses.
Neutron stars form as more ordinary stars (about 8 to 30 times the mass of our Sun) that burn up the last of their fuel, leaving a core about 1.1 to 2.3 times the mass of our Sun that’s too cold to withstand the pressure to resist its own gravity.
Add a little more mass – say, by cramming two neutron stars together – and not even the lackluster wobble of its own quantum fields can resist gravity’s urge to crush the living physics from the dead star. From a dense clump of particles we get, well, whatever the untold horror that happens to be at the heart of a black hole.
The basic theory of the process is pretty clear, set general boundaries how heavy a neutron star can be before it collapses. For cold, non-rotating spheres of matter, that upper limit is just under three solar masses, but that also implies complications that could make the journey from the neutron star to the black hole anything but easy.
For example, early last year Physicists announced the observation of a burst of gamma rays called GRB 180618A, which was discovered back in 2018. In the afterglow of the burst, they discovered the signature of a magnetically charged neutron star named a magnetarone with a mass close to that of the two colliding stars.
Barely a day later, that heavyweight neutron star was gone, no doubt succumbing to its extraordinary mass and morphing into something even light cannot escape.
How it managed to resist gravity for so long is a mystery, although its magnetic fields may have played a role.
These two new discoveries could also provide some clues.
The more accurate term for the pattern observed in the gamma-ray bursts recorded by Compton in the early 1990s is a quasi-periodic oscillation. The mix of frequencies rising and falling in the signal can be deciphered to describe the final moments of massive objects as they orbit each other and then collide.
From what the researchers can tell, the collisions each produced an object about 20 percent larger than that current heavyweight record holder neutron star – a pulsar calculated at 2.14 times the mass of our sun. They were also twice the diameter of a typical neutron star.
Interestingly, the objects spun at an extraordinary speed of almost 78,000 times per minute, much faster than that Record pulsar J1748–2446adwhich only manages 707 revolutions per second.
The few spins each neutron star made in its short, split-second lifetime may have been driven by just enough angular momentum to combat their gravitational implosion.
How this might apply to other neutron star mergers, further blurring the boundaries of stellar collapse and black hole formation, is a question for future research.
This study was published in Nature.
