The oscillation frequencies of two short gamma-ray bursts are the best evidence yet for the formation of “impossible” hypermassive neutron stars that can briefly defy gravity before collapsing to form a black hole.
A neutron star forms when a large star runs out of fuel and explodes, leaving behind a super-dense residue that can pack the sun’s mass into the space of a city. Usually a neutron star can only hold a little more than twice the mass of the sun before it undergoes gravitational collapse to form a black hole. However, when two regular neutron stars in a binary system merge, their combined mass can exceed this limit – but only briefly, and the scene is hard to spot.
“We have to start with two light neutron stars in the binary in order to form a hypermassive neutron star, otherwise there would be a direct collapse into a black hole,” Cecilia Chirenti, who led the research, told Space.com . Chirenti is an astrophysicist at the University of Maryland, NASA’s Goddard Space Flight Center in Maryland, and the Center for Mathematics, Computation and Cognition at the Federal University of ABC in Brazil.
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When pairs of neutron stars collide, they release an explosion of light called a kilonova, an explosion of gravitational waves and a short gamma burst (GRB), which is an explosion of gamma rays which usually lasts less than two seconds. And if, as computer simulations predicthypermassive neutron stars may form initially before collapsing into a black hole, evidence of these gravity-defying bodies could be found in unexplained oscillations in the frequency of gamma rays.
Chirenti’s team sifted through recordings of over 700 short GRBs to find two short GRBs that stood out as being different. These two GRBs were both detected by the Burst and Transient Source Experiment (BATSE) on NASA’s now-retired Compton Gamma-Ray Observatory satellite in the early 1990s. Named GRB 910711 and GRB 931101B, both events displayed somewhat (but not precisely) rhythmic flickers in frequency of their gamma rays.
Simulations predict that these quasi-periodic oscillations would be the natural result of the formation of a hypermassive neutron star, which would have a mass between 2.5 and 4 solar masses. Such a hypermassive neutron star would not immediately collapse because different parts of the neutron star spin at very different speeds, which prevents collapse.
However, a hypermassive neutron star would not be entirely stable either. The material on its surface would move, disturbing the orientation of the magnetic poles of the star, which emit the jets of gamma rays, in a jerky fashion. Previous research on GRB oscillations had been fruitless because they looked exclusively for periodic oscillations; Chirenti’s team realized that the dynamic properties of a hypermassive neutron star would instead lead to quasi-periodic oscillations. The two candidates they identified, GRB 910711 and GRB 931101B, do the trick.
And a hypermassive neutron star still won’t live very long. The gravitational waves emitted during the merger rob the hypermassive neutron star of some of its angular (rotational) momentum, reducing its spin enough for gravity to take over. “According to simulations, the hypermassive neutron star will spin rapidly, possibly losing material and oscillating before collapsing into a black hole with an accretion disk,” Chirenti said.
The lifetime of a hypermassive neutron star would be several hundred milliseconds. That seems like a pretty short time, but consider hypermassive neutron stars would be the fastest spinning stars in the world. universe, completing a revolution in 1.5 milliseconds or less. A hypermassive neutron star could spin hundreds of times before collapsing.
Although finding just two candidates in a sample of over 700 short GRBs might indicate that hypermassive neutron stars might be rare, Chirenti doesn’t see it that way.
“There could be other aspects related to the generation of the GRB that could make it difficult to detect the signature of a hypermassive neutron star,” she said.
The new research represents just one of the ways scientists seek to understand what happens when neutron stars merge. “There are several ways to probe the end states of neutron star mergers that the community is pursuing,” Wen-fai Fong, a Northwestern University astronomer who was not involved in the new research, told Space.com. . “The potential existence of evidence for a supermassive neutron star in archival data is extremely exciting and complementary to existing efforts today of new short gamma-ray bursts across the electromagnetic spectrum.”
One way to broaden the search for hypermassive neutron stars is to detect the gravitational waves emitted during their formation. According to the simulations, the gravitational waves should also oscillate, but at a frequency too high for the current harvest of detectors measure. However, the frequency modulation of gravitational waves “should be detectable by the next generation of gravitational wave detectors in 10 to 15 years,” Chirenti said.
The results were published in January in the journal Nature (opens in a new tab); Chirenti also presented the findings at the 241st meeting of the American Astronomical Society, held this week in Seattle and virtually. The full article can be read at arXiv Preprint Server.
Space.com contributing writer Robert Lea provided reporting for this story. Follow Keith Cooper on Twitter @21stCenturySETI. follow us on Twitter @Spacedotcom and on Facebook.