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.
AND neutron star It forms when a massive star runs out of fuel and explodes, leaving behind a super-dense remnant that can fill the mass of the sun into the void of a city. Generally, a neutron star can contain only slightly more than twice the mass of the sun before it undergoes a gravitational collapse. black holes. However, when two normal neutron stars in a binary system merge, their combined masses may exceed this limit – but this is only for a short time and the phase is difficult to detect.
“To create a hypermassive neutron star, we need to start with two light neutron stars in the binary system, otherwise it would collapse directly 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 Federal ABC University in Brazil.
Related: 8 ways we know black holes really exist
When paired neutron stars collide, they emit a burst of light called a kilonova. gravity waves and short gamma ray burst (GRB), which is a blast gamma rays this usually takes less than two seconds. And if, as computer simulations forecastingHypermassive neutron stars may initially form before collapsing into a black hole, evidence of which GravityChallenging bodies can be found in unexplained oscillations in the frequency of gamma rays.
Chirenti’s team examined the records of more than 700 short GRBs to find two short GRBs that stood out as distinct. Both of these two GRBs Burst and Transient Welding Test (BATSE) On NASA’s now-retired Compton Gamma-Ray Observatory satellite in the early 1990s. Both events, designated GRB 910711 and GRB 931101B, exhibited somewhat (but not quite) rhythmic vibrations in the frequency of gamma rays.
The simulations predict that these quasi-periodic oscillations will be a natural consequence of the formation of a hypermassive neutron star, which could have anywhere from 2.5 to 4 solar masses. Such a hypermassive neutron star would not collapse immediately because different parts of the neutron star rotate at very different speeds, which prevents collapse.
However, a hypermassive neutron star would not be completely stable either. The material on its surface would shift, nervously disorienting the star’s magnetic poles, which emit gamma-ray jets. Previous searches for GRB oscillations were inconclusive as they only looked 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 have identified, GRB 910711 and GRB 931101B, fit the bill.
And a hypermassive neutron star won’t live very long anyway. Gravitational waves emitted during the merger strip away some of the angular (rotational) momentum of the hypermassive neutron star and reduce its spin enough for gravity to take over. “According to the simulations, the hypermassive neutron star will spin rapidly, maybe lose matter and oscillate before collapsing into a black hole with an accretion disk,” Chirenti said.
The lifetime of a hypermassive neutron star would be a few hundred milliseconds. That seems like a pretty short time, but consider that hypermassive neutron stars would be the fastest spinning stars in the universe. Universecompletes one cycle in 1.5 milliseconds or less. A hypermassive neutron star can spin hundreds of times before collapsing.
Although the finding of only two candidates in a sample of more than 700 short GRBs suggests that hypermassive neutron stars may be rare, Chirenti doesn’t see it that way.
“There may be other aspects of the formation of GRB that could make the signature of a hypermassive neutron star difficult to detect,” he said.
The new research represents just one of the ways scientists are looking to understand what happens when neutron stars merge. “There are several avenues for investigating the latest states of neutron star mergers that the community is tracking,” Wen-fai Fong, an astronomer at Northwestern University who was not involved in the new research, told Space.com. “The potential presence of evidence of a supermassive neutron star in the archive data is extremely exciting and complements current efforts to launch new short gamma-ray bursts across the electromagnetic spectrum.”
One way to expand the search for hypermassive neutron stars is to detect gravitational waves propagating when they form. According to the simulations, gravitational waves should also oscillate, but at a frequency too high for the current crop. detectors measure. However, Chirenti said the frequency modulation of gravitational waves “should be detectable by the next generation of gravitational wave detectors in 10 to 15 years.”
The results were published on 1 January. in the magazine Nature (opens in new tab); Chirenti also presented the results at the 241st meeting of the American Astronomical Society, held this week in Seattle and virtually. The full article can be read here: arXiv preprint server.
Space.com contributor author Robert Lea provided the story for this story. Follow Keith Cooper on Twitter @21stCenturySETI. Follow us from twitter @Spacedotcom and he Facebook.
