Evidence for Hypermassive Neutron Stars Discovered in Gamma-ray Bursts

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Evidence for Hypermassive Neutron Stars Discovered in Gamma-ray Bursts

The oscillating 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 remnant that can pack the mass of the sun into the space of a city. Usually, a neutron star can only contain a bit more than two times 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 stage is difficult to spot.

The formation of a hypermassive neutron star requires two light neutron stars to be in a binary system, otherwise, there would be a direct collapse to a black hole. The collision of these paired neutron stars releases an explosion of light called a kilonova, a burst of gravitational waves and a short gamma-ray burst (GRB), which is a blast of gamma-rays that typically lasts less than two seconds. The evidence for these gravity-defying bodies could be found in unexplained oscillations in the frequency of the gamma-rays.

The research team sifted through records of more than 700 short GRBs to find two that stood out as being different. These two GRBs were detected by the Burst and Transient Source Experiment (BATSE) on NASA’s now-retired Compton Gamma-Ray Observatory satellite in the early 1990s. They displayed somewhat (but not precisely) rhythmic flickers in frequency of their gamma-rays.

Simulations predict that these quasi-periodic oscillations would be a natural outcome of the formation of a hypermassive neutron star, which would have a mass anywhere between 2.5 and 4 solar masses. Such a hypermassive neutron star would not collapse straight away because different parts of the neutron star spin at vastly different rates, which prevents collapse. However, a hypermassive neutron star would not be entirely stable, either. Material on its surface would shift, disturbing the orientation of the star’s magnetic poles, which emit the gamma-ray jets, in a jittery fashion.

A hypermassive neutron star still won’t live very long. 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. The hypermassive neutron star will be rotating fast, maybe losing matter and oscillating before collapsing to a black hole with an accretion disk. A hypermassive neutron star’s lifetime would be several hundred milliseconds, although it could spin several hundred times before it collapses.

Although finding just two candidates in a sample of over 700 short GRBs could indicate that hypermassive neutron stars might be rare, the research team doesn’t see it that way. There could be other aspects related to the generation of the GRB that could make it hard to detect, and further research is needed to confirm the existence of these gravity-defying bodies.

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