The Quest for Hypermassive Neutron Stars: Evidence from Gamma-Ray Bursts

The AO Editors

Utilizing gravitational waves, a new discovery identified a hyper massive neutron star.

Image by Jurik Peter/Shutterstock

The recent detection of two short gamma-ray bursts with oscillating frequencies may provide the most substantial evidence for the formation of "impossible" hypermassive neutron stars. Neutron stars are created when a massive star depletes its fuel and explodes, leaving behind a superdense remnant that can contain the sun's mass in an area no larger than a city. Typically, a neutron star can only hold slightly more than twice the sun's mass before collapsing into a black hole. Nonetheless, when two neutron stars in a binary system merge, their combined mass can briefly surpass this limit, although this stage is challenging to observe.

When two neutron stars collide, they produce a kilonova (a burst of light), a flurry of gravitational waves, and a short gamma-ray burst (GRB), which typically lasts less than two seconds. Suppose computer simulations are accurate and hypermassive neutron stars can briefly form before collapsing into black holes. In that case, the evidence for these gravity-defying bodies could be found in the unexplained oscillations in the frequency of gamma rays from a GRB.

The lead astrophysicist, Cecilia Chirenti, and her team combed through records of more than 700 short GRBs to identify two noteworthy events. The Burst and Transient Source Experiment (BATSE) on NASA's now-retired Compton Gamma-Ray Observatory satellite detected GRB 910711 and GRB 931101B in the early 1990s. Simulations predict that the gamma-ray frequency fluctuations observed in both events are a natural consequence of the formation of a hypermassive neutron star.

A hypermassive neutron star is predicted to have a mass between 2.5 and 4 solar masses and would not immediately collapse due to its various components' vastly different spin rates. However, it would not be completely stable. Material on its surface would shift, disrupting the orientation of the star's magnetic poles, which emits gamma-ray jets in a jittery manner. GRB 910711 and GRB 931101B, the two candidates identified by Chirenti's team, match the dynamic properties of a hypermassive neutron star, leading to quasi-periodic oscillations as opposed to the previously searched for periodic changes.

Even though a hypermassive neutron star won't live very long, with a lifespan of several hundred milliseconds, it would be the universe's fastest-spinning star, completing one revolution in 1.5 milliseconds or less and spinning several hundred times before collapsing. While the presence of only two candidates in a sample of over 700 short GRBs may suggest that hypermassive neutron stars are uncommon, Chirenti believes that other factors associated with the GRB's generation make it challenging to detect the signature of a hypermassive neutron star.

The search for hypermassive neutron stars can be expanded by detecting the gravitational waves they emit during formation. Simulations indicate that gravitational waves should oscillate but at a frequency that is too high for current detectors to detect. However, the modulation of gravitational waves' frequency "should be detectable by the next generation of gravitational-wave detectors in 10 to 15 years," Chirenti said.