Scientists detect superheavy neutron star that existed for only a fraction of a second

A mix of computer simulations and gamma-ray burst observations shed new light on merging neutron stars.
Chris Young
An artist's impressions of two neutron stars merging.
An artist's impressions of two neutron stars merging.

NASA 

Astronomers trawled through archival observations of short gamma-ray bursts (GRBs) and detected the rapid evolution of two merging neutron stars into a superheavy neutron star, which then collapsed into a black hole.

This entire process lasted only a fraction of a second, a blog post from NASA explains, and it can teach us a great deal about the transient nature of neutron stars and the evolution of colossal black holes.

Two neutron stars merge to create a black hole

Gamma-ray bursts are the most powerful and energetic explosions witnessed by humans, and they are thought to be the most powerful since the Big Bang itself. A neutron star, meanwhile, forms when a massive star runs out of fuel and collapses in on itself, forming an incredibly dense, compact star. Above a certain mass, neutron stars — which typically pack a mass greater than our sun into the size of Manhattan — collapse into black holes.

The scientists looked for GRB signals in 700 short GRBs detected by NASA's Neil Gehrels Swift Observatory, the Fermi Gamma-ray Space Telescope, and the Compton Gamma Ray Observatory. Compton no longer exists as it was deorbited and burned up in Earth's atmosphere in the year 2000.

The researchers found gamma-ray patterns indicative of two neutron stars colliding and eventually forming a black hole in two bursts observed by Compton in the early 1990s.

"We know that short GRBs form when orbiting neutron stars crash together, and we know they eventually collapse into a black hole, but the precise sequence of events is not well understood," said Cole Miller, a professor of astronomy at UMCP and a co-author of the paper. "At some point, the nascent black hole erupts with a jet of fast-moving particles that emits an intense flash of gamma rays, the highest-energy form of light, and we want to learn more about how that develops."

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According to NASA, mega neutron stars spin almost 78,000 times a minute, meaning they almost double the speed of J1748–2446ad, the fastest pulsar ever observed. This rapid rotation allows them to very briefly support themselves against further collapse. However, they only exist for a few tenths of a second before collapsing into a black hole.

Future gravitational wave detectors will shed new light on neutron stars

Short GRBs often shine for as little as two seconds, but they are so powerful they can be detected more than a billion light-years away. Merging neutron stars, meanwhile, produce gravitational waves that are detectable by ground-based observatories on Earth.

Through computer simulations, the researchers learned they could apply an expected change in the frequency of gravitational waves — as the neutrons star merged — to their GRB observations. This allowed them to detect the merging stars, which would have been too faint for most gravitational wave observatories. The two short GRBs that gave these massive merging stars away occurred on July 11, 1991, and November 1, 1993.

By the 2030s, gravitational wave observatories are expected to be much more sensitive, meaning they will be able to provide new insight into the fleeting existence of neutron stars. Until then, gamma-ray observations paired with computer simulations provide the most in-depth knowledge about cosmic objects.

The findings are detailed in a paper in the journal Nature.

Study abstract:

Short gamma-ray bursts (GRBs) are associated with binary neutron star mergers, which are multimessenger astronomical events that have been observed both in gravitational waves and in the multiband electromagnetic spectrum1. Depending on the masses of the stars in the binary and on details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may be formed after the merger, existing for approximately 10–300 ms before collapsing to a black hole2,3. Numerical relativity simulations across different groups consistently show broad power spectral features in the 1–5-kHz range in the post-merger gravitational-wave signal4,5,6,7,8,9,10,11,12,13,14, which is inaccessible by current gravitational-wave detectors but could be seen by future third-generation ground-based detectors in the next decade15,16,17. This implies the possibility of quasiperiodic modulation of the emitted gamma rays in a subset of events in which a neutron star is formed shortly before the final collapse to a black hole18,19,20,21. Here we present two such signals identified in the short bursts GRB 910711 and GRB 931101B from archival Burst and Transient Source Experiment (BATSE) data, which are compatible with the predictions from numerical relativity.