Scientists Find Collision of Two Neutron Stars in Old Compton Data

A research team represented by Cecilia Chirenti at the 241st meeting of the American Astronomical Society in Seattle demonstrated the value of preserving data from retired scientific instruments by finding evidence of the collision of two neutron stars. The neutron stars fused into a superheavy neutron star that existed briefly before collapsing into a black hole.

“We looked for these signals in 700 short GRBs detected with NASA’s Neil Gehrels Swift Observatory, Fermi Gamma-ray Space Telescope, and the Compton Gamma Ray Observatory,” Chirenti said during a discussion panel in the below video.

The Compton Gamma Ray Observatory produced the data in the early 1990s. This orbiting observatory was deployed during a space shuttle mission in April 1991. While combing through the data, the research team found gamma ray patterns in two short bursts that indicated the fusing of the two neutron stars.

Neutron stars are typically the “leftover” cores of massive stars that already used up all their fuel and shed their outer layers in a supernova. The remaining “corpse” of the star collapses into a super-dense object that can continue to generate radiation until it runs out of the energy left over from when it could fuse hydrogen into heavier elements.

Exceptionally big neutron stars might collapse into black holes. However, some of them might be just small enough to avoid that fate. The superheavy neutron star that was the result of the fusion of two previous neutron stars simply had enough mass to tip it over the edge.

Superheavy neutron stars typically have twice the mass of ordinary neutron stars packed into an area that is twice the length of Manhattan Island. The Compton data and computer simulations revealed mega neutron stars tipping the scales by 20% more than the most massive, precisely measured neutron star known – dubbed J0740+6620 – which weighs in at nearly 2.1 times the Sun’s mass.

The mega neutron stars can rotate much faster than most pulsars, a form of neutron star that send out focused beams of intense radiation as they rotate. The mega neutron stars detected by the Compton Gamma Ray Observatory can rotate nearly 78,000 times a minute, nearly twice as fast as the pulsar with the fastest known rotation.

Computer simulations show that gravitational waves can jump to 1,000 hertz when two neutron stars collide and fuse. Existing gravitational wave observatories are not yet sensitive enough to detect the gravitational waves. However, scientists anticipate that these observatories will be capable of picking up gravitational waves in the kilohertz range by the 2030s.

Chirenti’s research team found a proxy for the gravitational waves in gamma-ray signals. Two short GRBs recorded by Compton’s Burst And Transient Source Experiment (BATSE) on July 11, 1991, and Nov. 1, 1993 matched the expected gamma-ray signals for merging neutron stars, which became a superheavy neutron star, which then collapsed into the black hole as expected.

In some cases, mega neutron stars can produce short gamma-ray bursts (GRBs) that put out more energy in two seconds than all the stars in the Milky Way Galaxy produce in a year. (Basically, just be glad that these energetic neutron stars aren’t close to Earth, eh?) Their spins delay their final collapse into black holes by a few tenths of a second before gravity has its way with them.

“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 published in the scientific journal Nature. “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.”

The discovery showed the value of occasionally going back to comb through old scientific data – even data produced during the early 1990s with instrumentation like the Compton Gamma Ray Observatory. They might help to confirm existing models about the evolution of neutron stars, even the ones that might merge with other neutron stars and collapse into a black hole.