Short gamma-ray bursts (GRBs) are associated with binary neutron star mergers, multimessenger astronomical events observed in gravitational waves, and the multiband electromagnetic spectrum. Depending on the masses of the stars in the binary and on the details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may form after the merger.
Astronomers examining ancient data on explosive explosions known as short gamma-ray bursts (GRBs) have found light patterns that suggest a supermassive neutron star existed briefly before collapsing into a black hole. The collision of two neutron stars likely created this volatile, massive object.
Cecilia Chirenti, a researcher at the University of Maryland, College Park (UMCP) and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said: “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. We found these gamma-ray patterns in two bursts observed by Compton in the early 1990s. ”
The Compton measurements and computer simulations showed that the masses of meganeutron stars were 20% greater than that of the most massive accurately measured neutron star, J0740+6620, which is nearly 2.1 times the mass of the Sun. In addition, compared to regular neutron stars, supermassive neutron stars are nearly twice as large or about twice as long as Manhattan Island.
The meganeutron stars rotate about 78,000 times per minute, almost twice as fast as J1748-2446ad, the fastest pulsar ever observed. This fast spin prevents the items from collapsing just a few tenths of a second before forming a black hole faster than the speed of light.
Cole Miller, a professor of astronomy at UMCP and a co-author of the paper, said: “We know that short GRBs form when orbiting neutron stars collide, and we know that they eventually collapse into a black hole, but the exact sequence of events is not well understood. At some point, the nascent black bursts hole out 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 evolves.”
Computer simulations of these mergers show that gravitational waves show a sudden frequency jump of more than 1000 hertz when the neutron stars merge. These signals are too fast and too weak for existing gravitational wave observatories to detect. But Chirenti and her team reasoned that similar signals could appear in the gamma rays from short GRBs. These signals are known as quasi-periodic oscillations or QPOs for short.
QPOs can be composed of several near frequencies that vary or disappear over time. The QPOs of gamma rays and gravitational waves originate in the maelstrom of swirling matter as the two neutron stars merge.
While no gamma-ray QPOs were found in the Swift and Fermi bursts, two short GRBs recorded by Compton’s Burst And Transient Source Experiment (BATSE) on July 11, 1991, and November 1, 1993, complied.
The advantage of locating these elusive patterns — the signature flicker that indicated the presence of massive neutron stars — belonged to the BATSE instrument’s wider field of view. The team estimates there is less than a 1 in 3 million chance that all of these signals appear by chance alone.
Chryssa Kouveliotou, chair of the physics department at George Washington University in Washington, said: “These results are significant because they pave the way for future measurements of hypermassive neutron stars by gravitational wave observatories.”
- Chirenti, C., Dichiara, S., Lien, A. et al. Kilohertz quasi-periodic oscillations in short gamma-ray bursts. Nature (2023). DOI: 10.1038/s41586-022-05497-0