Astronomers have examined data about 700 short gamma-ray bursts (GRBs) detected with NASA’s Neil Gehrels Swift Observatory, Fermi Gamma-ray Space Telescope, and Compton Gamma Ray Observatory. They have found flickering gamma-ray patterns — called quasiperiodic oscillations — in two short GRBs indicating the brief existence of a superheavy neutron star shortly before it collapsed into a black hole.
A neutron star (blue sphere) spins in the center of a colorful disk of gas, some of which follows the magnetic field (blue lines) and flows (blue-white arcs) onto the object’s surface. Image credit: NASA’s Goddard Space Flight Center Conceptual Image Lab.
A neutron star forms when the core of a massive star runs out of fuel and collapses. This produces a shock wave that blows away the rest of the star in a supernova explosion.
Neutron stars typically pack more mass than our Sun into a ball about the size of a city, but above a certain mass, they must collapse into black holes.
Both data from Compton’s Burst And Transient Source Experiment (BATSE) and computer simulations revealed neutron stars tipping the scales by 20% more than the most massive, precisely measured neutron star known — MSP J0740+6620 — which weighs in at nearly 2.1 times the Sun’s mass.
These superheavy neutron stars also have nearly twice the size of a typical neutron star.
They spin nearly 78,000 times a minute — almost twice the speed of PSR J1748-2446ad, the fastest pulsar on record.
This rapid rotation briefly supports the objects against further collapse, allowing them to exist for just a few tenths of a second, after which they proceed to form a black hole faster than the blink of an eye.
“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 University of Maryland’s Professor Cole Miller.
“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.”
Merging neutron stars produce gravitational waves, ripples in space-time that can be detected by a growing number of ground-based observatories.
Computer simulations of these mergers show that gravitational waves exhibit a sudden jump in frequency — exceeding 1,000 Hz — as the neutron stars coalesce. These signals are too fast and faint for existing gravitational wave observatories to detect.
But the study authors reasoned that similar signals — quasiperiodic oscillations — could appear in the gamma-ray emission from short GRBs.
While no gamma-ray quasiperiodic oscillations materialized in the Swift and Fermi bursts, two short GRB events — named GRB 910711 and GRB 931101B — were found in the Compton/BATSE data collected on July 11, 1991, and November 1, 1993.
“These results are very important as they set the stage for future measurements of hypermassive neutron stars by gravitational wave observatories,” said Dr. Chryssa Kouveliotou, an astronomer at George Washington University.
The study was published this week in the journal Nature.
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C. Chirenti et al. Kilohertz quasiperiodic oscillations in short gamma-ray bursts. Nature, published online January 9, 2023; doi: 10.1038/s41586-022-05497-0
