The oscillating frequencies of two short gamma-ray bursts are the best evidence yet for the formation of ‘impossible’ hypermassive neutron star that can briefly defy gravity before collapsing to form a black hole.
Moreover, the neutron star is extremely slow in completing its rotation – one rotation in every 76 seconds. The object also gives off seven different types of radio pulses. Since such slower rotation of the object is unusual to other known neutron stars, it can be expected to help unlock the mystery of fast radio bursts (FRBs).
A neutron star forms when a large star runs out of fuel and explodes, leaving behind a super-dense remnant that can pack the mass of the sun into the space of a city. Usually, a neutron star can only contain a bit more than two times the mass of the sun before it undergoes gravitational collapse to form a black hole. However, when two regular neutron stars in a binary system merge, their combined mass can exceed this limit — but only briefly, and the stage is difficult to spot.
Most of these pulsars rotate at an extreme speed, ranging from the scale milliseconds to a few seconds. The previously known slowest spinning record was taking 23.5 seconds per rotation. Now, this newly discovered neutron star seems to be rotating at a snail’s pace – one cycle in every 75.88 seconds. This pulsar is quite ‘unusual’ by being the lowest radio-emitting neutron star discovered yet.
“We need to start with two light neutron stars in the binary in order to form a hypermassive neutron star, otherwise there would be a direct collapse to a black hole,” Cecilia Chirenti, who led the research, told Space.com. Chirenti is an astrophysicist at the University of Maryland, NASA’s Goddard Space Flight Center in Maryland and the Center for Mathematics, Computation and Cognition at the Federal University of ABC in Brazil.
Astronomers don’t consider its idle speed as a mere curiosity. Rather, they wonder about it as an impossibility. For a long time, it has been thought that neutron stars can produce their radio emissions due to their fast rotations. Their emissions will eventually stop if they slow down their rotation over time.
Simulations predict that these quasi-periodic oscillations would be a natural outcome of the formation of a hypermassive neutron star, which would have a mass anywhere between 2.5 and 4 solar masses. Such a hypermassive neutron star would not collapse straight away because different parts of the neutron star spin at vastly different rates, which prevents collapse.
The hypermassive star was produced by the merger of two smaller neutron stars. Normally such collisions result in neutron stars so massive that they collapse into a black hole almost instantaneously under their own gravity. But the latest observations revealed the monster star hovering in view for more than a day before it faded out of sight.
However, a hypermassive neutron star would not be entirely stable, either. Material on its surface would shift, disturbing the orientation of the star’s magnetic poles, which emit the gamma-ray jets, in a jittery fashion. Previous searches for GRB oscillations had come up empty because they were looking exclusively for periodic oscillations; Chirenti’s team realized that the dynamic properties of a hypermassive neutron star would lead to quasi-periodic oscillations instead. The two candidates they identified, GRB 910711 and GRB 931101B, fit the bill.
The unexpected sightings were made using Nasa’s orbiting Neil Gehrels Swift Observatory, which detected the initial gamma-ray burst coming from a galaxy about 10.6bn light years away. A robotic observatory, the Liverpool Telescope, situated in the Canary Islands, then automatically swivelled to view the aftermath of the merger. These observations revealed telltale signatures of a highly magnetised, rapidly spinning neutron star.
A hypermassive neutron star’s lifetime would be several hundred milliseconds. This sounds like a pretty short time, but consider that hypermassive neutron stars would be the fastest spinning stars in the universe, completing one revolution in 1.5 milliseconds or less. A hypermassive neutron star could spin several hundred times before it collapses.
This suggests that the neutron star itself launched the gamma-ray burst, rather than it occurring after its gravitational collapse. Until now, the exact sequence of events has been hard to figure out.
The new research represents just one way scientists are looking to understand what happens when neutron stars merge. “There are several ways to probe the end states of neutron star mergers which the community has been pursuing,” Wen-fai Fong, an astronomer at Northwestern University who wasn’t involved in the new research, told Space.com. “The potential existence of evidence for a supermassive neutron star in archival data is extremely exciting and complementary to existing efforts today of new short gamma-ray bursts across the electromagnetic spectrum.”