“Highly unusual” neutron star signals detected in Western Australia

Astronomers in Australia have observed a neutron star spinning more slowly  than any other previously detected.

Details of the “highly unusual” neutron star’s discovery is published in Nature Astronomy.

There have been  more than 3,000 radio-emitting neutron stars discovered.  The newly found star’s spin is well outside what astrophysicists predict of neutron star behaviour.

Neutron stars form when large stars die in supernovae. After ejecting all of its outer layers, the dead star’s core collapses – more than the mass of our Sun is compacted into a ball about 20km across.

This makes neutron stars among the densest objects in the universe. Matter becomes so dense that electrons and protons are squished together to form neutrons (hence the star’s name).

Such extreme physics lends itself to tremendously fast neutron star spins.

Not so with the new object, given the roll-off-the-tongue name ASKAP J1935+2148, a neutron star between 14,000 and 17,600 light years from Earth.

ASKAP J1935+2148 was spotted using the Australian Square Kilometre Array Pathfinder radio telescope operated by the national science agency CSIRO.

The neutron star was discovered accidentally when astronomers were looking at the gamma-ray burst GRB 221009A in October 2022.

Astronomer in grey t shirt
CSIRO’s Dr Emil Lenc. Credit: Andrew Zic / CSIRO.

“We were simultaneously monitoring a source of gamma rays and seeking a fast radio burst when I spotted this object slowly flashing in the data. Three very different things in one field-of-view,” says co-lead author Emil Lenc, a researcher at CSIRO. “ASKAP is one of the best telescopes in the world for this sort of research, as it is constantly scanning so much of the sky, allowing us to detect any anomalies.”

The object is also strange because it shows 3 distinct emission states: a strong pulse lasting tens of seconds, a weak pulse lasting hundreds of milliseconds, and a resting mode with no pulses.

“It is highly unusual to discover a neutron star candidate emitting radio pulsations in this way,” says lead author Manisha Caleb from the University of Sydney.

Astronomer leaning against sandstone wall
Dr Manisha Caleb. Credit: Fiona Wolf / University of Sydney.

The pulse could also be the result of a white dwarf with a very strong magnetic field, but no such object has been discovered nearby. A slow-spinning neutron star is the best explanation – but it could also be a neutron-white dwarf binary system.

Either way, further research into the bizarre signals will provide valuable insights into the extreme physics of these unusual cosmic objects.

“It might even prompt us to reconsider our decades-old understanding of neutron stars or white dwarfs; how they emit radio waves and what their populations are like in our Milky Way galaxy,” Caleb adds.

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