Astronomers on Wednesday announced their discovery of a dead star spinning at a bafflingly slow rate of one rotation every 54 minutes, appearing to defy rules that govern such stars.

Manisha Caleb, an Indian astronomer in Australia and her colleagues, have found the slowest-spinning neutron star ever documented since astronomers began in the late 1960s cataloguing these intriguing objects that shoot out radio emissions with every rotation.

A neutron star is the remnant of a massive star that has exploded as a supernova after exhausting its fuel and left behind a city-sized, rapidly spinning core packed with subatomic particles called neutrons. All the 3,000-plus neutron stars observed until now take only seconds to tiny fractions of a second to complete one rotation.

“We were monitoring something else in the sky when we noticed this bright radio emission flash that repeated every 54 minutes,” Caleb, an astrophysicist at the University of Sydney, told The Telegraph. “The emission had signatures of a neutron star but its slow rotation was extraordinary,” Caleb said.

Physics rules that relate to magnetic fields and radio emissions and dictate how neutron stars — also called pulsars — should behave, do not apply to the object. Caleb and her colleagues described their discovery in a paper in the journal Nature Astronomy on Monday.

The object is about 16,000 light-years from Earth. For comparison, the solar system is about 26,000 light years away from the Milky Way galaxy’s centre.

“The exact mechanism leading to the slow rotation remains unclear for now,” said Caleb, who studied physics at the Stella Maris College in Chennai, before completing a Masters in spacecraft technology and satellite communications in the UK and a PhD in astrophysics in Australia.

The researchers were monitoring a distant source of gamma rays in space using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia when a team member, Emil Lenc, spotted the unusually slow repeating flash in the data.

The ASKAP can look at a large part of the sky allowing it to capture signals researchers are not even looking for. “ASKAP is among the best telescopes for this sort of research, it is constantly scanning so much of the sky, allowing us to detect anomalies,” said Lenc, a scientist at the Australia Telescope National Facility.

While a slow-spinning neutron star is for now the best possible explanation of the observation, the researchers said they have not ruled out the possibility that the object could be a binary system — a neutron star and a white dwarf orbiting another star. A white dwarf is the remnant core of a Sun-like star that has used up all its fuel.

In the binary scenario, Caleb said, the period of 54 minutes would represent the orbital period of the two objects — a neutron star or a white dwarf paired with a normal star — as they spiral around each other. The radio emission then would likely emerge from the interaction between the two objects.

She said the discovery might prompt a reconsideration of astronomers’ decades-old understanding of how neutron stars or white dwarfs emit radio waves and their populations in the Milky Way galaxy.

Researchers in Germany, the Netherlands, South Africa, the UK and the US were also involved in the discovery.