Artist’s impression of ASKAP J1839-0756, a neutron star emitting radio waves from its magnetic poles
James Josephides
A fallen star about 13,000 light years away is so unusual that the researchers who discovered it say it shouldn’t exist.
It was first detected in January 2024 by the ASKAP radio telescope in Western Australia and is likely to be a type of pulsar never seen before.
When supermassive stars reach the end of their lives and explode in a supernova, the remnants form a super-dense object called a neutron star. Pulsars are rapidly spinning neutron stars that emit radio waves from their magnetic poles as they spin. Most pulsars spin at speeds of more than one revolution per second, and we receive a pulse at the same frequency every time a radio beam is directed at us.
But in recent years, astronomers have begun to find dense objects that emit pulses of radio waves at a much slower rate. This has puzzled scientists, who thought the radio wave flashes should stop when it slows down for more than a minute per rotation.
These slowly spinning objects are known as radio transients. Last year, the team he leads Manisha Caleb At the University of Sydney, Australia, he announced the discovery of a transitory with a period of 54 minutes.
Now, Caleb and his colleagues say that a new object they discovered a year ago, called ASKAP J1839-0756, is rotating at a slow rate of 6.45 hours per rotation.
It is also the first transient to be found with an interpulse: a weaker pulse halfway between the main pulses, coming from the opposite magnetic pole.
At first, the team thought ASKAP J1839-0756 might be a white dwarf, a smaller star like our dead sun. “But we have never seen an isolated white dwarf emitting radio pulses before and our calculation suggests that it is too large to be an isolated white dwarf based on the properties of the pulse,” he says. Joshua LeeUniversity of Sydney team member.
The team then thought it might be a magnetar, a neutron star with an enormous magnetic field, 10 trillion times stronger than the most powerful MRI machines on Earth.
A magnetar with a similar rotation period of 6.67 hours has been found before, but so far it has only emitted X-rays, not radio waves.
Caleb says that if the star is an isolated magnetar, it would be the first to emit radio waves with a slow period.
“This new object is completely rewriting what we’ve known about the radio emission mechanisms of neutron stars for the past 60 years,” says Caleb. “It’s certainly one of the strangest objects of recent times, because we didn’t think these things existed. But now we are finding it. If it is magnetic, it is certainly unique among the population of neutron stars.
He says the idea that pulsars stop emitting radio waves when they spin too slowly needs to be reconsidered.
“In recent years we’ve been seeing objects that cross that death boundary, but they’re still emitting radio (frequency),” Caleb says. “So they’re like zombie stars, where you don’t expect them to be alive, but they’re still alive, and they’re pulsating.”
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