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| The record-breaking star was a "lucky" find for the ASKAP telescope. (Supplied: ARC Centre of Excellence for Gravitational Wave Discovery) |
By science reporter Jacinta Bowler
In our galaxy, about 13,000 light years away, a dead star called ASKAP J1839-075 is breaking all the rules … extremely slowly.
Dead stars — or neutron stars — normally spin at breakneck speeds, but a team of astronomers has clocked the new-found star taking a leisurely six-and-a-half hours to undertake just one spin, which is thousands of times slower than expected.
"This could really change how we think about neutron star evolution," Yu Wing Joshua Lee, an astronomer from the University of Sydney and the first author on the new study, said.
The discovery has been published in Nature Astronomy.
The researchers believe ASKAP J1839-075, is a "pulsar", a high-energy neutron star that releases short bursts of radio waves.
But conventional wisdom is that when pulsars slow down they stop emitting radio waves, meaning ASKAP J1839-075 should be invisible to radio telescopes.
So what's going on?
What is a neutron star?
Neutron stars are one of the most extreme objects in the Universe.
These small, dense stars are created when the core of a supermassive star collapses and triggers a fiery explosion known as a supernova.
When the star collapses it may go from a million kilometre radius to just 10 kilometres.
This extreme crumpling increases the speed, like a figure skater spinning faster when their arms move close to their body.
Spinning extremely fast is therefore part and parcel of being a neutron star. A full spin usually takes these collapsed stars just milliseconds or seconds to occur.
If our Sun was to go through the process of becoming a neutron star, it's current 27-day rotation could become 1,000 rotations a second.
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| As pulsars rotate, we see flashes of radio waves from Earth, similar to a lighthouse. (Supplied: NASA/Goddard Space Flight Center) |
Using radio telescopes, astronomers can 'see' pulses of radio waves from Earth as the neutron star spins, with the movement regularly described as like a "cosmic lighthouse".
Later in the collapsed star's life, it was thought that when it lost energy and began to slow down, the bursts of radio waves scientists detected from Earth would also stop.
"Once they pass the [speed] threshold, we thought they'd be silenced forever," Mr Lee said.
But in the past few years, astronomers discovered pulsars that seemed to contradict that hypothesis.
"This makes us rethink our previous theories on how these sources form."
What rule-breakers have researchers found?
Early in 2022 researchers found pulsars that rotated on minutes-long time scales rather than seconds, and by June last year, researchers had discovered an object which took almost an hour between pulses. These usually slow objects were called "long-period radio transients".
But ASKAP J1839-075's leisurely 6.45-hour spin was unheard of.
"The previous record was 54 minutes, so it was a huge jump," Mr Lee said.
"The team was really surprised."
According to Gemma Anderson, an astronomer at Curtin University who was not involved in this paper but who was part of the team who found the first long-period radio transient, 6.45 hours stretches "our understanding of physics".
"A normal pulsar couldn't spin this slowly and produce radio light," she said.
"Some kind of extreme particle acceleration … is occurring that is causing it to be so radio bright on these long time scales."
A lucky find
Mr Lee was searching for "peculiar radio transients" by trawling through archival data from a sky survey taken by CSIRO's ASKAP radio telescope in outback Western Australia.
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| Yu Wing Joshua Lee was the first author of the new paper. (Supplied: University of Sydney) |
With little prior knowledge of where these transients pop up, the strategy is to pick a random point in the sky to see if anything interesting shows up, Mr Lee said.
Within the archival data, the team discovered a pulsar-like blip from early January 2024. The signal was already starting to fade when the survey begun, so the team could only study the second half.
"If the observation was scheduled 15 minutes later, then we would have completely missed it," Mr Lee said.
"It is quite lucky that we discovered it."
It took 14 more observations to uncover its repeating pulses, and understand more about what sort of object it could be.
All of the long-period radio transients discovered so far have involved Australian teams and according to Dr Anderson, Australia is particularly well placed to find them because of our current generation of radio telescopes.
"[The Murchison Widefield Array and the ASKAP telescope] are the two discovery machines for these types of objects," she said.
The SKA-Low telescope, which is aiming to be fully operational by the end of this decade, will be even more powerful.
So what's the explanation?
While finding these rule-breakers has only taken a few years, understanding what could be causing these mysteriously slow pulses is proving more challenging.
Previous papers have suggested that some other type of star, like white dwarfs (created when stars less massive than our Sun run out of fuel and collapse) or special pulsars called magnetars could be behind the slow pulses.
However, the long-period radio transients found so far all emit radiation a little differently.
"They all have different properties," Mr Lee said.
"We don't know whether they belong to the same family, or the same type of object with different mechanisms."
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| ASKAP is the radio telescope used to discover the neutron star. (ABC News: Tom Hartley) |
Dr Anderson noted that there may be two distinct classes of object, one group caused by white dwarfs, and one caused by magnetars.
In the case of ASKAP J1839-075, the evidence suggests that it's unlikely to be a white dwarf.
"This [research] nicely explains the different possible scenarios, but finds [in this case the] isolated neutron star or magnetar scenario is the most likely," Dr Anderson said.
The telltale signs were in ASKAP J1839-075's distinct radio emissions, as well as a lack of a stars visible in optical telescopes, which would normally be seen if the star was a white dwarf.
Even if the star is a magnetar, its slow spin is still almost unheard of, as most magnetars rotate once every two to 10 seconds, and more research will need to be done to understand how they work.
This is unlikely to be the last long-period radio transient scientists find, according to Dr Anderson, although the brightest and most obvious ones have probably already been found.
With the easiest finds out of the way, Dr Anderson suggests that researchers may turn to understanding more about how these rule-breaking stars could have formed.
"Perhaps this is opening an even larger discovery space where there are lots of objects producing these [transients]," she said.
"It's just we had never looked at the galaxy in this way with our radio telescopes before."
First published at ABC News, January 16, 2025
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