There are strange animals in the cosmic zoo that astronomers stumble across intriguingly. Not long ago, a team in Australia found an extremely unusual magnetar, one of the Star Zoo’s stranger residents. Called GPM J1839-10, it lies about 15,000 light-years away toward the constellation of Scutum.
Indeed, GPM J1839-10 appeared in observations that began several decades ago, hiding in plain sight. Astronomers described it as an “enigmatic, ephemeral object” that appears and disappears, emitting energy three times an hour. It wasn’t until 2022, when a team from Curtin University observed it with the Murchison Wide-Field Array radio telescope in Wajarri Yamaji Country in outback Western Australia, that they identified it as a possible long-period magnetar. “This remarkable object challenges our understanding of neutron stars and magnetars, which are among the most exotic and extreme objects in the Universe,” said team leader Natasha Hurley-Walker.
It is only the second long-term magnetar ever found. Hurley-Walker’s student Tyrone O’Doherty found the first. His discovery surprised everyone. “We were stumped,” Hurley-Walker said. “So we started looking for similar objects to see if it was a one-off event or just the tip of the iceberg.”
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An artist’s impression of the Murchison Widefield Array radio telescope observing the ultra-long-period magnetar, 15,000 light-years from Earth in the constellation Scutum. Photo credit: ICRAR
Magnetar the Great
Astronomers have been studying magnetars for years. They are extremely magnetic dead stars, releasing energy in bursts lasting seconds to a few minutes. They are likely formed when massive stars die in supernovae and the remainder collapse to form a neutron star. There is also evidence that colliding neutron stars could produce magnetars.
The core of a magnetar is a spinning neutron star only about 20 kilometers across. They probably have solid surfaces. The core usually has a mass of 100 million tons or more. It has an incredibly strong magnetic field (hence the name “Magnetar”). As it spins, the magnetar emits periodic radio bursts and other emissions.
An artist’s view of a highly magnetized neutron star, a magnetar. Credit: Carl Knox/OzGrav
Mapping these outbursts is like listening to a ticking clock, but using radio telescopes to pick up the signals. Most magnetars lose their magnetic field after about 10,000 years, making them short-lived phenomena in a cosmic sense. This new device emits bursts of energy for five minutes every 22 minutes. This makes it the magnetar with the longest period that has been found. It could also be an aging company that is in the process of ceasing to advertise its presence.
Find GPM J1839-10 again and again
As part of their research, the astronomy team looked for evidence of GPM J1839-10 in the observation logs of other radio observatories over the past few decades. That’s when they realized it had been observed since 1988. Only no one knew exactly what it was.
“It showed up in observations by the Giant Meterewave Radio Telescope (GMRT) in India, and the Very Large Array (VLA) in the US had observations going back to 1988,” Hurley-Walker said. “That was an incredible moment for me. I was five years old when our telescopes first recorded pulses from this object, but no one noticed and it remained hidden in the data for 33 years. They missed it because they didn’t expect to find something like this.”
The team performed follow-up observations using radio telescopes in Australia and South Africa and the orbiting XMM-Newton X-ray telescope. It showed up in data from a radio telescope as well as in the infrared range from a telescope in the Canary Islands. However, no X-ray emissions were found, suggesting that the object does not emit at these energies.
The archive search helped the team find out as much as possible about this object. Hurley-Walker described it as “below the line of death,” where a star’s magnetic field is too weak to emit high-energy radio emissions. So what happens to GPM J1839-10 since it emits signals that radio telescopes can detect?
Wait, it gets even weirder
Hurley-Walker explained that GPM J1839-10 is spinning too slowly and shouldn’t be emitting radio waves. This is because the periodic radio emissions from magnetars are a result of rotating dipolar magnetic fields and other mechanisms. Magnetar models assume fast spins, so radio emissions from slow rotators are unexpected.
“Assuming it’s a magnetar, that object shouldn’t be able to generate radio waves,” she said. “But we see them. And we’re not just talking about a small radio output here. It emits a five-minute burst of radio wavelength every 22 minutes and has been doing so for at least 33 years. Whatever mechanism is behind it is extraordinary.”
Does this object challenge the conventional understanding of magnetars? Possibly. There is certainly food for thought for astronomers studying the formation and evolution of magnetars from the shells of stars that died as supernovae. It could also help determine if colliding neutron stars play a role. And it could provide some insight into fast radio bursts that astronomers are observing across the universe.
Of course, discovering more of these long-period magnetars would help astronomers understand whether they are indeed typical magnetars — or just another new find in the cosmic zoo.
An animation describing the detection, the object’s behavior, and what it might look like. Photo credit: ICRAR.
For more informations
Hidden invisibly: astronomers find new type of stellar object
A long-term radio transient that has been active for three decades