New stellar object challenges our knowledge of neutron stars

An international team of astronomers, led by the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), have discovered what they believe to be an ultralong-period magnetar — a rare type of neutron star with extremely strong magnetic fields that can produce powerful bursts of energy.

Until recently, all known magnetars released energy at intervals ranging from a few seconds to a few minutes. The newly discovered object emits radio waves every 22 minutes, making it the longest-period magnetar ever detected, according to the team’s study in the journal Nature.

Astronomers discovered the object using the Murchison Widefield Array (MWA), a radio telescope on Wajarri Yamaji Country in outback Western Australia. The magnetar, named GPM J1839−10, is located 15,000 light-years away from Earth in the Scutum constellation.

The stellar object is only the second of its kind ever detected, after the first was discovered by Curtin University undergraduate research student Tyrone O’Doherty. Initially scientists could not explain what they had found, with a paper published in January 2022 describing an enigmatic transient object that would intermittently appear and disappear, emitting powerful beams of energy three times per hour.

“We were stumped,” said lead author Dr Natasha Hurley-Walker, O’Doherty’s honours supervisor. “So we started searching for similar objects to find out if it was an isolated event or just the tip of the iceberg.”

Between July and September 2022, the team scanned the skies using the MWA telescope. They soon found GPM J1839−10, which emitted bursts of energy that last up to five minutes — five times longer than the first object.

Other telescopes followed up to confirm the discovery and learn more about the object’s unique characteristics, including three CSIRO radio telescopes in Australia, the MeerKAT radio telescope in South Africa, the Gran Telescopio Canarias (GTC) 10 m telescope and the XMM-Newton space telescope. The team also began searching the archives of the world’s premier radio telescopes for previous observations of GPM J1839−10, based on the object’s celestial coordinates and characteristics.

“It showed up in observations by the Giant Metrewave Radio Telescope (GMRT) in India, and the Very Large Array (VLA) in the USA had observations dating as far back as 1988,” Hurley-Walker said.

“That was quite an incredible moment for me. I was five years old when our telescopes first recorded pulses from this object, but no one noticed it, and it stayed hidden in the data for 33 years.

“They missed it because they hadn’t expected to find anything like it.”

Not all magnetars produce radio waves; some exist below the ‘death line’, a critical threshold where a star’s magnetic field becomes too weak to generate high-energy emissions. Hurley-Walker said GPM J1839−10 technically lies below the death line, as it spins far too slowly to be able to produce radio waves — “but we’re seeing them”.

“We’re not just talking about a little blip of radio emission,” Hurley-Walker said. “Every 22 minutes, it emits a five-minute pulse of radio wavelength energy, and it’s been doing that for at least 33 years.

“Whatever mechanism is behind this is extraordinary.”

According to the researchers, the discovery has important implications for our understanding of the physics of neutron stars and the behaviour of magnetic fields in extreme environments. It also raises new questions about the formation and evolution of magnetars and could shed light on the origin of mysterious phenomena such as fast radio bursts.

“This remarkable object challenges our understanding of neutron stars and magnetars, which are some of the most exotic and extreme objects in the universe,” Hurley-Walker said.

The research team plans to conduct further observations of the magnetar to learn more about its properties and behaviour. They also hope to discover more of these enigmatic objects in the future, to determine whether they are indeed ultralong-period magnetars or perhaps something even more phenomenal.

Image caption: Artist’s impression of the ultralong-period magnetar, which has emitted a five-minute pulse of radio wavelength energy for at least the last 33 years. Image credit: ICRAR.