The cosmic zoo has strange beasts that astronomers stumble across in the most fascinating ways. Not long ago a team in Australia found a highly unusual magnetar, one of the weirder denizens of the starry zoo. It’s called GPM J1839-10 and it lies some 15,000 light-years away in the direction of the constellation Scutum.
GPM J1839-10 actually showed up in observations beginning several decades ago, hiding in plain sight. Astronomers described it as an “enigmatic transient object” that would appear and disappear, emitting energy three times per hour. It wasn’t until 2022, when a team from Curtin University observed it with the Murchison Wide-Field Array radio telescope on 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 some of the most exotic and extreme objects in the Universe,” said team lead Natasha Hurley-Walker.
It’s only the second long-period magnetar ever found. Hurley-Walker’s undergraduate student Tyrone O’Doherty found the first one. His discovery took everyone by surprise. “We were stumped,” Hurley-Walker said. “So we started searching for similar objects to find out if it was an isolated event or just the tip of the iceberg.”
Magnetar the Magnificent
Astronomers have studied magnetars for years. They’re extremely magnetic dead stars that release energy in bursts ranging from seconds to a few minutes in length. They probably originate as massive stars die in supernovae and with the leftover remnant collapsing to form a neutron star. There’s also some evidence that colliding neutron stars could create magnetars.
The core of a magnetar is a spinning neutron star only about 20 kilometers across. They are likely to 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 bursts of radio and other emissions.
Charting those outbursts is like listening to a ticking clock but using radio telescopes to capture the signals. Most magnetars lose their magnetic fields after about 10,000 years, which makes them short-lived phenomena in cosmic terms. This new one emits energy bursts for five minutes every 22 minutes. That makes it the longest period magnetar found. It might also be an aging one, about to stop advertising its presence.
Finding GPM J1839-10 Again and Again
As part of their research, the astronomy team looked for evidence of GPM J1839-10 in the observational logs of other radio observatories over the past decades. That’s when they found it had been observed since 1988. It was just that nobody knew exactly what it was.
“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,” said Hurley-Walker. “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.”
The team did follow-up observations using radio telescopes in Australia, South Africa, and from the XMM-Newton X-ray telescope in orbit. It showed up in the radio telescope data, as well as in infrared from a telescope in the Canary Islands. No x-ray emissions were found, however, indicating that the object doesn’t emit at those energies.
The archival search helped the team find out as much as they could about this object. Hurley-Walker described it as “below the death line”, where a star’s magnetic field is too weak to emit high-energy radio emissions. So, what’s happening with GPM J1839-10 since it is emitting signals that radio telescopes can detect?
Wait, It Gets Weirder
Hurley-Walker explained that GPM J1839-10 is spinning too slowly and shouldn’t be sending out radio waves. That’s because the periodic radio emissions from magnetars is a result of rotating dipolar magnetic fields and other mechanisms. Models of magnetars assume fast spins, so radio emissions from slow rotators are unexpected.
“Assuming it’s a magnetar, it shouldn’t be possible for this object to produce radio waves,” she said. “But we’re seeing them. And we’re not just talking about a little blip of radio emission. 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.”
Does this object challenge the conventional understanding of magnetars? Perhaps. It certainly does give astronomers something to think about as they study the formation and evolution of magnetars from the husks of stars that died as supernovae. It might also help determine if colliding neutron stars play a role. And, it could shed some insight into fast radio bursts that astronomers detect throughout the universe.
Of course, finding more of these long-period magnetars would help astronomers understand whether they’re actually typical magnetars—or yet another new find in the cosmic zoo.