The Cosmic Zoo contains strange monsters that astronomers encounter in the most amazing way. Not so long ago, a team in Australia found a very unusual magnetar, one of the strangest residents of the starry zoo. It is called GPM J1839-10 and is located about 15,000 light-years away in the direction of the constellation of Scutum.
GPM J1839-10 has already appeared in observations starting several decades ago, hiding in plain sight. Astronomers have described it as a “transient mysterious object” that appears and disappears, emitting energy three times an hour. It wasn’t until 2022, when a team from Curtin University observed it using the Murchison Wide-Field Array radio telescope in Wajarri Yamaji Country in Outback Western Australia, that they identified it as a 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 leader Natasha Hurley-Walker.
It is the second long-period magnetar ever found. Undergraduate Tyrone O’Doherty found Hurley-Walker’s first student. His discovery surprised everyone. “We were baffled,” said Hurley Walker. “So we started looking for similar objects to see if it was an isolated event or just the tip of the iceberg.”
Astronomers have studied magnetars for years. They are highly magnetic dead stars that release energy in bursts of seconds to a few minutes in length. It likely arose when massive stars died in supernovae and the remaining remnants collapsed to form a neutron star. There is also some evidence that colliding neutron stars can create magnetars.
The core of the magnetar is a rotating neutron star with a diameter of only about 20 kilometers. Likely to have solid surfaces. The mass of the pulp is usually 100 million tons or more. It has an incredibly strong magnetic field (hence the name “magnetar”). As it rotates, the magnetar emits periodic bursts of radio and other emissions.
Charting these explosions is like listening to a ticking clock but using radio telescopes to pick up the signals. Most magnetars lose their magnetic fields after about 10,000 years, which makes them a short-lived phenomenon in cosmological terms. This new one emits five-minute bursts of energy every 22 minutes. This makes it the longest magnetically found period. It may also be an elder, about to stop making his presence known.
Searching for GPM J1839-10 over and over again
As part of their research, the astronomy team looked for evidence of GPM J1839-10 in observational records of other radio observatories over the past decades. That’s when they found it had been observed since 1988. No one knew exactly what it was.
“It showed up in observations made by the Giant Metrewave Radio Telescope (GMRT) in India, and the Very Large Array (VLA) in the US had observations going back to 1988,” said Hurley-Walker. “That was an amazing moment for me. I was five years old when our telescopes first recorded pulses from this object, but it went unnoticed, hidden in the data for 33 years. They missed it because they didn’t expect to find anything like it.”
The team followed up observations using radio telescopes in Australia and South Africa and from the orbiting XMM-Newton X-ray telescope. It appeared in radio telescope data, as well as in infrared radiation from a telescope in the Canary Islands. However, no X-ray emissions were found, indicating that the object does not emit at those energies.
Archival research helped the team discover as much information as possible about this object. Hurley-Walker described it as “below the death line”, where the star’s magnetic field is too weak to emit high-energy radio emissions. So what happens to GPM J1839-10 as 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 should not be emitting radio waves. That’s because the periodic radio emissions from magnetars are the result of rotation of dipole magnetic fields and other mechanisms. The magnet models assume that the magnets rotate quickly, so radio emissions from slow engines are not expected.
“Assuming it is a magnetar,” she said, “it should not be possible for this object to produce radio waves.” “But we see them. And we’re not just talking about a tiny glimpse of a radio emission. Every 22 minutes, it emits a five-minute pulse of wavelength energy, and it’s been doing so for at least 33 years. Whatever the mechanism behind this is, it’s extraordinary.”
Does this object defy traditional understanding of magnets? maybe. It certainly gives astronomers something to think about as they study the formation and evolution of magnetars from shells of stars that died as supernovae. It may also help determine whether neutron star collisions play a role. And it could shed some light on the fast radio bursts that astronomers are detecting throughout the universe.
Of course, finding more of these long-period magnetars will help astronomers understand if they are in fact typical magnetars–or another new discovery in the cosmic zoo.