Something in Earth's cosmic neighborhood is emitting weird signals of a kind we've never seen before.
Just 4,000 light-years away, something is flashing radio waves. For roughly 30 to 60 seconds, every 18.18 minutes, it pulses brightly, one of the most luminous objects in the low-frequency radio sky. It matches the profile of no known astronomical object, and astronomers are gobsmacked. They have named it GLEAM-X J162759.5-523504.3.
"This object was appearing and disappearing over a few hours during our observations," said astrophysicist Natasha Hurley-Walker of the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) in Australia.
"That was completely unexpected. It was kind of spooky for an astronomer because there's nothing known in the sky that does that. And it's really quite close to us – about 4,000 light-years away. It's in our galactic backyard."
At the moment, they believe it is most likely one of two things, both 'dead' stars: a type of ultra-magnetic neutron star called a magnetar, or, with a smaller likelihood, a highly magnetized white dwarf. If it's the former, it would be the first time we've detected a magnetar with a very long pulsation period, known as an ultra-long period magnetar.
Objects that pulse on a regular or not-so-regular basis are actually fairly common in space. Anything that changes unexpectedly and dramatically in brightness is known as a transient, and includes everything from supernovae and black holes rupturing stars, down to stellar flares.
Pulsars fall into a similar basket – these are neutron stars that rotate extremely fast, ejecting bright beams of radio emission from their poles, so that they sweep past Earth, much like a lighthouse. The period of these rotations, and therefore the pulses, is on the scale of seconds down to milliseconds.
However, astronomers have seen nothing like GLEAM-X J162759.5-523504.3. It was spotted in data from the Murchison Widefield Array in Western Australia, a low-frequency radio telescope consisting of thousands of spider-like dipole antennas spread out across the desert.
In data collected by the MWA between January and March of 2018, employing a new technique developed by astronomer Tyrone O'Doherty of Curtin University, the astronomers found 71 pulses from the same spot in the sky.
Analyzing the signal, they tracked down its location and worked out that the object, whatever it might be, is smaller than the Sun, and very radio-bright. They also discovered that the emission is highly polarized, or twisted, suggesting that its source has an extraordinarily strong magnetic field.
This suggests that we could be looking at a magnetar. As already mentioned, these are a type of neutron star, which are already fascinating – the collapsed, dead cores of once-massive stars, up to around 2.3 times the mass of the Sun, packed into an ultradense sphere just 20 kilometers (12.4 miles) across.
To get a magnetar, you need to add to this an absolutely insane magnetic field. These magnetic structures are around 1,000 times more powerful than a typical neutron star's, and a quadrillion times more powerful than Earth's. We don't know how or why they form, but recent evidence suggests that they might evolve from pulsars.
Ultra-long period magnetars could be the evolved form, having slowed down their rotation significantly over time, but were thought impossible to actually detect.
This was a bit of a bummer, really, because magnetars have been suggested as the source of mysterious bright radio signals called fast radio bursts; but many fast radio bursts have been traced to locations incompatible with young magnetars. Ultra-long period magnetars would solve this problem nicely.
This brings us to GLEAM-X J162759.5-523504.3, with its small size, highly polarized signal and shockingly bright emission.
"Nobody expected to directly detect one like this because we didn't expect them to be so bright," Hurley-Walker said. "Somehow it's converting magnetic energy to radio waves much more effectively than anything we've seen before."
It's possible that the object is something else, such as a white dwarf. But the profile so far best fits what we'd expect to see from an ultra-long period magnetar, the researchers said.
It's worth noting that, for the eight years that the MWA has been operational, GLEAM-X J162759.5-523504.3 has only been found to be active for that two-month period in 2018. There are many potential reasons for this, including the possibility that its activity is outside our current detection threshold, or that it experienced an unusual outburst. Both of these reasons could explain why we haven't detected anything like it before.
The researchers are continuing to monitor the region to see if the object kicks back into gear again. They also suggest that it might be beneficial to study it in other radio wavelengths. Meanwhile, they will be continuing to look for other objects like it. We just have so many questions.
"More detections will tell astronomers whether this was a rare one-off event or a vast new population we'd never noticed before," Hurley-Walker said.
The research has been published in Nature.