Around 4,000 years ago some sort of dense, intensely magnetized object emitted enormous amounts of energy. Every 18 minutes this huge energy beam was pointed towards Earth's current location. Having crossed the vastness of space, in 2018, some of it landed on a radio telescope in the Western Australian outback, surprising astronomers. Despite some similarities to the signals produced by pulsars, the radio wave bursts look different from anything we have seen before and require a fundamentally new explanation, which astronomers don't have yet — but they have ruled out aliens.
In 1967, astronomers were shocked to pick up radio signals that appeared and disappeared every few seconds or milliseconds, initially calling them LGMs for Little Green Men. They have since been explained as rapidly spinning neutron stars, known as pulsars, the debris of supernova explosions.
PhD student Tyrone O'Doherty and Dr Natasha Hurley-Walker of Curtin University, Australia had a somewhat similar experience on finding the blips from the object now known as (GLEAM-X) J162759.5-523504.3, which they have reported in a new paper published in Nature.
The longest pulsar signal repeats on a timescale of 118 seconds, and substantially longer periods are considered impossible. (GLEAM-X) J162759.5-523504.3's cycle is 1,091 seconds of which signals lasted 30-60 seconds. Moreover, its brightness is similar to the brightest pulsar known, in the Crab Nebula.
(GLEAM-X) J162759.5-523504.3's radiation is intensely linearly polarized, indicating the presence of a powerful magnetic field.
“This object was appearing and disappearing over a few hours during our observations,” Hurley-Walker said in a statement. “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 lightyears away. It’s in our galactic backyard.”
Attempts to recapture (GLEAM-X) J162759.5-523504.3's signal have not succeeded. Digging through years of data from the Murchison Widefield Array (MWA), O'Doherty and Hurley-Walker found 71 pulses over two periods when it was “on” over almost three months. Other telescopes have failed to detect it, but that is unsurprising. The MWA's unique combination of sensitivity and wide field mean it has picked up numerous unexpected objects other telescopes would only find if they were focused on the right location.
This information launched the quest to explain something so unexpected. Ruling out aliens was the easy part. Technological signals cover only a narrow part of the spectrum, but (GLEAM-X) J162759.5-523504.3 is broad. To produce a signal over so many frequencies requires emitting truly staggering amounts of energy, which would be wasteful for any civilization so advanced they could even do it.
On the other hand, Hurley-Walker told IFLScience, a slow pulsar is also not possible. “If this were a pulsar it would need a magnetic field a 100 times stronger than anything else in the universe,” she said. “It would also decay very quickly.” Faster pulsars have more power, which is inconsistent with the bright and slow combination (GLEAM-X) J162759.5-523504.3 displays.
The possibility of two objects in an elongated orbit, producing bursts of energy when they approach each other, was also considered. Hurley-Walker told IFLScience she does not rule this out entirely, but wide consultation has failed to produce a model that works.
That leaves the team favoring a magnetar whose spectacularly powerful magnetic field has “[b]ecome twisted and complicated,” Hurley-Walker said. “When it untwists it produces the burst of energy we see before building up again.”
(GLEAM-X) J162759.5-523504.3 lies 2.5 degrees off the galactic plane. Its distance was calculated by dispersion, where longer experience more slowing when passing through interstellar material. Hurley-Walker told IFLScience nothing has been found to match this location, but being so close to the galactic plane, the area is crowded. In a media conference, Hurley-Walker praised O'Doherty for choosing the galactic plane for his search, rather than easier, but less promising, parts of the sky.