 |
(GLEAM-X) J162759.5-523504.3's location in relation to the
plane of the Milky Way galaxy. Being so close to the galactic plane makes
identifying the source of the radiation particularly difficult because the area
is so crowded. Image Credit: Natasha Hurley-Walker (ICRAR/Curtin) and the GLEAM
Team. |
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 said, 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 said. 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 “become 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 said 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.
Reference: Research Paper