Strange celestial object found emitting bizarre pattern of radio signals
This 'spooky' object turns itself on and off in a way completely unlike anything astronomers have found so far.
Right in our own galactic backyard is an unusual object behaving in such a strange way that astronomers aren't quite sure exactly what they've found.
When we gaze into space, we see planets and stars. We can spot nebulae and galaxies with the right set of binoculars or a backyard telescope. However, these methods only give us a small part of the picture of what's really going on out there in the cosmos. Using special telescopes that pick up radio waves, astronomers can see a whole other part of the universe invisible to our eyes. One type of object that is easily found by mapping the cosmos in radio waves is the so-called transients.
The core of the Milky Way, as seen from Earth. The starburst icon (right of centre) shows the position of a mysterious repeating transient that is invisible at these wavelengths of light. Credit: Dr. Natasha Hurley-Walker (ICRAR/Curtin)
Transients earned their name because to us here on Earth, the radio signals they emit are only seen for a relatively short time, or they go on and off at regular intervals.
When astronomers find a radio transient, they can usually list it under one of two categories — fast or slow.
Pulsars are a good example of a fast transient. These are the rotating remnants of dead stars, which emit beams of intense radiation from their magnetic poles. If the magnetic poles of a pulsar do not match the pole it rotates around, those beams of radiation sweep around space, looking a bit like a cosmic lighthouse. When those beams sweep past Earth, we can detect them as pulses of radio waves. Pulsars rotate incredibly fast. The repeated radio blips from one can be seen once per second or so, or even up to hundreds of times per second. The timing varies from pulsar to pulsar.
The supernova that produced the pulsar would be considered a slow transient. It indicates the end of the lifespan of a massive star as it explodes and leaves its crushed-down core behind in its place. The light and radio emissions from this explosion brighten over days and then fade away over months, likely to never be seen again.
However, a new radio source has been found that defies these two categories.
This object, named GLEAM-X J162759.5-523504.3, was detected in 2018, during the GaLactic and Extragalactic All-sky MWA extended survey (GLEAM-X), conducted using the Murchison Widefield Array (MWA) radio telescope in Western Australia. Scanning the skies, the survey team picked up something surprising. This radio source appears for between 30 to 60 seconds, then goes out and returns just over 18 minutes later! Even more strange, when the object's signal was picked up, it was the brightest radio source in the sky.
This radio view of the Milky Way galaxy shows different frequencies of radio waves in different shades of colour - the lowest frequencies in red, middle frequencies in green, and the highest frequencies in blue. The white starburst shows the location of an unusual transient that currently defies definition. Credit: ICRAR
"This object was appearing and disappearing over a few hours during our observations," Dr. Natasha Hurley-Walker, from the Curtin University node of the International Centre for Radio Astronomy Research, explained in an ICRAR press release. "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."
In an article penned for The Conversation, Hurley-Walker revealed that there's even more to the mystery surrounding this object. In addition to its unusually slow pattern of radio signals, it only showed up in observations for a period of around three months, between January and March of 2018. It wasn't seen before that, and it apparently hasn't been seen since.
IS IT ALIENS?
Finding something that astronomers had never seen before, Hurley-Walker had a very profound reaction.
"At this point I broke out in a cold sweat," she wrote for The Conversation. "There is a worldwide research effort searching for repeating cosmic radio signals transmitted at a single frequency. It's called the Search for Extra-Terrestrial Intelligence. Was this the moment we finally found that the truth is … out there?"
After searching through the data for more sightings of this object, though, she was able to relax. It wasn't aliens.
"I had detected the source across a wide range of frequencies, so the power it would take to generate it could only come from a natural source; not artificial (and not aliens)!" she wrote.
WHAT COULD IT BE?
In their study of GLEAM-X J162759.5-523504.3, published in the journal Nature last week, Hurley-Walker and her team came up with two potential explanations for this mysterious object.
One possibility is an unusual white dwarf. White dwarfs are essentially the naked cores of stars like our Sun. They are what's left behind after those stars balloon into Red Giants, and their outer layers blow off due to the stellar wind, forming an immense expanding shell around the remnant. A spinning white dwarf behaves similarly to a pulsar, beaming out radiation from its poles. However, they rotate at a much slower rate. As a result, the radio signals picked up from rotating white dwarfs tend to repeat on the order of hours or days.
However, it could also be a special type of magnetar that no one has seen yet.
A magnetar is a neutron star that, for some reason, develops a supercharged magnetic field. A typical neutron star has a magnetic field around a trillion times stronger than Earth's magnetic field. A magnetar's magnetic field is another 1,000 times stronger on top of that!
This graphic explores the different types of neutron stars. Credit: NASA/JPL-Caltech
As of 2020, NASA says only 30 magnetars have been found. Six of those are known to behave like pulsars, with radio signals picked up from them every one to ten seconds, depending on the magnetar.
Based on all their evidence, the research team believes that GLEAM-X J162759.5-523504.3 may be an ultra-long period magnetar.
"It's a type of slowly spinning neutron star that has been predicted to exist theoretically," Hurley-Walker said in the ICRAR press release. "But nobody expected to directly detect one like this because we didn't expect them to be so bright."
This screen capture from an ICRAR video explaining the discovery shows a typical pulsar (left) along with the two objects the researchers believe could produce these strange signals. Credit: ICRAR
In their paper, the researchers noted that a pulsar's or magnetar's brightness depends on its spin, or more precisely, on how fast that spin is slowing down. This is called the object's spin-down luminosity, and the strength of the radio emissions from a pulsar or magnetar is supposed to be only a tiny fraction of that energy. The object's magnetic field contributes a small amount of energy to the brightness as well.
If that's the case, though, with GLEAM-X J162759.5-523504.3 spinning over 100 times slower than a typical magnetar, it shouldn't be as bright as it is.
"Somehow it's converting magnetic energy to radio waves much more effectively than anything we've seen before," Hurley-Walker explained.
This artist's impression shows what this object may look like if it is an ultra-long period magnetar, showing its powerful magnetic field lines looping out of the stellar remnant's surface, along with the swirling beams of radiation that it emits as it rotates. Credit: ICRAR
The discovery of this unusual object came about after changing how the Murchison Widefield Array (MWA) was used to scan the universe. Since radio astronomers usually look for very high-speed signals, they focus their radio arrays on searching narrow ranges of frequencies for relatively short amounts of time. However, those types of scans would completely miss something like GLEAM-X J162759.5-523504.3 due to the very low frequency of its radio pulses.
It was a new technique developed by undergraduate student (at the time) Tyrone O'Doherty that allowed the team to find this strange object.
"It's exciting that the source I identified last year has turned out to be such a peculiar object," O'Doherty, co-author of the research study and now a Ph.D. student at Curtin University, said in the press release.
