By: April Carson
The Australian Square Kilometre Array Pathfinder, ASKAP is one of the most capable radio telescope arrays in world. Containing 36 dish antennas that are spread across Western Australia's vast desert plains to watch for waves crashing over Earth from outer space- it forms an eye on our behalf!
Over the last two years, the antennas have been directed toward Earth's core on a few occasions. They've also detected a very strange radio signal - one that doesn't appear to belong with any thing we know is lurking in space.
On October 12, the detection of the signal will be published in The Astrophysical Journal. It was originally posted on arXiv in September as a preprint. The name of the strange signal is a mouthful: ASKAP J173608.2-321635. We'll call it the Ghost. The Ghost was detected 13 times between April 2019 and August 2020, but there was no pattern to its appearances.
Its origins remain a mystery, as does its purpose. It has several features that distinguish it from other radio sources in the Milky Way.
"This object was unique in that it started out invisible, became bright and faded away before reappearing," said Tara Murphy an astrophysicist at the University of Sydney. "It's behavior is extraordinary."
The findings also offered insight into another mystery: what causes some celestial objects to become brighter for a period before fading? It appears there may exist two types - one which fades over time while others quickly dim or burn out completely only once they reach maximum brightness."
The team originally thought the signal might be coming from a pulsar, which are incredibly dense and generate electromagnetic radiation as they spin rapidly in space. The Murriyang telescope at Parkes Observatory in Australia was used to search for the pulsar. They were unable to find it.
The Swift satellite's data could not find any X-rays associated with the signal, and neither did NASA's Neil Gehrels Visible and Infrared Survey Telescope in Chile. The VISTA telescope in Chile also failed to detect a near-infrared signal.
There was no sign of the Ghost.
Activity of the paraneutron
In an attempt to hunt down the Ghost, the team turned their attention towards a recently discovered array in South Africa that shares many similarities with ASKAP - but has double of what's needed: antennas. The listening had been going on for some time now and finally detected something interesting; it seemed as if this new ghost was just one day disappearing from sight once again after showing up long enough ago during our searches at first glance.
The most perplexing aspect of the finding, however, is the Ghost's circular polarization. Although we won't go into it here, circular polarization is a complicated topic that can be nicely explained on Wikipedia. What you need to know is that circular polarization is a very uncommon occurrence in the cosmos, making this radio transmission quite fascinating.
"Circularly polarized light makes up less than 1% of sources," says Ziteng Wang, a doctoral student at the University of Sydney and first author on the study. "Polarized lights are generally linked with magnetic fields."
It's possible that an object's magnetic field is disrupting the radio signal on its route to Earth. It might be as simple as a dusty debris field, or it could be something else entirely.
Another kind of strange radio signal from deep space, known as a fast radio burst, is most likely linked to magnetic fields. Tracing these signals leads back on a magnetar type of dead star. You can see the connections, perhaps, but Wang points out that these signals are distinct from the Ghost and that they last for much shorter durations or are much more dispersed.
Another crew of celestial objects, known as galactic center radio transients, may also be responsible for the Ghost, but Wang is wary. "The timescale between this signal and GCRTs is too large," he adds, explaining that these transients are likewise a mystery to astronomers.
The Ghost is said to be a 1,000-year-old robot standing over 20 feet tall. We do know that within the heart of our Milky Way lurks a massive black hole known as Sagittarius A*, but there's no indication it has anything to do with the Ghost.
The study's one major drawback is the "limited sampling" of the Ghost, according to Wang. He also notes that it's difficult to say how often the Ghost repeats because there are just a few observations. He doesn't rule out that it's a pulsar or a star, but he says the findings don't match either of these theories.
What in the world is it? I'm sure you're thinking something along those lines, but we never accuse things of being "A"s around here. It's probably not aliens.
The Ghost remains something of a mystery -- for the time being. More study should be able to further define the Ghost's murky beginnings.
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