Across the vast desert plains of Western Australia, on the lands of the Wajarri Yamatji people, lies one of the most capable radio telescope arrays in the world. Containing 36 dish antennas, the Australian Square Kilometre Array Pathfinder, ASKAP, is an eye to the universe. The three dozen antennas watch for radio waves that crash over the Earth.
Over the last two years the antennas have, on occasion, been pointed toward the heart of the Milky Way, our home galaxy. And, on occasion, they've detected a highly unusual radio signal -- one that does not seem to fit with any object we currently know is lurking in the cosmos.
The detection of the signal appears in the Astrophysical Journal on Oct. 12. It was first published as a preprint on arXiv in September.
The name of the strange signal is a mouthful: ASKAP J173608.2-321635. We're going to call it the Ghost. Between April 2019 and August 2020, the Ghost was spotted 13 times but without any consistent timing.
It exhibits a range of characteristics that make it highly unusual and unlike other radio sources from the depths of the Milky Way.
"This object was unique in that it started out invisible, became bright, faded away and then reappeared," said Tara Murphy, an astrophysicist at the University of Sydney and co-author on the paper, in a press release. "This behaviour was extraordinary."
Originally, the team thought the radio signal might be emanating from a pulsar, a kind of neutron star that is incredibly dense and throws off electromagnetic radiation while spinning rapidly in space. The team went searching for the pulsar using the Murriyang telescope at the Parkes observatory in Australia. They came up empty.
Further searches through data obtained by NASA's Neil Gehrels Swift Observatory found no X-rays associated with the signal and data from the VISTA telescope, in Chile, also showed no near-infrared signal.
The Ghost left little trace.
In an attempt to hunt down the Ghost, the team turned to the Meerkat array in South Africa, which is very similar to ASKAP -- with double the antennas.
Listening in with Meerkat, the signal reappeared. But the Ghost had morphed into something new. No longer was the radio signal lasting for weeks, it now disappeared within a day.
This irregular buzzing is one confusing aspect of the discovery, but perhaps the most unusual feature is the Ghost's circular polarization. Polarization relates to how the radio wave moves through space and time -- we're not going to get into it here, but this entry on Wikipedia is actually quite good at explaining it. What you need to know is circular polarization is a rare phenomenon in the cosmos, making this radio signal rather interesting.
"Much less than 1% of sources are circularly polarized," says Ziteng Wang, a doctoral student at the University of Sydney, Australia and first author on the study, adding "usually polarized sources are associated with magnetic fields."
Potentially, the magnetic field of an object is messing with the radio signal on its way to Earth. That might be something as common as a dusty debris field or it could be something else entirely.
Magnetic fields are likely associated with, known as a fast radio burst. Tracing these signals leads back to a type of dead star known as a magnetar. You can see the similarities, perhaps, but Wang points out that these signals are different from the Ghost and that FRBs last for much shorter time periods or repeat on much clearer timeframes. Nevertheless, magnetic fields appear to be a powerful way to mess with a radio signal.
There is another group of objects, known as galactic center radio transients, that might explain the Ghost, too, but Wang has reservations about this hypothesis. "The timescale of this signal and GCRTs are different," he says, noting that these transients are still a mystery to astronomers, too, and if the Ghost is another GCRT, we're not much closer to working out what that really means.
We do know that, lurking within the heart of our Milky Way is a gigantic black hole known as Sagittarius A*, but there's no indication it has anything to do with the Ghost, either.
One of the limitations of the study is the "sparse sampling" of the Ghost, Wang says. He also notes that it's hard to say exactly how often the Ghost might repeat, because the amount of observations is still quite small. He doesn't rule out that it's a pulsar or star, but says the observations don't fit with either of these objects.
So what is it? I know what you're thinking, but we never jump to the "A" word around here. It's almost certainly not aliens.
It definitely is a mystery -- for now. Further observations should be able to refine the characteristics of the Ghost a little more, bringing its murky origins into clearer view.