Across the vast desert plains of Western Australia, on the lands of the Wajarri Yamatji people, is one of the world’s most capable radio telescope arrays. With 36 satellite dishes, the Australian Square Kilometer Array Pathfinder, ASKAP, is an eye for the universe. The three dozen antennas are listening for radio waves hitting the Earth.
For the past two years, the antennas have on occasion pointed toward the heart of the Milky Way, our home galaxy. And, on occasion, they have detected a very unusual radio signal, one that doesn’t seem to match any object that we currently know of lurking in the cosmos.
the Detection of the signal appears in the Astrophysical Journal. on oct. 12. It was first published as a preprint in arXiv in September.
The name of the strange sign is a mouthful: ASKAP J173608.2-321635. We’ll call it the Ghost. Between April 2019 and August 2020, the Ghost was seen 13 times, but without constant timing.
It has a number of characteristics that make it very unusual and different from other radio sources deep in the Milky Way.
“This object was unique in that it started out invisible, turned bright, faded, and then reappeared,” said Tara Murphy, an astrophysicist at the University of Sydney and a co-author on the paper, in a press release. “This behavior was extraordinary.”
Originally, the team thought that the radio signal could come from a pulsar, a kind of neutron star that is incredibly dense and emits electromagnetic radiation as it spins rapidly in space. The team went to search for the pulsar using the Murriyang telescope at the Parkes Observatory in Australia. They arrived empty.
Further searches through data obtained by NASA’s Swift Neil Gehrels Observatory found no X-rays associated with the signal, and data from the VISTA telescope in Chile did not show any near-infrared signals either.
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 twice as many antennas.
Hearing with Meerkat, the signal reappeared. But the Ghost had been transformed into something new. The radio signal no longer lasted weeks, now it disappeared in a day.
This uneven hum is a confusing aspect of the discovery, but perhaps the most unusual feature is the Phantom’s circular polarization. Polarization is related to how the radio wave moves through space and time; we won’t go into that here, but this Wikipedia entry is pretty good explaining it. What you need to know is that circular polarization is a rare phenomenon in the cosmos, which makes this radio signal quite 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 of the study, adding that “generally polarized sources are associated with magnetic fields.”
Potentially, an object’s magnetic field is interfering with the radio signal on its way to Earth. That could be something as common as a dusty debris field or it could be something else entirely.
Magnetic fields are likely associated with, known as fast radio burst. Tracking these signals leads 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 Phantom and that FRBs last for much shorter periods of time or repeat in much clearer time frames. However, magnetic fields appear to be a powerful way to interfere with a radio signal.
There is another group of objects, known as galactic center radio transients, that could also explain the ghost, but Wang has reservations about this hypothesis. “The timescale of this signal and the GCRTs are different,” he says, noting that these transients are also a mystery to astronomers, and if the Ghost is another GCRT, we’re not much closer to figuring out what that really means.
We know that, lurking in the heart of our Milky Way, there is a gigantic black hole known as Sagittarius A *, but there is also no indication that it has anything to do with the Phantom.
One of the study’s limitations is the “sparse sampling” of the Phantom, Wang says. He also notes that it is difficult to say exactly how often the Phantom might repeat itself, because the number of observations is still quite small. He does not rule out that it is a pulsar or a star, but says that the observations do not match 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 is almost sure no aliens.
Definitely it is a mystery for now. Other observations should be able to refine the characteristics of the ghost a bit more, bringing its murky origins into clearer view.