As our eyes become more sensitive to the sky, we will see more and more things that we have never seen before.
This is what happened to a new source of radio signals that was discovered near the center of our galaxy. It is called ASKAP J173608.2-321635 and astronomers are unable to determine what type of cosmic object would best suit its strange properties.
"We have presented ASKAP J173608.2-321635 as a highly-polarized variable radio source near the Galactic Center. It is not compatible with any multi-wavelength counterpart," says Ziteng Wang, an Australian team of astronomers.
"ASKAP J173608.2-321635" may be part of a new group of objects discovered by radio imaging surveys.
ASKAP J173608.2-32163 were discovered with the Australian Square Kilometre Astronomy Pathfinder (ASKAP), which is one of the most sensitive radio telescopes ever made and designed to peer deep into radio Universe.
It has already been able to find things that we've never seen before. This includes Odd Radio Circles (we don’t yet know what they are), unknown galaxies and mysterious fast radio bursts.
ASKAP J173608.2-32163 could be a type of known cosmic object. However, if it does, it could extend the definition of what object it is.
Wang said, "We've never seen anything similar."
It is highly variable and emits radio waves for several weeks before disappearing quickly. It is also highly polarized, meaning that the direction of the oscillations of the electromagnetic waves is twisted both linearly as circularly.
ASKAP J173608.2-32163 can also be a difficult beast to spot. Whatever the object is, it had never been seen prior to the ASKAP detections. These were made during a pilot survey in the sky to search for transient radio sources. The signal was detected 13 times between April 2019 and August 2020.
In April and July 2020, follow-up observations using a different radio telescope Murriyang in Parkes (Australia) yielded no results. In February 2021, however, the MeerKAT radio telescope from South Africa was successful. In April 2021, the Australia Telescope Compact array (ATCA), also detected the object.
This confirms and validates ASKAP detections. However, it also suggests that the source of the signal is very elusive since there have been no MeerKAT or ATCA detections before that date. The source did not appear in Xray and near infrared observations nor in radio data archives that were compiled by multiple instruments.
This leaves us with a fascinating mystery. This suggests that there is some scattering and magnetization. However, it is possible that the source itself may be highly magnetized.
It's difficult to determine the source of the star. There are many types of stars known to have radio wavelength differences, including stars that flare often, close binaries with active and active chromospheres or stars that eclipse one another. This is unlikely due to the non-detection of near-infrared and X-ray wavelengths.
Flaring stars often have X-ray emission which corresponds to radio emission. The vast majority of stars also have near-infrared emissions that should be visible.
A pulsar is not likely. It is a type neutron star that emits broad beams of radio radiation, similar to a cosmic lighthouse. ASKAP J173608.2-32163 detected fading. Pulsars are known to have a regular periodicity on a timescale that is hours. There was also a three-month period without detections. This is also inconsistent with the pulsars.
All gamma-ray bursts and X-ray binaries were also ruled out.
The object shares some characteristics with a mysterious signal that was spotted near the galactic centre. These are called Galactic Center Radio Transients or GCRT. Three of them were discovered in 2000s and many more are awaiting confirmation.
Although these sources are still to be discovered, they share many features with ASKAP J173608.2-32163.
ASKAP J173608.2-32163 could be a GCRT. ASKAP's detection may help us locate more sources and determine what they are.
Researchers write that ASKAP J173608.2-321635 can be difficult to detect and can shut off in a matter of hours. Our limited sampling (12 epochs spread over 16 months) suggests there may be other similar sources.
"Increasing the survey frequency and comparing the search results to other regions will allow us to understand how unique ASKAP J173608.2-321635 really is, and whether it is related with the Galactic plane. This should eventually help us to deduce its true nature."
The Astrophysical Journal reports the findings.
A previous version of this article appeared in September 2021.