As our eyes in the sky become more and more sensitive, we will find more and more things that we had never seen before.
Such is the case with a newly discovered source of radio signals, located not far from the center of the galaxy. It is called ASKAP J173608.2-321635 and astronomers have not been able to figure out what type of cosmic object best fits its strange properties.
His work has been accepted for publication in The Astrophysical Journal, and is available on the arXiv prepress server.
“We have presented the discovery and characterization of ASKAP J173608.2-321635: a highly polarized variable radio source located near the galactic center and without any clear wavelength counterpart,” explains a team of astronomers led by Ziteng Wang from the University of Sydney in Australia.
“ASKAP J173608.2-321635 may represent part of a new class of objects that are discovered through radio image surveys.”
ASKAP J173608.2-32163 was discovered using the Australian Square Kilometer Array Pathfinder (ASKAP), one of the most sensitive radio telescopes ever built, designed to look deeply into the radio universe.
He’s already proven adept at finding things we’ve never seen before, like Odd Radio Circles (we don’t know what they are yet), undiscovered galaxies, and mysterious fast bursts.
ASKAP J173608.2-32163 may turn out to be a known type of cosmic object, but if it does, it could end up stretching the definition of any object.
It is highly variable, emitting radio waves for weeks at a time and then disappearing on fast time scales. The signal is also highly polarized, i.e. the orientation of the electromagnetic wave oscillation is twisted, both linearly and circularly.
ASKAP J173608.2-32163 is also a pretty tricky beast to spot. The object, whatever it was, had not been seen before ASKAP detections, conducted during a pilot sky survey to look for transient radio sources. Between April 2019 and August 2020, the signal appeared in the data 13 times.
Follow-up observations in April and July 2020 with a different radio telescope, Murriyang in Parkes, Australia, yielded nothing. But the MeerKAT radio telescope in South Africa was successful in February 2021. The Australia Telescope Compact Array (ATCA) also made a detection in April 2021.
This supports and validates ASKAP detections, but also suggests that the source is quite elusive: there were no MeerKAT or ATCA detections before that date. Neither did the source appear in nearby X-ray and infrared observations, nor in radio data files collected by multiple instruments that the researchers checked.
Which leaves a pretty fascinating mystery. Polarization suggests scattering and magnetization, possibly in part due to the dust and magnetic fields of the interstellar medium between us and the source, although it is possible that the source is also highly magnetized.
All in all, it is very difficult to find out what the source may be. There are several types of stars that are known to vary in radio wavelengths, such as stars that bloom frequently or close binaries with active chromospheres or that eclipse each other. Failure to detect nearby X-ray and near-infrared wavelengths makes it unlikely.
Flaming stars typically have an X-ray emission that corresponds to radio emission, and the vast majority of stars have near-infrared emission ratios that should be detectable.
Nor is a pulsar likely: a type of neutron star with beams of radio light, like a cosmic beacon. The pulsars have regular periodicity, on a time scale, and ASKAP J173608.2-32163 was detected to have faded, which is not consistent with the pulsars. In addition, there was a three-month period without detections, which is also incompatible with pulsars.
X-ray binaries, gamma-ray bursts, and supernovae were also ruled out.
However, the object shares some properties with a mysterious signal type that is close to the galactic center. They are known as Galactic Center Radio Transients (GCRTs), three of which were identified in the 2000s and many more are pending confirmation.
These sources have not yet been explained, but they have several features in common with ASKAP J173608.2-32163.
If ASKAP J173608.2-32163 is a GCRT, ASKAP detection can help us find more such sources and find out what they are.
“Since ASKAP J173608.2-321635 is not usually detected and can be disabled on time scales from several weeks to such a fast day, our sparse sampling (12 epochs over 16 months) suggests that there could be other similar sources in these fields, ”the researchers write.
“Increasing the cadence of the survey and comparing the results of this search with other regions will help us understand how truly unique ASKAP J173608.2-321635 is and whether it is related to the galactic plane, which should ultimately help us. ‘help deduce its nature’.
The investigation has been accepted The Astrophysical Journal, and is available at arXiv.