“We’ve never seen anything like it,” said researcher Ziteng Wang.

Wang is part of the research team that will detect the mysterious signals in 2020 using the Australian Square Kilometre Array Pathfinder (ASKAP for short) for the first time. “We scanned the sky with ASKAP in 2020 and 2021 to find unusual new objects,” said study researcher Tara Murphy. And so we succeeded. “When we looked at the heart of the Milky Way, we discovered ASKAP J173608.2-321635, named after its coordinates. The object was unique in that it was first invisible, then brightened, then faded and then reappeared. That was exceptional.”

And as exciting as these radio signals seemed to be in 2020, so frustrating was the search for their origin next. Because it has still not been found and thus ASKAP J173608.2-321635 is inexplicable to this day.

Very weird
“The strangest feature of this new signal is that it has a very high polarization,” Wang said. “That means its light oscillates in only one direction, but that direction rotates over time. The brightness of the object also varies enormously – by a factor of 100 – and the signal appears to appear and disappear completely randomly. We have never seen anything like it.”

And all the possible explanations that astronomers have thought of for the radio signals to date fall short. “At first we thought the source was a pulsar – a rapidly rotating, dead star that has an enormous density – or else a star spewing huge solar flares. But the signals this object gives are not what we would expect from these types of celestial objects.” And so the mystery continues.

That mystery began in 2020 when researchers using ASKAP spotted the mysterious radio signal six times over a nine-month period. Attempts to locate its origin in visible light were unsuccessful. The same goes for follow-up observations with the Parkes radio telescope; he couldn’t find the source of the signals either. “We then tried the much more sensitive MeerKAT radio telescope in South Africa,” Murphy said. Since the signal appeared intermittently and the telescope cannot continuously search for it, it was decided to have it peer toward the heart of our Milky Way for 15 minutes each week. “Hopefully we’d see the signal again.” And the researchers were lucky. “The signal returned,” Murphy said. But that didn’t solve the mystery. In fact; it actually just got bigger. “We found that the behavior of the well was radically different: the well disappeared within a day, while during our previous observations with ASKAP it was visible for weeks in a row.”

And that didn’t exactly make it easier to understand the nature of the object. “The information we have does have some parallels with another class of mysterious objects known as Galactic Centre Radio Transients (GCRTs),” said researcher David Kaplan. These as yet unidentified objects also intermittently generate radio waves. “But while our new object – ASKAP J173608.2-321635 – shares similarities with GCRTs, there are also differences. And we don’t quite understand these sources yet, so that all adds to the mystery.”

In an effort to unravel the mystery, researchers are keeping a close eye on ASKAP J173608.2-321635 for the foreseeable future. And if that doesn’t provide more insight into the origin of the radio signals, there is always the Square Kilometre Array (SKA): a huge radio telescope that will be built in Australia and South Africa in the coming years (and in which the Netherlands has also invested considerably). “With SKA, maps of the Universe will be made every day and we expect this telescope to help us solve mysteries like ours,” Murphy said. Whoever thinks that astronomers can sit back after that, however, is wrong. It is also expected that the observations of the Square Kilometre Array will also reveal many new mysteries.

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