(CN) — An international team of astronomers have discovered a mysterious pattern of radio signals emanating from deep inside the center of the Milky Way. The signals do not match any currently known patterns and may originate from an entirely new class of stellar object.
The scientists named their discovery ASKAP J173608.2-321636, which was detected at 888 megahertz sitting four degrees from the Galactic Center of the Milky Way. They detected the source six times over a nine-month period in 2020 as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients survey, which they describe in a new study published Tuesday in Astrophysical Journal.
“The strangest property of this new signal is that it is has a very high polarization,” said Ziteng Wang, lead author of the study and a doctoral student in the School of Physics at the University of Sydney. “This means its light oscillates in only one direction, but that direction rotates with time. The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it.”
Thanks to advances in radio astronomy, experts can detect a diverse array of cosmic objects whose brightness varies over time, such as pulsars, supernovae, stars and magnetars, among others. But this new discovery has researchers stumped because it doesn’t behave like other known stellar object types. The authors already ruled out the most likely origins, such as stars, normal neutron stars and X-ray binaries, which is a normal star paired with a collapsed star that produces X-rays that can be observed from Earth.
“At first we thought it could be a pulsar — a very dense type of spinning dead star — or else a type of star that emits huge solar flares. But the signals from this new source don’t match what we expect from these types of celestial objects,” Wang said in a statement.
Wang, along with a team including researchers from Australia, Germany, the U.S., Canada, South Africa, Spain and France, originally located the object using the CSIRO’s ASKAP radio telescope in Western Australia. ASKAP is designed to search for variable and transient phenomena over a wide range of parameters. Astronomers in South Africa followed up with observations from the more sensitive South African Radio Astronomy Observatory’s MeerKat telescope.
“Looking towards the centre of the Galaxy, we found ASKAP J173608.2-321635, named after its coordinates. This object was unique in that it started out invisible, became bright, faded away and then reappeared. This behaviour was extraordinary,” Dr. Tara Murphy, Wang’s supervisor at the Sydney Institute for Astronomy, said in a related statement.
The Galactic Center is an ideal region of space to search for these variable and transient radio sources because of its high stellar density and ongoing star formation, according to the study.
Despite detecting six radio signals from the source over a nine-month period in 2020, researchers found nothing when they attempted to locate the object in the visible light spectrum. They then turned to the Parkes radio telescope and again came up empty handed.
“We then tried the more sensitive MeerKAT radio telescope in South Africa. Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again,” Murphy said. “Luckily, the signal returned, but we found that the behaviour of the source was dramatically different — the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations.”
Unfortunately, that left researchers with more questions than answers, and didn’t reveal much more about the signals’ source than was already known. Wang’s co-supervisor, Dr. David Kaplan from the University of Wisconsin-Milwaukee, said the new object shares properties with other previously discovered variable and transient stellar objects, known as Galactic Radio Transients, but added that, unfortunately, many of those objects are not yet well understood.
The authors are hopeful that next-generation telescopes coming online in the next decade or so will help shine a light on some of the mysteries accruing under current-gen equipment. However, they also acknowledge that more powerful equipment will likewise usher in new mysteries in what appears to be a never-ending game of cosmic whack-a-mole.
“Within the next decade, the transcontinental Square Kilometre Array (SKA) radio telescope will come online. It will be able to make sensitive maps of the sky every day,” Murphy said. “We expect the power of this telescope will help us solve mysteries such as this latest discovery, but it will also open vast new swathes of the cosmos to exploration in the radio spectrum.”
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