Saturday, January 8, 2022

Strange Radio Waves Detected from The Galactic Center

What arises out of nowhere in the sky, emits random radio signals, and then vanishes for months? Astronomers have been perplexed since discovering strange radio waves emanating from near the center of our galaxy in January of last year. The signal is so unusual that it may be from a new class of celestial object.

Using the CSIRO radio telescope in Western Australia, a group of scientists from around the world discovered an object in the center of the Milky Way. In a press statement, Ziteng Wang, the lead author of the study and a Ph.D. student at the University of Sydney, said they first thought it might be a pulsar, a spinning dead star. But its signal didn’t match what they expected from those types of celestial objects.


The strangest feature of this new signal is that it has an extremely high polarization. This means that its light oscillates in only one direction, but then that direction rotates with time. Also, the brightness of the object fluctuates greatly as well, by a factor of 100. The signal appears to stay on for days or weeks at a time, and then other times, it can come on and off in a single day, which is extraordinarily quick for an astronomical object.


Throughout 2020 and 2021, Mr. Wang and his team have been using ASKAP to survey the sky for unique new objects as part of a study called variables and slow transients (VASTs).


When they looked towards the galaxy’s center, they discovered ASKAP J173608.2-321635, named after its coordinates. This object was unusual because it began as invisible, then became visible, brightened, faded away, and then reappeared. This behavior was extraordinary.

Over nine months in 2020, the astronomers detected six radio transmissions from the source and attempted to locate the object in visible light, but were unsuccessful.

They opted to use the CSIRO radio telescope, which is famous for detecting pulsars, but they were unable to identify the source. They then tried South Africa’s MeerKAT radio telescope, which not only detects pulses but also takes pictures of signals. Because the signal was irregular, they checked it every few weeks for 15 minutes in the hopes of seeing it again.


Fortunately, the signal reappeared, but they discovered that the source’s behavior had changed considerably. The source vanished in a single day, even though it had lasted for weeks in the earlier ASKAP observations. This new discovery, however, revealed little about the secrets of this transient radio source.


So, the first possibility was that it could be a pulsar, but the signals from this new source did not match that of a pulsar. The second possibility the team looked into was whether a massive flare from a star caused the signal. This object was so bright that they could have seen it in visible light if it was a star. But they couldn’t see it because it was completely invisible.


The only other possibility is that it is one of the emerging mysterious objects known as galactic center radio transients (GCRTs), one of which is dubbed the “cosmic burper”.


While the new object shares some properties with GCRTs, there are also differences. Every galactic center radio transient observed so far is slightly different; some emit regular radio wave pulses, while others do not. So although all these objects are lumped in the same category, not much is known about them to tell if they are related at all. They could be unknown variable polarized objects near the center of the galaxy.


Transient objects are pretty difficult to identify because they only turn on for a short period. And unlike transient objects that generate pulses of energy in the X-ray, optical or infrared wavelengths, objects that solely produce radio waves are very difficult to detect. However, the development of radio telescopes like ASKAP Pathfinder and MeerKAT has allowed astronomers to look farther into the universe than ever before.


The ASKAP and MeerKAT telescopes are the first stages of the world’s largest radio telescope, the Square Kilometre Array (SKA). When the SKA is operational in the next decade, it may be feasible to discover even fainter transient objects. There could be tens, hundreds, or thousands in our galaxy that have stayed hidden. The SKA will be able to make sensitive maps of the sky every day. Its power will not only help scientists in solving mysteries such as this latest discovery, but it will also open up large new swaths of the universe to radio spectrum exploration.

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