Once again, we find ourselves contemplating a signal found in radio observations. This particular signal comes from somewhere in the direction of the center of our own galaxy, the Milky Way. While using CSIRO’s ASKAP radio telescope, located in Western Australia, to observe the sky, an international team of scientists picked up the signal. Follow-up observations were then made using the MeerKAT telescope in South Africa. The object is now called ASKAP J173608.2-321635, and the paper on the finding was published in The Astrophysical Journal.

Now before anyone runs off yelling, “Aliens!”, it’s (probably) not aliens. We know. We want to find them, too.

So what makes this signal interesting if it’s not aliens? As graduate student and lead author Ziteng Wang explains: The strangest property of this new signal is that it has a very high polarisation. 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.

Wang’s doctoral adviser Dr. Tara Murphy further notes: This object was unique in that it started out invisible, became bright, faded away and then reappeared. This behaviour was extraordinary.

The signal was detected over a nine month period in 2020. It’s possible that this signal is the result of a new class of stellar object, and while it’s not as cool as aliens, that’s still an enticing possibility. However, while looking in visible light, the team found nothing. They also found nothing while using the Parkes radio telescope, which was when they turned to MeerKat, a more sensitive radio telescope. As Dr. Murphy continues: Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again. 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.

All these observations opened up the possibility that ASKAP J173608.2-321635 is part of an “emerging class of mysterious objects known as Galactic Centre Radio Transients” (GCRTs). There is even one of these objects someone has called a ‘cosmic burper’, which reminds us that perhaps astronomers can still not be trusted to name things. But while the ASKAP signal shares some similarities with the GCRTs potentially discovered to date, there are also differences between them.

It may take an even more sensitive radio telescope, such as the Square Kilometre Array, to come online and make sense of this latest discovery. For as Dr. Murphy says: 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.

And we hope other teams will find matching objects in their data so that comparisons can be made. Meanwhile, enjoy the latest astrophysical mystery.

More Information
The University of Sydney press release
“Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder,” Ziteng Wang et al., 2021 October 12, The Astrophysical Journal