A Mysterious Radio Signal Near the Milky Way's Core Has Baffled Scientists for Years

Six Flickers, Then Silence

Something near the heart of our galaxy switched on, broadcast a strange radio signal, and then went quiet — six times. The source was detected six times between January and September 2020 at 888 megahertz, in the Galactic plane and about four degrees from the Galactic Centre. Then, when astronomers went looking for it through every other lens available to science, they found nothing at all. The object has since earned an informal nickname — "Andy's Object" — after its discoverer, Ziteng (Andy) Wang, from the University of Sydney in Australia. Its official name, ASKAP J173608.2-321635, is simply a coordinate label: a placeholder for something that has not yet been explained.

"This object was unique in that it started out invisible, became bright, faded away, and then reappeared. This behaviour was extraordinary," said Professor Tara Murphy, a co-author of the study from the University of Sydney. Extraordinary is an understatement. The brightness of the object varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. No known cosmic phenomenon behaves quite like that.

What Makes This Signal So Strange

The main discovery paper was published in The Astrophysical Journal by Ziteng Wang and colleagues in 2021. The team reported the discovery of ASKAP J173608.2-321635 as a highly polarised transient point source using the Australian Square Kilometre Array Pathfinder, or ASKAP. The source was found in the ASKAP Variables and Slow Transients survey, known as VAST. What they found defied easy categorization from the start.

"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," said Wang. On top of that, when visible, it showed about 25 percent circular polarisation, a striking feature because circularly polarised radio emission can point toward coherent or strongly magnetised processes. And polarised radio sources are not common. "Polarized radio sources are extremely rare," Wang, Murphy, and their co-author David Kaplan noted. "We might find fewer than ten circularly polarized sources out of thousands. Almost all of them are sources we understand well, such as pulsars or highly magnetized red dwarf stars."

Every Explanation Falls Short

X-ray follow-up found no counterpart. Near-infrared observations found no counterpart. Astronomers pointed multiple world-class instruments at the source's location and came up empty. When the astronomers tried to find the source in visual light, using optical telescopes, they saw nothing. The Parkes radio telescope in Australia also came up empty. But then, the MeerKAT radio telescope in South Africa, an array of telescopes more sensitive than Parkes, did detect it — albeit intermittent and different.

Possible identifications discussed include a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center Radio Transient — none of these fully explains the observations. The magnetar hypothesis is particularly troubled: the lack of X-rays is a problem, because known magnetars are usually much more visible in high-energy observations, and the source's slow, irregular appearance also does not map neatly onto the regular pulses expected from a rotating neutron star.

A Window Into the Unknown

The authors did not claim discovery of a new class outright. They wrote that the source may represent part of a new class of objects being discovered through radio imaging surveys. That careful, cautious language reflects both scientific rigor and genuine puzzlement. This may be a new class of object because no counterpart has been detected at multiple wavelengths, which "rules out flaring stars, binary systems, neutron stars, gamma-ray bursts, or supernovae as its source."

What makes this story matter beyond astronomy circles is what it reveals about the limits of human knowledge. The universe near the center of our own galaxy — just 26,000 light-years away — can still produce something that stumps every instrument and every theory we have. In a 2024 preprint, Kierra Weatherhead and colleagues reported new spectrum and polarisation measurements , suggesting the scientific community has not given up the hunt. As next-generation radio telescopes come online in the years ahead, Andy's Object remains one of the most tantalizing open questions in modern astrophysics — a signal that spoke six times, then went silent, and still has not told us what it is.