Nobody really knows what's at the heart of the Milky Way. It's the galaxy we call home, but its center is so densely packed with billions of stars that even our most powerful telescopes can't see it. It's a commonly held belief that right at the center of the galaxy is a supermassive black hole – a phenomenon that scientists and astronomers call “Sagittarius A* – but they can't directly observe the black hole. We assume that it's there because of the behavior of stars and other observable stellar objects around it.
Famously, black holes don't “send out” anything. Instead, they pull everything in. Nothing can escape from a black hole of the size of Sagittarius A* – not even light. That's why scientists are so puzzled by a newly-discovered radio signal that's coming from the galactic center. The signal seems to have been aimed at Earth so precisely that one scientist said it was as if someone “buzzed” us. That's the kind of sentence that gets UFO enthusiasts excited, but in this case, scientists are just as excited as anybody who believes in extraterrestrial life. This signal has an origin point, whether it's artificial or not, and it's the job of scientists to find it. The problem is that finding it is likely to be much harder said than done.
The mysterious signal was detected by the Australian Square Kilometre Array Pathfinder, or ASKAP for short. It’s an enormous assemble of 36 dish antennas standing in the vast, empty desert of Western Australia. The sole task of ASKAP is to keep an eye (or ear) on the universe and look out for radio waves. The antennas are redirected to monitor different areas of space throughout the year, but they sweep past the galactic core regularly. Although the discovery of the unexplained signal wasn’t announced to the public until October 12th, the scientists who monitor ASKAP say they've encountered the same signal more than once. Curiously, though, it isn't there every time they look. Whatever this mysterious source is, the radio signal it sends out is unlike anything that's ever been detected before. If something is hiding in the centre of the Milky Way and sending this signal to us, it appears to be an utterly unique object.
Scientists aren't known for giving catchy titles to stellar phenomena. They've called the enigmatic signal “ASKAP J173608.2-3216325.” We prefer to use its nickname – “the Ghost.” The official announcement tells us that the Ghost signal was picked up thirteen times in just fourteen months between April 2019 and August 2020. Scientists then spent more than a year trying to find an adequate explanation for its existence before revealing its existence to the general public. They've had no luck, and their job has been made harder because the signal doesn't appear to follow a pattern. The signal is inconsistent, with no set guarantees on when it will appear or how long it will last. That makes it exceptionally unlikely to be a quasar, a neutron star, or any object moving in a regular orbit.
Tara Murphy, who wrote the scientific paper announcing the signal's discovery and is an astrophysicist with the University of Sydney, has probably spent more time studying the signal than anybody else. She's baffled by it because the signal is so weak that it's almost invisible when it first appears before becoming steady brighter, fading away altogether, and then reappearing. Her first theory was that it might be an exceptionally large pulsar – a specific type of neutron star that spins rapidly and gives off electromagnetic radiation – but if it were a pulsar, we should be able to detect it with instruments we have on Earth. Tara and her team attempted to do so at Australia's Parkes Observatory using the Murriyang telescope. They found nothing. She then contacted NASA and asked them to look for X-rays using the Neil Gehrels Swift Observatory. None were found. The VISTA telescope in Chile looked for infrared or near-infrared signals and drew a blank. The only conclusion that can be drawn from that is that there's no pulsar.
To make matters worse, South Africa's Meerkat array also turned its attention to the problem and added to the confusion. Meerkat is more sensitive than the Australian array and was able to detect variations in the Ghost signal that lasted for a day rather than a week, and showed signs of circular polarisation. Fewer than one percent of all known signals in the universe display this feature, and they're almost always connected to magnetic fields. This signal has to be coming from something huge and something that has a magnetic field but isn't a star and doesn't have a predictable pattern of behavior. That strongly raises the possibility of it having an artificial origin – but you won't catch any scientists saying that out loud any time soon.
This is yet another reminder of how little we know about the mysteries that are out there in the stars. We tend to think of stars as little more than celestial decorations – pretty to look at but trivial. We sing about them in nursery rhymes. We even use them as a source of entertainment. It's not a coincidence that “Starburst” – one of the most popular online slots games in the world – is set in deep space and uses star expansion as a feature. Most online slots players probably don't think of themselves as armchair astronomers, but there has to be something that draws them to the game. Space isn't there for entertainment, though. It offers us jackpots that an online slots player couldn't even dream of – but we lack the technology to go chasing after them.
Human beings won’t make it to the center of the galaxy in our lifetimes. It’s possible that we’ll never make it there at all – our planet might die before we develop technology capable of traveling that far. We might never find out whether this radio signal is an attempt at communication or not. We do know there’s something out there, though – and it’s the desire to know what that “something” is that keeps us all hooked on space.