Bizarre radio signal repeating every 18 minutes discovered in Milky Way
Astronomers have discovered a bizarre radio signal coming from somewhere in our galaxy that can’t be explained by any known object. When it’s active the source gives off energetic radio bursts lasting up to a minute, every 20 minutes – which should be impossible based on what we know about it.
The object was discovered by a team using the Murchison Widefield Array (MWA) telescope in Western Australia, which scans large areas of the sky in radio waves. Curtin University Honors student Tyrone O’Doherty was searching for transient objects within the plane of the Milky Way, comparing pairs of images taken 24 hours apart to find things that change in brightness in that time.
And sure enough, one signal stood out with a huge spike of radio waves. When the team searched older data from the same region, they discovered more pulses with stunning regularity. Whatever it was, the object gave off bursts every 18.18 minutes like clockwork, with each pulse lasting between 30 and 60 seconds.
“This object was appearing and disappearing over a few hours during our observations,” said Dr. Natasha Hurley-Walker, lead researcher on the study. “That was completely unexpected. It was kind of spooky for an astronomer because there’s nothing known in the sky that does that. And it’s really quite close to us – about 4,000 light-years away. It’s in our galactic backyard.”
Deepening the mystery, the object doesn’t just do this constantly – it went through an active period in January 2018, took most of February off, then turned back on for most of March. During each of those 30-day active periods it stuck to its strict schedule, but it didn’t appear in the data in the five years prior or the four years since.
So what is it? A suspiciously repeating radio signal from deep space will always raise the question of aliens, but the researchers say that’s unlikely. The signal covers a very wide range of frequencies, which points to a natural origin. And some of its other characteristics provide clues as to what it might be.
Analysis shows that the light coming from the object is 90 percent polarized, indicating that it has very strong, highly ordered magnetic fields. And its repetition means that it’s most likely rotating. These are features of pulsars and magnetars, and the new object is probably one of these – albeit a very unusual one.
Both objects are types of neutron stars, compact cores left behind after a massive star dies. Pulsars emit beams of radiation that sweep across the sky like a lighthouse, making it look like the light is flashing on and off. Magnetars meanwhile have extremely strong magnetic fields. On rare occasions, it’s possible for a neutron star to be both a pulsar and a magnetar, and since this new object has characteristics of both, that’s a possibility.
But there’s one major problem – it spins way too slowly. Pulsars spin on the order of milliseconds to a few seconds, while magnetars can rotate as slowly as once every 10 seconds. This new signal’s 18-minute rotation is far too long to neatly fit into the box.
“The thing is, if you do all of the mathematics you find that they shouldn’t have enough power to produce these kind of radio waves every 20 minutes,” said Hurley-Walker. “It just shouldn’t be possible, they should be quiet. So what we think is that the magnetic field lines are somehow twisted. The neutron star has undergone some kind of outburst or activity that is causing a temporary production of radio waves, that makes it strong enough to produce something every 20 minutes.”
The team suggests that the object is an “ultra-long-period magnetar,” a slow-spinning variation that’s been hypothesized but never detected.
“Nobody expected to directly detect one like this because we didn’t expect them to be so bright,” said Hurley-Walker. “Somehow it’s converting magnetic energy to radio waves much more effectively than anything we’ve seen before.”
Interestingly, magnetars are also a leading candidate for another cosmic mystery, fast radio bursts (FRBs). These signals are short-lived pulses of radio waves that can either be one-off events or repeat either regularly or randomly. While no source of these signals has yet to be confirmed, magnetars have all the right ingredients, and one in our galaxy was recently spotted giving off suspiciously FRB-like signals.
There’s a chance, the team says, that ultra-long-period magnetars could be responsible for FRBs as well, but we’ve missed them because astronomers haven’t been watching them on the time scales that they repeat over.
As is always the case, the mystery will only be solved with more observations. The team is planning to use MWA to keep an eye on the new object in case it turns back on, as well as scanning the Galactic Plane for signs of any others lurking out there. Searches of other archival data could also reveal similar signals.
The research was published in the journal Nature. The team describes the work in the video below.