Astronomers finally discover the cosmic source of a mysterious, pulsating radio signal

After searching through 24 hours of radio data that spanned the galaxy, a group of astronomers detected 71 distinct pulses that spanned from January to March 2018.

The pulses were unlike any they had seen before — low-frequency, bright, and lasting for 30 to 60 seconds each time. The mysterious pulses also seemed to be coming from within the Milky Way. After some deduction, the astronomers now believe the pulses may have resulted either from a strange magnetar star or a slowly spinning neutron star that would turn on and off.

How they did it — Using data from the Murchison Widefield Array (MWA), the astronomers behind the study investigated a radio signal that was left unidentified. MWA is a low-frequency radio telescope in Western Australia that picks up signals between 80 and 300 megahertz across the southern sky.

Through their search, they identified a low-frequency radio signal that occurred on an hourly basis, with each pulse ranging from 30 to 60 seconds, and sometimes it would have sub-pulses that only lasted 10 to 30 seconds.

The signal also showed linear polarization, meaning that the magnetic field of light coming from the source of the signal was confined to a single plane.

The pulses came through during two periods, each a 30-day interval. The first spanned from January 3 to February 2, 2018, and the second spanned from February 28 to March 28, 2018, with 26 days in between where the team detected no pulses.

“This object was appearing and disappearing over a few hours during our observations,” Natasha Hurley-Walker, an astrophysicist at Curtin University, said in a press statement. “That was completely unexpected. It was kind of spooky for an astronomer because there’s nothing known in the sky that does that.”

What’s new — The pulses resembled a gravitational lensing event, where an object in the foreground magnifies the light from a background object. But, the linear polarization suggested that the object has a strong magnetic field and its brightness ruled out radio emissions from exoplanets, white dwarf stars, or flare stars.

Meanwhile, the regular pulses revealed that the object was rotating. It was also smaller than the Sun, but much brighter than our host star.

After examining the radio signal, and ruling out different possibilities, the scientists behind the study suggest that the object is in fact a magnetar star located around 4,000 lightyears away.

But it was unlike any other object astronomers had observed before, releasing a giant burst of energy three times an hour. The magnetar’s beam of radiation was one of the brightest radio sources in the sky, lasting for about a minute each time.

“It’s a type of slowly spinning neutron star that has been predicted to exist theoretically,” Hurley-Walker said. “But nobody expected to directly detect one like this because we didn’t expect them to be so bright. Somehow it’s converting magnetic energy to radio waves much more effectively than anything we’ve seen before.”

What is a magnetar?

Magnetar stars are rather intense.

They are a type of neutron star, the collapsed core of a giant, dead star, with an extremely powerful magnetic field. These stars are known to erupt without warning, sometimes for hours and sometimes for months, before disappearing again into the cosmic abyss.

Magnetars have not been fully understood just yet, but these magnetic objects offer a fascinating insight into how stars work since they aren’t powered by the conventional methods of other stars. Instead, magnetars have magnetic fields a thousand times stronger than ordinary neutron stars. The Sun's magnetic field is only about 5 Gauss, while a magnetar’s magnetic field can measure up to a million billion Gauss.

WHAT’S NEXT — There are about 30 known magnetars across the Milky Way, and there could be hundreds more out there. But detecting the explosive tantrums of these magnetic stars has proven tricky.

Magnetars are commonly detected through X-ray observations. Meanwhile, four of the five known magnetars that have produced detectable radio signals have done so only after emitting X-ray outbursts first. But not all X-ray-emitting magnetars produce detectable radio emissions.

However, this new study sheds light on a new type of magnetar that can be detected through low-frequency radio signals. Learning more about these intense magnets can help scientists better understand the lifecycle of giant stars. The team behind the magnetar’s discovery is currently monitoring its vicinity to see if it turns on again, and hoping to discover more objects that are like it.

Abstract: The high-frequency radio sky is bursting with synchrotron transients from massive stellar explosions and accretion events, but the low-frequency radio sky has, so far, been quiet beyond the Galactic pulsar population and the long-term scintillation of active galactic nuclei. The low-frequency band, however, is sensitive to exotic coherent and polarized radio-emission processes, such as electron-cyclotron maser emission from faring M dwarfs1 , stellar magnetospheric plasma interactions with exoplanets2 and a population of steep-spectrum pulsars3 , making Galactic-plane searches a prospect for blind-transient discovery. Here we report an analysis of archival low-frequency radio data that reveals a periodic, low-frequency radio transient. We fnd that the source pulses every 18.18 min, an unusual periodicity that has, to our knowledge, not been observed previously. The emission is highly linearly polarized, bright, persists for 30–60 s on each occurrence and is visible across a broad frequency range. At times, the pulses comprise short-duration (<0.5 s) bursts; at others, a smoother profle is observed. These profles evolve on timescales of hours. By measuring the dispersion of the radio pulses with respect to frequency, we have localized the source to within our own Galaxy and suggest that it could be an ultra-long-period magnetar.