What to make of a newly discovered pulsing radiation source?
An image of the Milky Way from the Murchison Widefield Array, with the lowest frequencies in red, middle frequencies in green and the highest frequencies in red. The star marks the newly discovered object. Image credit: Natasha Hurley-Walker (ICRAR/CURTIN) and the GLEAM Team.
An Australian-led team scanning the cosmic melee for radio waves has discovered a mysterious object unlike anything astronomers have seen before. The object, GLEAM-XJ162759.5-523504, releases a giant burst of energy that crosses our line of sight and, roughly three times an hour, is one of the brightest radio sources in the sky.
“This object was appearing and disappearing over a few hours during our observations,” says Natasha Hurley-Walker of Curtin University, leader of the team that made the discovery. “That was completely unexpected. It was kind of spooky for an astronomer because there’s nothing known in the sky that does that.”
Unfortunately, it’s fairly clear these signals aren’t the work of little green men. As Hurley-Walker explains, the pulses of radiation come across a wide range of frequencies, which rules out an artificial signal, pointing instead to some kind of natural process we don’t yet fully understand.
The location of the source of the radio waves in the sky in January 2022, marked with a star. At this time of year, it’s above the horizon during the day. Image credit: Stellarium
So, what actually is GLEAM-XJ162759.5-523504?
We know it’s a radio transient, and these aren’t unknown to science.
“A radio transient is something that we see in radio waves that switches on and then off again,” says Hurley-Walker, “and there have actually been a lot of these detected over the years.”
But a key feature of radio transients, she says, is that they “come and go”. They appear, disappear, and leave observing astronomers stumped, because there’s not enough observational data to understand what might have made the signal.
“That’s a shame,” says Hurley-Walker, “because we would really like to understand what’s generating these kinds of things. They’re often going to come from very high energy processes in the universe. And being able to understand that would allow us to probe really extreme physics, like the intersection between quantum mechanics and general relativity.”
The object in question was discovered by Tyrone O’Doherty, a Curtin University honours student supervised by Hurley-Walker, using the Murchison Widefield Array (MWA) telescope in outback Western Australia, in one of the most radio-silent parts of the continent. O’Doherty developed a new software that looks at pairs of observations and the differences between them, helping to identify unusual bursts of radiation.
The team were amazed by what they discovered. GLEAM-XJ162759.5-523504 is incredibly bright – “really extreme”, according to Hurley-Walker.
But even more peculiar was the relatively slow pulse-rate of this strange object. “Slow transients”, like supernovae, might appear over the course of a few days and disappear after a few months. “Fast transients”, like pulsars, flash on and off within seconds or milliseconds. Something that turns on for a minute, however, is really quite weird.
“If you do all of the mathematics, you find that these things shouldn’t have enough power to produce these kinds of radio waves every 20 minutes,” says Hurley-Walker.
So, what do they reckon is causing this strange pattern of energy pulses?
“What we think is that the magnetic field lines are somehow twisted and that this 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.”
One option is that it could be a predicted astrophysical object – never before actually observed in the skies – called an “ultra-long period magnetar”.
An artist’s impression of what the object might look like if it is indeed a magnetar. Magnetars are highly magnetic neutron stars, some of which sometimes produce radio emissions. Known magnetars rotate every few seconds, but theoretically ‘ultra-long period magnetars’ could rotate much more slowly. Image credit: ICRAR
“It’s a type of slowly spinning neutron star that has been predicted to exist theoretically,” she says. “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.”
Another option is that it could be a white dwarf, a collapsed star remnant that might be producing a pulsar.
“But that’s quite unusual as well, we only know of one white dwarf pulsar, and nothing as bright as this.”
Or, it could be something entirely new to science. Finding an entirely new cosmic object is an exceptionally rare gift for any astronomer, let alone an honours student on the cusp of his academic career, as O’Doherty was at the time of discovery.
“It was quite surreal to have found something like this,” he says.
So, what next? The team are currently monitoring the object, which has ceased pulsing, to see if it switches back on.
Tile 107, or “the Outlier” as it is known, is one of 256 tiles of the MWA located 1.5km from the core of the telescope. The MWA is a precursor instrument to the SKA. Image credit: Pete Wheeler, ICRAR
“It’s such an exciting thing to find a new class of object, so I’m pretty sure if we find it switching back on again, the eyes of the world will turn to that little patch of sky,” says Hurley-Walker. In the meantime, she also hopes to find more of them.
The Square Kilometre Array (SKA), the still under-construction mega-telescope that will encompass sites in Australia and South Africa, will offer scientists an unprecedented window into the vast cosmic wilds, comprising the largest radio telescope array ever constructed. For Hurley-Walker, O’Doherty and their team, the SKA will massively enhance their ability to look for more of these mysterious, pulsing objects – and potentially other paradigm-shifting discoveries.
“It’s really important to keep our minds open to the possibilities that we haven’t considered,” says Hurley-Walker. “No one really thought of looking for objects on this timescale because we couldn’t think of any mechanisms that produce them – and yet they exist.
“We will be making discoveries like this all the time. The universe is full of wonders.”
Originally published by Cosmos as Mysterious object unlike anything astronomers have seen before