Astronomers in the Australian outback have discovered something new. Three times an hour, it becomes one of the brightest objects in the sky. The team that discovered it thinks it’s a magnetar — and it’s right in our cosmic backyard.
As the mysterious object rotates, highly polarized or twisted beams of radiation shoot from its poles. Every 18.18 minutes, for 30 to 60 seconds, a beam crosses our line of sight, and the object starts to flash. “It was kind of spooky for an astronomer because there’s nothing known in the sky that does that,” said team leader Dr. Natasha Hurley-Walker in a statement. Slow transients, like a supernova, might happen on a scale of days to months. Faster ones like pulsars flash on and off within milliseconds. “It’s just every 18.18 minutes, like clockwork,” she said.
Because of this strangely long interval, Dr. Hurley-Walker said the observations match with predictions of an exotic astrophysical object called an ultra-long period magnetar. “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.”
“Because we didn’t expect this kind of radio emission to be possible,” said Dr. Hurley-Walker, “the fact that it exists tells us that some kind of extreme physical processes must be happening,” she said.
This image shows the Milky Way as viewed from Earth. The star icon shows the position of the mysterious repeating transient. Credit: Dr Natasha Hurley-Walker (ICRAR/Curtin).
Dr Hurley-Walker is now monitoring the object with the Murchison Widefield Array (MWA) in Western Australia to see if it flashes the porch light again. “If it does,” she said, “there are telescopes across the Southern Hemisphere and even in orbit that can point straight to it.”
Squish upon a star
Neutron stars rise from the ashes of a core-collapse supernova. Starquakes rock their crusts while incomprehensible levels of force crush the core of a star into a tiny ball. But if a neutron star is rotating fast enough when it’s born, it can develop an internal dynamo effect that supercharges its magnetic field. The beautiful and terrible result is called a magnetar.
These monstrous neutron stars cram more than the mass of the Sun into a sphere about twenty kilometers across. But even at fifty times that distance from a magnetar, the devastating magnetic fields are incompatible with the chemistry of all known life.
Within a magnetic field of magnetar strength, physics as we know it becomes something… different. “X-ray photons readily split in two or merge together. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic wavelength of an electron.”
The Milky Way is somewhere between one and two hundred thousand light-years across. At four thousand light-years away, this magnetar is in our cosmic backyard. It’s extremely powerful, and uncomfortably close. Happily, it won’t last long. After just ten thousand years or so, a magnetar’s magnetic field decays. After that, its outbursts will cease.
Dark skies, deep time
Curtin University student Tyrone O’Doherty was using a method of his own design when he spotted the new magnetar on the block in data from the MWA telescope. “It’s exciting that the source I identified last year has turned out to be such a peculiar object,” said O’Doherty, now studying for a PhD.
The MWA is a low-frequency radio telescope consisting of thousands of spider-legged dipole antennas. To do its work, it uses the dark skies and radio silence of the Australian outback. MWA Director Professor Steven Tingay said the telescope is a precursor instrument for the Square Kilometre Array (SKA).
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. Photographed by Pete Wheeler, ICRAR
“Key to finding this object, and studying its detailed properties, is the fact that we have been able to collect and store all the data the MWA produces for almost the last decade at the Pawsey Research Supercomputing Centre. Being able to look back through such a massive dataset when you find an object is pretty unique in astronomy,” he said.
Once operational, the SKA will encompass a vast network of telescopes throughout Australia and South Africa. Its incredible resolution will allow researchers to create a three-dimensional map of galaxies out to the edge of the visible universe, and to look back to the time of First Light. The array is expected to make its first observations in 2027.
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