We’ve received a strange signal from across the galaxy, and astronomers are scrambling to figure out what it means.
They know what is emitting the signals. It is a neutron star called ASKAP J193505.1+214841.0 (ASKAP J1935+2148 for short) located in the plane of the Milky Way, about 15,820 light-years from Earth.
But the signals themselves are like no other we’ve ever seen. The star goes through periods of strong pulses, periods of weak pulses, and periods of no pulses at all.
What we don’t know, according to a team led by astrophysicist Manisha Kaleb of the University of Sydney in Australia, is why. The strange object poses a fascinating challenge to our models of neutron star evolution – which, let’s be honest, are currently quite far from complete.
A neutron star is what remains after the death of a star in a certain mass range, between about 8 and 30 times the mass of the Sun. The star’s outer material is shot out into space, ending in a supernova explosion.
The remnant core of the star collapses under the action of gravity, forming an ultra-dense object with a mass up to 2.3 times the mass of the Sun, in a sphere only 20 kilometers (12 mi) in diameter.
The resulting neutron star can be represented in various ways. There’s the main neutron star that just hangs around without doing much. Here’s the pulsar, spewing beams of radio emission from its poles as it spins, flashing like a cosmic beacon.
And there’s a magnetar, a neutron star with an extremely powerful magnetic field that moves and erupts when the outward pull of that magnetic field collides with the gravity holding the star together.
There may also be rare crossover between neutron star types, suggesting that they may be different stages of neutron star evolution. In general, however, pulsars, magnetars, and neutron stars tend to behave in relatively predictable ways.
ASKAP J1935+2148 is not behaving in ways that are normal for a neutron star of any known type. It was first identified by chance during observations of another target, and subsequent observations were made using the Australian Square Kilometer Array Pathfinder (ASKAP) and the MeerKAT radio telescope in South Africa.
The researchers also delved into previous ASKAP observations covering the same patch of sky.
They found that ASKAP J1935+2148 had a regular pulse period of 53.8 minutes… but that seemed to be the only normal thing about its pulsations. They found that one pulsation mode was extremely bright, with a strongly linear polarization. But then it will die down completely, without any measurable ripples for a period of time.
Finally, the star was detected resuming its pulsating activity – but as much as 26 times fainter than its previous bright mode, and with light that is circularly polarized.
In recent years, several strange objects have been discovered emitting repetitive signals in the southern sky. While they don’t all behave the same way, they may be related.
GLEAM-X J162759.5-523504.3 is an object near the galactic center that was caught spewing strange bright flashes for just three months before going quiet again. GPM J1839-10 was found to behave like a strangely slow pulsar, emitting five-minute bursts of radio waves every 22 minutes. And GCRT J1745-3009 is a pulsating object near the galactic center with a period of 77 minutes.
We don’t know for sure what any of these objects are, but neutron stars seem likely. And ASKAP J1935+2148, Caleb and her colleagues suggest, could be something of a bridge between the different states.
The differences between its pulsation modes are likely related to magnetospheric changes and processes, suggesting that all objects belong to a new class of magnetars, possibly while evolving into pulsars.
“ASKAP J1935+2148 is likely part of an older population of magnetars with long rotation periods and low X-ray brightness, but sufficiently magnetized to be able to produce coherent radio emission,” the researchers wrote in their paper.
“It is important to explore this hitherto unexplored region of neutron star parameter space to get a complete picture of neutron star evolution, and this can [be] an important source for this.”
The findings are published in Natural astronomy.
