We’ve received a strange signal from across the galaxy, and astronomers are struggling to understand what it means.
They know what is sending 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 nothing we’ve ever seen before. 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 Caleb 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 face it, are currently far from complete.
A neutron star is what remains after a star within a certain mass range dies, between about 8 and 30 times the mass of the Sun. The star’s outer material is blown into space, culminating in a supernova explosion.
The star’s remaining core collapses under gravity, forming an ultra-dense object with a mass up to 2.3 times the mass of the Sun, in a sphere just 20 kilometers (12 miles) in diameter.
The resulting neutron star can then present itself in different ways. There’s the basic neutron star, which just hangs around and doesn’t do much. There is the pulsar, which as it rotates blasts radio beams away from its poles and flashes like a cosmic lighthouse.
And then you have the magnetar, a neutron star with an extremely powerful magnetic field, which convulses and erupts as the outward pull of that magnetic field wars with the force of gravity that holds the star together.
There may also be a rare crossover between the types of neutron stars, indicating that they may be different stages of neutron star evolution. In general, however, pulsars, magnetars and neutron stars behave relatively predictably.
ASKAP J1935+2148 does not behave in a way that is normal for a neutron star of any established species. It was first identified incidentally during observations of another target, and follow-up 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 over the same patch of sky.
They found that ASKAP J1935+2148 has a regular pulsation period of 53.8 minutes… but that seemed to be the only normal thing about its pulsations. One pulsation mode, they found, was extremely bright, with very linear polarization. But then it would disappear completely, without any measurable pulsations for a period of time.
Ultimately, it was discovered that the star resumed its pulsatile activity – 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 found spewing repeating signals in the southern sky. Although 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 spewed out bizarrely bright flashes for only three months before going quiet again. GPM J1839-10 turned out to behave like a bizarrely 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 these objects are, but neutron stars seem likely. And ASKAP J1935+2148, Caleb and her colleagues suggest, could be a kind of bridge between the different states.
The differences between the pulsation modes are likely related to magnetospheric changes and processes, indicating that all objects belong to a new class of magnetars, possibly as they evolve into pulsars.
“ASKAP J1935+2148 is likely part of an older population of magnetars with long spin periods and low X-ray luminosity, but sufficiently magnetized to produce coherent radio emission,” the researchers write in their paper.
‘It is important that we explore this previously unexplored region of neutron star parameter space to gain a complete picture of neutron star evolution. [be] an important resource to do this.”
The findings were published in Nature Astronomy.