We’ve received a strange signal from across the galaxy, and astronomers are trying to understand what it means.
They know what is sending signals. It is a neutron star named 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 pulsations, periods of weak pulsations, and periods of no pulsations 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 be honest, are currently far from complete.
A neutron star is what is left after a star in a certain mass range dies, about 8 to 30 times the mass of the Sun. The star’s outer material is ejected into space, culminating in a supernova explosion.
The remaining core of the star collapses under gravity to form an ultra-dense object with a mass of 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 then present itself in various ways. There’s a basic neutron star that just hangs around and doesn’t do much. There’s a pulsar that sweeps beams of radio emission from its poles as it spins, flashing like a cosmic beacon.
And there’s the magnetar, a neutron star with an extremely strong magnetic field that yanks and explodes as the outward pull of that magnetic field battles the gravity that holds the star together.
There may also be some 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 does not behave in ways that are normal for a neutron star of any established kind. It was first identified by chance during an observation of another target, and subsequent observations were made using Australia’s Square Kilometer Array Pathfinder (ASKAP) and the MeerKAT radio telescope in South Africa.
The researchers also delved into previous ASKAP observations covering the same part of the 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. They found that one pulsation mode was extremely bright with a highly linear polarization. But then it would die out completely, for a period of time with no measurable pulsations at all.
Eventually, the star was detected resuming its pulsating activity—but a whopping 26 times fainter than its former bright mode, and with light that is circularly polarized.
In recent years, several strange objects have been found in the southern sky that emit repeating signals. Even if they don’t all behave the same, they could be related.
GLEAM-X J162759.5-523504.3 is an object near the galactic center that was caught spewing bizarrely bright flashes for just three months before going quiet again. GPM J1839-10 was found 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 any of these objects are, but neutron stars seem likely. And ASKAP J1935+2148, as Caleb and her colleagues suggest, could be a kind of bridge between different states.
The differences between its pulsation modes are likely linked to magnetospheric changes and processes, suggesting that all objects belong to a new class of magnetars, perhaps as they evolve into pulsars.
“ASKAP J1935+2148 is likely part of an older population of magnetars with long rotation periods and low X-ray luminosity, but sufficiently magnetized to be able 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 get a complete picture of neutron star evolution, and this can [be] an important source for it’.
The findings were published in Astronomy of nature.