A strangely intermittent radio signal from space has puzzled astronomers

When astronomers turn our radio telescopes toward space, we sometimes detect sporadic bursts of radio waves coming from across the vast universe. We call them “radio transients”: some burst only once and are never seen again, and others flicker on and off in predictable patterns.

We think most radio transients come from rotating neutron stars known as pulsars, which emit regular bursts of radio waves, like cosmic lighthouses. Normally, these neutron stars spin at incredible speeds, taking only a few seconds or even a fraction of a second to complete each rotation.

We recently discovered a radio transient that looks nothing like anything astronomers have seen before. Not only does it have a cycle of almost an hour (the longest ever seen), but during several observations we saw that it sometimes emitted long, bright flashes, sometimes fast, weak pulses – and sometimes nothing at all.

We can’t quite explain what’s going on here. It is most likely a very unusual neutron star, but we cannot rule out other possibilities. Our research has been published in Nature Astronomy.

A lucky find

Introducing ASKAP J1935+2148 (the numbers in the name refer to its location in the sky). This periodic radio transient was discovered using CSIRO’s ASKAP radio telescope on Wajarri Yamaji Country in inland Western Australia.

The telescope has a very wide field of view, allowing it to survey large volumes of the universe very quickly. This makes it very suitable for detecting new and exotic phenomena.

Using ASKAP, we were simultaneously monitoring a source of gamma rays and looking for pulses from a fast radio burst when we noticed ASKAP J1935+2148 slowly flashing in the data. The signal stood out because it consisted of “circularly polarized” radio waves, meaning the direction of the waves rotates as the signal travels through space.

Our eyes cannot distinguish between circularly polarized light and normal unpolarized light. However, ASKAP functions like Polaroid sunglasses, filtering out the glare from thousands of common sources.

After the initial detection, we carried out further observations over several months using ASKAP and also the more sensitive MeerKAT radio telescope in South Africa.

The slowest radio transient ever found

ASKAP J1935+2148 belongs to the relatively new class of long-lasting radio transients. Only two others have ever been found, and the 53.8 minute period of ASKAP J1935+2148 is by far the longest.

However, the exceptionally long period is just the beginning. We have seen ASKAP J1935+2148 in three different states or modes.

In the first condition we see bright, linear (rather than circular) polarized pulses that last 10 to 50 seconds. In the second state, there are much weaker, circularly polarized pulses that last only about 370 milliseconds. The third state is a silent or extinguished state, without pulses.

These different modes, and the switching between them, could be the result of an interplay of complex magnetic fields and plasma currents from the source itself with strong magnetic fields in the surrounding space.

Similar patterns have been observed in neutron stars, but our current understanding of neutron stars suggests that they should not have such a long period.

Neutron stars and white dwarfs

The origin of a signal with such a long period remains a profound mystery, with a slowly spinning neutron star the prime suspect. However, we cannot rule out that the object is a white dwarf: the Earth-sized cinder of a burned-out star that has used up its fuel.

White dwarfs often have slow rotation periods, but we don’t know of any way you could produce the radio signals we see here. Furthermore, there are no other highly magnetic white dwarfs nearby, making the neutron star explanation more plausible.

One explanation could be that the object is part of a binary system in which a neutron star or white dwarf orbits another invisible star.

This object could make us rethink our decades-old understanding of neutron stars or white dwarfs, especially in terms of how they emit radio waves and what their populations are like in our Milky Way. Further research is needed to confirm what the object is, but either scenario would provide valuable insights into the physics of these extreme objects.

The search continues

We do not know how long ASKAP J1935+2148 has been emitting radio signals, as radio astronomy surveys do not usually look for objects with such long periods. Furthermore, radio emissions from this source are only detected for only 0.01% to 1.5% of the rotation period, depending on the emission state.

So we were lucky enough to come across ASKAP J1935+2148. It is very likely that there are many more similar objects elsewhere in our Galaxy, waiting to be discovered.