Editor’s note: Manisha Caleb is a professor at the University of Sydney; Emil Lenc is a researcher at the Commonwealth Scientific and Industrial Research Organization (CSIRO), also in Australia.
When we astronomers turn our radio telescopes toward space, we sometimes detect sporadic “bursts” of radio signals originating from deep within the Universe. We call them “radio transient signals”: some erupt just once, never to be seen again, while others “blink” in predictable patterns.
We believe that most radio transient signals come from rotating neutron stars known as pulsars, which emit regular flashes of radio waves, like cosmic beacons. Typically, these neutron stars spin at incredible speeds, taking just a few seconds or even a fraction of a second to complete each rotation.
We recently discovered a radio transient that looks like nothing astronomers have seen before. In addition to having a cycle lasting almost an hour (the longest ever seen), over the course of several observations we saw that sometimes it emitted long, bright flashes, sometimes quick, weak pulses, and sometimes nothing at all.
We can’t explain what’s going on here. Most likely it is a very unusual neutron star, but we cannot rule out other possibilities. Our search was published in the magazine Nature Astronomy.
A lucky find
Meet ASKAP J1935+2148 (the numbers in the name indicate its location in the sky). This periodic radio transient was discovered using the ASKAP radio telescope of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Wajarri Yamaji Country, in outback Western Australia.
This radio telescope has a very wide field of view, which means it can survey large swaths of the Universe very quickly. This makes it very suitable for detecting new and exotic phenomena.
Using ASKAP, we were simultaneously monitoring a gamma ray source and looking for pulses of a fast radio burst, when we detected ASKAP J1935+2148 slowly blinking in the data. The signal drew attention because it was made up of “circularly polarized” radio waves, meaning the direction of the waves rotates like a corkscrew as the signal travels through space.
Our eyes cannot differentiate between circularly polarized light and ordinary unpolarized light. However, ASKAP works like a pair of polarizing sunglasses, filtering out 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
ASKAP J1935+2148 belongs to the relatively new class of long-period 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 saw ASKAP J1935+2148 in three distinct states, or modes.
In the first state, we see bright, linearly (rather than circularly) polarized pulses lasting 10 to 50 seconds. In the second state, there are much weaker, circularly polarized pulses lasting just 370 milliseconds. The third state is a silent or extinguished state without any pulse.
These different modes, and the switching between them, may result from an interaction of complex magnetic fields and plasma flows 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 be able to 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 rotating neutron star being the main suspect. However, we cannot rule out the possibility that the object is a white dwarf – the Earth-sized “ash” of a star like the Sun that has exhausted its nuclear fuel.
White dwarfs generally have slow rotation periods, but we don’t know how one could produce the radio signals we’re seeing here. Furthermore, there are no other highly magnetic white dwarfs nearby, which makes 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 cause us to reconsider our decades-old understanding of neutron stars or white dwarfs, especially how they emit radio waves and what their populations are like in our galaxy. More research is needed to confirm what the object is, but either scenario would provide valuable information about the physics of these extreme objects.
The search continues
We don’t know how long ASKAP J1935+2148 has been emitting radio signals, as radio astronomy surveys don’t typically look for objects with such long periods. Furthermore, radio emissions from this source are only detected during just 0.01% to 1.5% of its rotation period, depending on its emission state.
Therefore, we were very lucky to spot ASKAP J1935+2148. It is very likely that there are many other objects like this in other parts of our galaxy, waiting to be discovered.
This article is republished from The Conversation. Read the original article.
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