About a year ago, astronomers announced that they had spotted an object that shouldn’t exist. Like a pulsar, it regularly emits timed bursts of radio emission. But unlike a pulsar, these bursts were separated by more than 20 minutes. If the 22-minute gap between bursts represents the object’s rotation period, it rotates too slowly to produce radio emissions by any known mechanism.
Now, some members of the same team have returned (with new collaborators) to discover something that is, if anything, acting even weirder. The new source of radio bursts, ASKAP J193505.1+214841.0, takes approximately an hour between bursts. It appears to have three different settings, sometimes producing weaker bursts and other times skipping them entirely. While researchers suspect that this, like pulsars, is also powered by a neutron star, it’s not even clear that it’s in the same class of objects they’ve previously discovered.
How do pulsars pulsate?
Contrary to the section title, pulsars do not actually pulsate. Neutron stars can create the illusion of having magnetic poles that are not aligned with their spin pole. The magnetic poles are a source of constant radio emissions, but as the neutron star rotates, the emissions from the magnetic pole sweep through space in a way similar to light from a rotating lighthouse. If Earth happened to be caught in this sweep, the neutron star would appear to blink on and off as it rotates.
The star’s rotation is also necessary to generate the radio emissions themselves. If the neutron star rotates too slowly, its magnetic field will not be strong enough to produce radio emissions. So, it is thought that if the pulsar’s rotation slowed down enough (causing its pulsations to separate for a very long time), it would simply stop, and we would stop observing any radio emissions from the object.
We have no clear idea of how long the time between pulsations can last before a pulsar stops working. But we know it will be much less than 22 minutes.
That’s why the 2023 discovery was so strange. Not only did the object, GPM J1839–10, take a long time between pulsations, but archival images show that it has been pulsating on and off for at least 35 years.
To find out what’s going on, we really have two options. The first is to make more and better observations of the source we know. The second is to find other examples of similar behavior. There is a possibility that we may now have a second object like this, although there are enough differences that it is not entirely clear.
Mysterious discovery
The object, ASKAPJ193505.1+214841.0, was discovered by accident when the Australian Pathfinder Telescope was used to observe the area due to the detection of a gamma-ray burst. It picked up a bright radio burst in the same field of view, but it was unrelated to the gamma-ray burst. More radio bursts appeared in subsequent observations, as did a few much weaker bursts. A search of the telescope’s archives also discovered a weaker explosion from the same location.
By examining the timing of the radio bursts, the team found that they could be explained by an object emitting bursts every 54 minutes, with bursts lasting from 10 seconds to just under a minute. However, verification of additional observations showed that there were often cases in which the 54-minute period did not end with a radio burst, indicating that the source sometimes exceeded the radio emissions entirely.
What’s even stranger is that the photons in the strong and weak bursts seem to have different polarizations. These differences arise from the magnetic fields present where the jets originate, suggesting that the two types of jets differ not only in overall energy but also that the object producing them has a different magnetic field.
So, the researchers suggest that the object has three modes: strong pulsations, faint pulsations, and an off mode, although they cannot rule out an off mode that produces weak radio signals below the detection capabilities of the telescopes we use. . Over eight months of sporadic observations, there was no clear pattern of explosions.
What is this thing?
Checks at other wavelengths indicate the presence of a magnetar and a supernova remnant near the mysterious object, but not in the same location. There’s also a brown dwarf nearby at that point in the sky, but they strongly suspect that’s just chance interference. So none of this tells us much about what produces these irregular explosions.
As with the previous discovery, there appear to be two possible explanations for the source of ASKAP. One is a neutron star that is still able to emit radio frequency radiation from its poles despite rotating very slowly. The second is a white dwarf with a reasonable rotation period but an unreasonably strong magnetic field.
To solve this problem, the researchers estimated the strength of the magnetic field needed to produce the larger jets and arrived at a value much higher than any previously observed originating on a white dwarf. So they strongly argue that the source is a neutron star. Whether this justifies the former source being a neutron star will depend on whether you feel that the two objects represent a single phenomenon despite their somewhat different behavior.
Anyway, we now have two of these slow-repeating mysteries to explain. It is possible that we will be able to learn more about this latest version if we can get some information about what the mode switch entails. But then we will have to see if what we learn applies to what we discovered previously.
Natural Astronomy, 2024. DOI: 10.1038/s41550-024-02277-s (About digital IDs).
