About a year ago, astronomers announced that they had observed an object that should not exist. Like a pulsar, it emitted regularly synchronized bursts of radio emissions. But unlike the pulsar, these bursts were separated by more than 20 minutes. If the 22-minute interval between bursts represents the rotation period of the object, then it is rotating too slowly to produce radio emissions by any known mechanism.
Now, some of the same team (along with new collaborators) are back with the discovery of something that, if nothing else, is behaving even more strangely. The new radio burst source, ASKAP J193505.1+214841.0, takes almost an hour between bursts. And it seems to have three different settings, sometimes producing weaker bursts and sometimes missing them altogether. Although the researchers suspect that, like pulsars, this one is powered by a neutron star, it’s not even clear if it’s the same class of object as their earlier discovery.
How pulsars pulsate
Contrary to the section title, pulsars do not actually pulsate. Neutron stars can create the illusion by having magnetic poles that are not aligned with their rotation pole. The magnetic poles are the source of constant radio emissions, but as the neutron star rotates, the emission from the magnetic pole travels through space in a manner similar to the light from a rotating lighthouse. If the Earth happens to be caught in this motion, then the neutron star will appear to blink on and off as it rotates.
The rotation of the star is also necessary for the generation of 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 believed that if the rotation of a pulsar is slowed down enough (with its pulses separated by too much time), it will simply shut down and we will stop observing any radio emissions from the object.
We don’t have a clear idea of ​​how long it can take between pulses before the pulsar shuts down. But we know it will take much less than 22 minutes.
That’s why the 2023 discovery was so strange. The object, GPM J1839–10, not only took a long time between pulses, but archival images showed that it had been pulsing on and off for at least 35 years.
To understand what’s going on, we really have two options. One is more and better observations of the source we know about. The second is to find other examples of similar behavior. There’s a chance we now have a second object like this, although there are enough differences that it’s not entirely clear.
An enigmatic find
The object, ASKAPJ193505.1+214841.0, was discovered by chance when Australia’s Square Kilometer Array Pathfinder telescope was used to make observations in the area due to the detection of a gamma-ray burst. It caught a bright radio burst in the same field of view, but unrelated to the gamma-ray burst. Additional radio bursts appeared in later observations, as well as several much weaker bursts. A search of the telescope’s archives also revealed a fainter burst from the same location.
By checking 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, inspection of additional observations showed that there were often cases where a 54-minute period did not end with a radio burst, suggesting that the source sometimes missed radio emissions entirely.
Even more strangely, the photons in the strong and weak bursts appear to have different polarizations. These differences arise from the magnetic fields present where the bursts originate, suggesting that the two types of bursts differ not only in total energy, but also that the object making them has a different magnetic field.
So the researchers suggest that the object has three modes: strong pulses, weak pulses and an off mode, although they cannot rule out an off mode producing weak radio signals that are below the detection capabilities of the telescopes we use. During about eight months of sporadic observations, there was no apparent pattern to the outbreaks.
what is that thing
Checks at other wavelengths show that there is a magnetar and a supernova remnant in the vicinity of the mystery object, but not in the same location. There is also a nearby brown dwarf at this point in the sky, but they strongly suspect that this is just a coincidental overlap. So none of this tells us more about what produces these chaotic bursts.
As with the earlier finding, there appear to be two possible explanations for the source of ASKAP. One is a neutron star that still manages to emit radio frequency radiation from its poles, even though it rotates extremely slowly. The second is a white dwarf that has a reasonable rotation period but an unreasonably strong magnetic field.
To address this problem, the researchers estimated the strength of the magnetic field needed to produce the larger bursts, and arrived at a value that was significantly higher than any previously observed to originate from a white dwarf. So they strongly argue that the source is a neutron star. Whether this is an argument for the earlier source being a neutron star will depend on whether you consider the two objects to represent the same phenomenon, despite their somewhat different behavior.
In any case, we now have two of these mysterious slowly repeating objects to explain. It’s possible we can learn more about this newer one if we can get information on what’s involved in its mode switching. But then we’ll have to figure out if what we learn applies to what we discovered earlier.
Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02277-w (About DOI).
