About a year ago, astronomers announced that they had observed an object that should not exist. Like a pulsar, it emitted regular bursts of radio emissions. But unlike a pulsar, those bursts were separated by more than 20 minutes. If the 22-minute gap between bursts represents the object’s rotation period, 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 anything, is acting 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 skipping them entirely. While the researchers suspect that, like pulsars, this one is powered by a neutron star, it’s not even clear that it’s the same class of object as their previous discovery.
How pulsars pulse
Contrary to the section head, pulsars do not actually pulsate. Neutron stars can create the illusion by having magnetic poles that are not aligned with their spin pole. The magnetic poles are a source of persistent radio emissions, but as the neutron star rotates, the emissions from the magnetic pole travel through space in a manner similar to light from a rotating lighthouse. If Earth happens to be caught up in that sweep, 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, then its magnetic field will not be strong enough to produce radio emissions. So it’s thought that if a pulsar’s rotation slows down enough (causing its pulses to be spaced too long), it will simply shut down and we’ll stop observing any radio emission from the object.
We don’t have a clear idea of how long the time between pulses can last before a pulsar shuts down. But we know it will be much less than 22 minutes.
That’s why the discovery of 2023 was so strange. The object, GPM J1839–10, not only took a long time between pulses, but archival images showed that it had been on and off for at least 35 years.
To understand what’s going on, we really have two options. One is the best and best observations of the source that we know of. The second is to find other examples of similar behavior. There’s a chance we now have a second object like this, though there are enough differences that it’s not entirely clear.
An enigmatic find
The object, ASKAPJ193505.1+214841.0, was discovered by chance when the Australian Square Kilometer Array Pathfinder telescope was used to survey the area due to the detection of a gamma-ray burst. He picked up a bright radio burst in the same field of view, but it was unrelated to the gamma-ray burst. Other radio bursts appeared in later observations, as well as some 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 they could be explained by an object emitting bursts every 54 minutes, with bursts lasting from 10 seconds to just under a minute. Checking additional observations, however, showed that there were often times when a 54-minute period did not end with a radio burst, suggesting that the source sometimes skipped radio emissions altogether.
Stranger still, the photons in the strong and weak bursts appeared to have different polarizations. These differences arise from the magnetic fields present where the explosions originate, suggesting that the two types of explosions 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 that produces weak radio signals that are below the detection capabilities of the telescopes we are using. . During about eight months of sporadic observations, there is no discernible pattern to the eruptions.
What is this thing?
Checks at other wavelengths show that there is a magnetar and a supernova remnant in the vicinity of the mysterious object, but not in the same location. There is also a nearby brown dwarf at that point in the sky, but they strongly suspect that this is just a random overlap. So none of this tells us more about what produces these erratic outbursts.
As with the previous finding, there appear to be two possible explanations for the source of ASKAP. One is a neutron star that is still managing to emit radio frequency radiation from its poles despite spinning extremely slowly. The second is a white dwarf that has a reasonable rotation period but an unreasonably strong magnetic field.
To get to this point, the researchers estimate the strength of the magnetic field needed to produce larger explosions and come up with a value that is significantly higher than any previously observed to originate in a white dwarf. So they strongly argue that the source is a neutron star. Whether this argues that the former source is a neutron star will depend on whether you think the two objects represent a single phenomenon, despite their somewhat different behaviors.
In any case, we now have two of these slowly repeating mysterious objects. It’s possible we’ll be able to learn more about this newest one if we can get some information on what goes into its mode switching. But then we will have to figure out if what we learn applies to what we discovered before.
Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02277-w (About DOIs).
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Image Source : arstechnica.com