About a year ago, astronomers announced that they had observed an object that should not exist. As a pulsar, it emitted regularly timed bursts of radio emissions. But unlike a pulsar, there were more than 20 minutes between these bursts. If the 22-minute gap between bursts represents the rotation period of the object, then the object 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, seems even stranger. The new source of radio pulses, ASKAP J193505.1+214841.0, lasts almost an hour between pulses. And it seems to have three different settings, sometimes creating weaker bursts and sometimes skipping them altogether. Although the researchers assume that, like pulsars, this one is powered by a neutron star, it is not even clear that it is 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 an illusion by 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 emission from the magnetic pole spreads through space much like light from a rotating lighthouse. If the Earth happens to be caught in this tug, then the neutron star will appear to spin on and off.
The rotation of the star is also necessary for the generation of radio emissions itself. If a neutron star rotates too slowly, its magnetic field will not be strong enough to produce radio emissions. So it’s thought that if the pulsar’s rotation slows down enough (causing its pulses to be separated by too long), it will simply shut down and we’ll stop seeing any radio emissions from the object.
We don’t have a clear idea of ​​how long it might take between pulses for a pulsar to shut down. But we know it will be 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 find out 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.
Mysterious find
The object ASKAPJ193505.1+214841.0 was discovered by chance when Australia’s Square Kilometer Array Pathfinder telescope was used to observe the area for a gamma-ray burst detection. It picked up a bright radio flash in the same field of view, but it was 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 found a fainter burst from the same location.
Checking the timing of the radio flashes, the team found that they could be explained by an object emitting flashes every 54 minutes, with flashes lasting from 10 seconds to less than a minute. However, inspection of additional observations showed that there were often cases where the 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 had different polarizations. These differences result from the magnetic fields present where the explosions originate, suggesting that the two types of explosions differ not only in total energy, but also in that the object producing them has a different magnetic field.
So the scientists suggest that the object has three modes: strong pulses, weak pulses and an off mode, although they cannot rule out that the off mode produces weak radio signals that are below the detection capability of the telescopes we use. . In about eight months of sporadic observations, there is no apparent pattern to the explosions.
What is this thing?
Checks at other wavelengths suggest that there is a magnetar and a supernova remnant near the mystery 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 coincidental overlap. So none of this tells us more about what causes these erratic outbursts.
As with the earlier finding, there appear to be two possible explanations for the source of ASKAP. One of these is a neutron star that can still emit radio frequency radiation from its poles despite rotating extremely slowly. The other is a white dwarf, which has a reasonable rotation period but a disproportionately strong magnetic field.
To get at this problem, scientists estimate the strength of the magnetic field needed to create larger explosions, and arrive at a value that is significantly higher than any previously observed to come from a white dwarf. So they strongly argue that the source is a neutron star. Whether this suggests that the former source was a neutron star will depend on whether you feel that the two objects represent a single phenomenon despite their somewhat different behavior.
In any case, we now have two of these mysterious slowly repeating objects to explain. It is possible that we will be able to learn more about this newer one if we can get some information regarding its mode switching. But then we will have to see if what we learn also applies to what we have discovered before.
Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02277-w (About DOI).