About a year ago, astronomers announced that they had observed an object that should not exist. Like a pulsar, it sent out regularly timed bursts of radio emissions. But unlike a pulsar, there were more than twenty minutes between these bursts. If the 22-minute period between bursts represents the object’s rotation period, then it is rotating too slowly to produce radio emissions by any known mechanism.
Now, some members of the same team (along with new hires) are back with the discovery of something that’s behaving even more strangely. The new source of radio bursts, ASKAP J193505.1+214841.0, takes almost an hour between bursts. And it appears to have three different settings, sometimes producing weaker bursts and sometimes skipping them altogether. Although the researchers suspect that this, like pulsars, is also powered by a neutron star, it is not even clear that it is the same class of objects as their earlier discovery.
How pulsars pulse
Contrary to the chapter heading, pulsars don’t actually pulsate. Neutron stars can create this illusion by having magnetic poles that are not aligned with their rotation pole. The magnetic poles are a source of constant radio emissions, but as the neutron star rotates, the emissions from the magnetic pole move through space in a manner similar to light from a rotating lighthouse. When Earth gets involved in that motion, the neutron star appears to blink on and off as it spins.
The star’s rotation is also necessary to generate radio emissions themselves. If the neutron star rotates too slowly, the 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 the pulses to be too much time apart), it will simply turn off and we’ll stop observing radio emissions from the object.
We have no clear idea of how long the time between pulses might last before a pulsar shuts down. But we do know it will be much less than 22 minutes.
That’s why the discovery in 2023 was so strange. The object, GPM J1839–10, not only had a long time between pulses, but archival footage showed that it had been pulsing on and off for at least 35 years.
To find out what’s going on, we actually have two options. One of them is more and better observations of the source we know. 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 so many differences that it’s not entirely clear.
A puzzling find
The object, ASKAPJ193505.1+214841.0, was accidentally discovered when Australia’s Square Kilometer Array Pathfinder telescope was used to observe the area due to detection of a gamma-ray burst. He caught a bright radio burst in the same field of view, but it was unrelated to the gamma-ray burst. Later observations revealed even more radio bursts, as well as some much fainter bursts. A search of the telescope’s archives also discovered a fainter burst from the same location.
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, checking additional observations revealed that there were often instances where a 54-minute period did not end with a radio burst, suggesting that the source sometimes skips radio transmissions altogether.
Even stranger, the photons in the strong and weak bursts appeared to have different polarizations. These differences arise from the magnetic fields present where the eruptions originate, indicating that the two types of eruptions not only differ in total energy, but also that the object that produces 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 that the off mode produces weak radio signals that are below the detection capabilities of the telescopes we use. . During about eight months of sporadic observations, no clear pattern to the eruptions can be discerned.
What is this thing?
Checks at other wavelengths indicate that there is a magnetar and a supernova remnant near 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 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 ASKAP source. One is a neutron star that still manages to emit radio frequency radiation from its poles despite spinning extremely slowly. The second is a white dwarf with a reasonable rotation period, but an unreasonably strong magnetic field.
To get to the bottom of this problem, the researchers estimated the strength of the magnetic field needed to produce the larger bursts and came up with a value that is significantly higher than any previously observed value coming from a white dwarf. So they argue strongly that the source is a neutron star. Whether that argues for the prior source being a neutron star will depend on whether you think the two objects represent a single phenomenon, despite their slightly different behavior.
Anyway, 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 some information about what’s involved in switching modes. But then we’ll have to find out if what we learn applies to the ones we discovered earlier.
Natural Astronomy, 2024. DOI: 10.1038/s41550-024-02277-w (About DOIs).