Slowly rotating neutron star: a new class of objects?
Surprise! A slowly spinning neutron star
The discovery of a neutron star emitting unusual radio signals rewrites our understanding of these unique star systems.
My colleagues and I (the MeerTRAP team) made the discovery while observing the Vela X-1 region of the Milky Way about 1,300 light-years from Earth, using the MeerKAT radio telescope in South Africa. We spotted a strange looking flash or “pulse” that lasted about 300 milliseconds.
The flash had some characteristics of a radio-emitting neutron star. But it was unlike anything we had seen before.
Intrigued, we scoured older data from the area in hopes of finding similar legumes. Interestingly, we identified more such pulses that were previously missed by our real-time pulse detection system (since we’re usually only looking for pulses that are around 20-30 milliseconds in duration) .
A quick analysis of the arrival times of the pulses showed that they repeated every 76 seconds or so, whereas most neutron star pulses repeat within seconds or even milliseconds.
Our observation showed that PSR J0941-4046 had some of the characteristics of a pulsar or even a magnetar. Pulsars are the extremely dense remnants of collapsed giant stars that typically radiate radio waves from their poles. As they spin, the radio pulses can be measured from Earth, much like seeing a lighthouse flashing periodically in the distance.
However, the longest known rotation period for a pulsar before that was 23.5 seconds, which means we could have found a whole new class of radio-emitting objects. Our conclusions are published May 30, 2022, at natural astronomy.
An anomaly among neutron stars?
Using all the data we have from the MeerTRAP and ThunderKAT projects at MeerKAT, we managed to locate the position of the object with excellent accuracy. After that, we made our most sensitive follow-up observations to investigate the source of the pulses.
The newly discovered object, named PSR J0941-4046, is a peculiar radio-emitting galactic neutron star that spins extremely slowly compared to other pulsars. Pulsar pulse frequencies are incredibly constant, and our tracking observations have allowed us to predict the arrival time of each pulse to the nearest 100 millionth of a second.
Besides the unexpected pulse rate, PSR J0941-4046 is also unique because it resides in the “graveyard” of neutron stars. This is a region of space where we don’t expect to detect radio emissions at all, as it is theorized that neutron stars here are at the end of their life cycle and therefore not active. (or less active). PSR J0941-4046 challenges our understanding of the birth and evolution of neutron stars.
It is also fascinating because it appears to produce at least seven distinct pulse shapes, whereas most neutron stars do not exhibit such a variety. This diversity in pulse shape, as well as pulse intensity, is likely related to the object’s unknown physical emission mechanism.
A particular type of pulse shows a strong quasi-periodic structure, suggesting that some sort of oscillation is causing the radio emission. These pulses can provide us with valuable information about the inner workings of the PSR J0941-4046.
These quasi-periodic pulses bear some resemblance to enigmatic fast radio bursts, which are short radio bursts of unknown origin. However, it is not yet clear whether PSR J0941-4046 emits the type of energy seen in fast radio bursts. If we find this to be the case, then it could be that PSR J0941-4046 is an “ultra long period magnetar”.
Magnetars are neutron stars with very strong magnetic fields. Only a handful are known to transmit in the radio part of the spectrum. Although we have not yet identified an ultra-long-period magnetar, they are theorized as a possible source of fast radio bursts.
Lucky to spot it
It is not known how long PSR J0941-4046 has been active and broadcasting in the radio spectrum, as radio soundings do not usually look for such long periods.
We don’t know how many of these sources might exist in the galaxy. Additionally, we can only detect radio emissions from PSR J0941-4046 for only 0.5% of its rotational period, so it is only visible to us for a fraction of a second. It’s lucky enough that we were able to spot it in the first place.
The detection of similar sources is difficult. And this challenge implies that there could be a larger undetected population waiting to be discovered. Our discovery also adds to the possibility of a new class of radio transients: the ultra-long-period neutron star. Future searches for similar objects will be critical to our understanding of the neutron star population.
Conclusion: The slowly rotating neutron star PSR J0941-4046 challenges our understanding of the birth and evolution of neutron stars.