The discovery of a neutron star emitting unusual radio signals is rewriting our understanding of these unique star systems.
My colleagues and I (the MeerTrap team) made this discovery using the Meerkat radio telescope in South Africa while observing the Vela-X1 region of the Milky Way, about 1,300 light-years away from Earth. We saw a strange looking flash or “pulse” that lasted about 300 milliseconds.
The flash had some of the characteristics of a radio-emitting neutron star. But it was not like anything we had seen before.
Curiously, we scoured older data from this region in the hope of finding similar pulses. Interestingly, we identified more pulses that were previously missed by our real-time pulse detection system (since we typically only search for pulses lasting 20-30 milliseconds).
A quick analysis of the pulses’ arrival times showed them to repeat about every 76 seconds – whereas most neutron star pulses cycle within a few seconds or even milliseconds.
Our observations showed that PSR J0941-4046 had some of the characteristics of a “pulsar” or “magnator”. Pulsars are extremely dense remnants of collapsed giant stars that usually emit radio waves from their poles.
Radio pulses can be measured from Earth as they rotate, much like how you see a lighthouse periodically in the distance.
However, before that the longest known rotation period for a pulsar was 23.5 seconds – meaning we may have found an entirely new class of radio-emitting object. Our findings are published today in nature astronomy,
An anomaly between neutron stars?
Using all the data we have available from the Miratrap and Thundercat projects in Meerkat, we managed to pinpoint the position of the object with excellent accuracy. We then carried out our more sensitive follow-up observations to study 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 rates are incredibly consistent, and our follow-up observations have allowed us to predict the arrival time of each pulse to 100 millionths of a second.
In addition to the unpredictable pulse rate, PSR J0941-4046 is also unique because it resides in a neutron star “graveyard”. This is a region of space where we do not expect to detect any radio emissions, as it is theorized that neutron stars here are at the end of their life cycles and therefore no longer active (or less active).
PSR J0941-4046 challenges our understanding of the birth and evolution of neutron stars.
It is also attractive because it appears to produce at least seven different pulse sizes, whereas most neutron stars do not exhibit such diversity. This variation in pulse size, and pulse intensity, is probably related to the object’s unknown physical emission mechanism.
A particular type of pulse shows a strongly “quasi-periodic” structure, which suggests that some sort of oscillation is driving the radio emission. These pulses can provide us with valuable information about the inner workings of PSR J0941-4046.
These quasi-periodic pulses are similar to mysterious fast radio bursts, which are short radio bursts of unknown origin. However, it is not yet clear whether PSR J0941-4046 emits the energy seen in the fast radio burst. If we find that it does, it could be that PSR J0941-4046 is an “ultra-long period magnetar”.
Magnetars are neutron stars with very strong magnetic fields, of which only a handful are emitted in the radio part of the spectrum. While we have yet to actually identify an ultra-long period magnetar, they are thought to be a potential source of fast radio bursts.
brief visits
It is not clear how long PSR J0941-4046 has been active and emitting across the radio spectrum, as radio surveys usually do not find such long periods.
We do not know how many of these sources may be present in the Milky Way. Plus, we can only detect radio emission from PSR J0941-4046 for 0.5 percent of its rotation period—so it’s only visible to us for a fraction of a second. It’s very fortunate that we were able to have it in the first place.
Finding similar sources is challenging, which means there may be a large undetermined population waiting to be discovered. Our discovery also adds to the possibility of a new class of radio transient: ultra-long period neutron stars.
Future exploration of similar objects will be important to our understanding of neutron star populations. 
Manisha Caleb, Lecturer, University of Sydney.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
