(ORDO NEWS) — 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 this discovery while observing the Vela-X 1 region of the Milky Way, about 1300 light-years from Earth, using the MeerKAT radio telescope in South Africa. We noticed a strange-looking flash or “pulse” that lasted about 300 milliseconds.
The flare had some of the characteristics of a radio-emitting neutron star. But it was not like what we had seen before.
Intrigued, we looked at older data from this region in the hope of finding similar impulses. Interestingly, we found more of these pulses that were previously missed by our real-time pulse detection system (since we typically look for pulses around 20-30 milliseconds long).
A quick analysis of the arrival times of the pulses showed that they repeat about every 76 seconds – while most neutron star pulses take a few seconds or even milliseconds.
Our observations have shown that PSR J0941-4046 has some of the characteristics of a “pulsar” or even a “magnetar”. Pulsars are extremely dense remnants of decayed giant stars that normally radiate radio waves from their poles.
As they rotate, radio pulses can be measured from Earth, much like you see a beacon flashing periodically in the distance.
However, the longest pulsar rotation period known so far was 23.5 seconds, which means we may have found an entirely new class of radio-emitting objects. Our results are published today in the journal Nature Astronomy.
An anomaly among neutron stars?
Using all the data available to us from the MeerTRAP and ThunderKAT projects on MeerKAT, we were able to determine the position of the object with excellent accuracy. After that, we made more sensitive follow-up observations to study the source of the impulses.
The newly discovered object, named PSR J0941-4046, is an unusual radio-emitting galactic neutron star that rotates extremely slowly compared to other pulsars. The frequency of pulsars is incredibly constant, and our subsequent observations have allowed us to predict the arrival time of each pulse to within 100 millionths of a second.
In addition to the unexpected frequency of pulsations, PSR J0941-4046 is also unique in that it is located in the “graveyard” of neutron stars. This is a region of space where we do not expect to detect any radio emission, since neutron stars are thought to be at the end of their life cycle here and therefore not active (or less active).
PSR J0941-4046 challenges our understanding of how neutron stars are born and evolve.
It is also interesting because it produces at least seven pulses of different shapes, while most neutron stars do not show such a variety. Such a variety of pulse shapes, as well as their intensity, are probably related to the unknown physical mechanism of the object’s radiation.
One particular type of pulse has a pronounced “quasi-periodic” structure, which suggests that the radio emission is caused by some kind of fluctuations. These pulses can give us valuable information about the internal workings of PSR J0941-4046.
These quasi-periodic pulses bear some resemblance to the 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 that is observed in fast radio bursts. If we find that this is the case, then it may turn out that PSR J0941-4046 is an “extra-long period” magnetar.
Magnetars are neutron stars with very powerful magnetic fields, of which only a few are known to emit in the radio part of the spectrum. Although we have not yet discovered an ultra-long period magnetar, they are hypothesized to be a possible source of fast radio bursts.
It is not known how long PSR J0941-4046 was active and emitted in the radio band, as radio surveys do not usually look for such long periods.
We do not know how many such sources may exist in the galaxy. In addition, we can detect radio emission from PSR J0941-4046 only during 0.5 percent of its rotation period – thus, it is only visible to us for a fraction of a second. It is a great fortune that we were able to discover it from the very beginning.
Finding such sources is challenging, which implies that there may be a larger undetected population waiting to be discovered. Our finding also increases the likelihood of a new class of radio transition objects: ultra-long period neutron stars.
Future searches for such objects will be vital to our understanding of the neutron star population.
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