A New Type of Neutron Star Observed

One of the perks of discovery that excites astronomers most is when they stumble across something odd, weird and not fitting the regular models of what they were originally searching for. There are good reasons why these models exist -  as many prior observations are refined through historical work to create data sets that feature baselines, regular parameters, and observable / testable phenomena - but that is not to say the outliers don’t pop up. 

Sometimes the outliers can be far fetched - too good to be true, artefacts of instrumentation issues or the like. And sometimes, well, they can be remarkably interesting - with the potential to change our understanding of science, introduce new objects we never knew about, and even flip those models on their heads.  

Today, a new paper has been published in Nature Astronomy by Dr Manisha Caleb from the University of Sydney which describes an outlier in the population of neutron stars - and it might be one of those cases that could change our understanding of this population across the Galaxy. 

In the new article, Dr Caleb and her collaborators describe the strange new source they found as a neutron star giving off pulsed radio emission as it rotates once every 76 seconds. Pulsed radio signals from neutron stars is nothing new - an entire class of neutron stars exists known as pulsars (with the majority of the neutron star population observed being pulsars) - but none are known to rotate this slowly. 

The discovery itself was somewhat serendipitous as Dr Caleb and her group used the MeerKAT radio telescope in South Africa (an array of 64 dishes, each 13.5-metres in diameter and spread over a larger region) to piggyback on observations being undertaken by another science group which was looking for different types of radio transients.

"Our data pipeline had originally identified this emission as radio frequency interference (RFI) due to its wide pulse duration, but luckily we always manually inspect the output," Dr Caleb told us.

The new object, named PSR J0901-4046, is somewhat weird when looking at the models of neutron stars. It is unlike the known pulsar population which have spin periods that start around 1.4 milliseconds and go up to 23.5 seconds. Nor is it just a regular neutron star, as it is emitting unique radio pulses, periodically. Its magnetic field is extremely powerful, about a quadrillion times that of your regular fridge magnet at home - giving some similarity to a handful of extremely magnetised objects known as Magnetars. 

What makes this neutron star really special, is that its long rotation period and complex magnetic field structure seem to point toward a new type of object. Dr Caleb and her team think it could belong in a theorised class of objects known as the ‘ultra-long period magnetars” - of which only one other source, found using the Murchison Widefield Array, might be associated. 

“Amazingly, we only detect radio emission from this source for 0.5% of its rotation period. This means that it is very fortuitous that the radio beam intersected with the Earth,” said Dr Caleb. “It is therefore likely that there are many more of these very slowly spinning sources in the Galaxy which has important implications for how neutron stars are born and age.”

PSR J0901-4046 is located in the constellation of Vela - an area of the sky that has been searched and catalogued over many decades in the radio regime - in fact, one of the most famous pulsars resides here (the Vela pulsar, and its associated supernova remnant). So how did this new object evade detection until this point in time?

It all came down to the way pulsar searches are conducted - often requiring astronomers to search for periods that they think the pulsar might be spinning in. And of course, according to the known population of pulsars so far, nothing really spins slower than 23.5 seconds. Until now.

“The majority of pulsar surveys do not search for periods this long and so we have no idea how many of these sources there might be,” said Dr Caleb. “In this case, the source was bright enough that we could detect the single pulses with the MeerTRAP instrument at MeerKAT.”

Neutron stars exist because of the quirky quantum behaviour of subatomic particles. They form after supergiant stars, about 8 - 25 times more massive than our Sun, become unstable at the ends of their lives and explode as spectacular supernovae. During these events, the remaining stellar core falls inwards due to gravity, and the only thing preventing the core from total collapse is an effect known as neutron degeneracy pressure - which pushes the particles away from one another and prevents neutrons to be squeezed in any closer than a certain, limited configuration within a finite volume.

The rotation of these objects is the result of a shrinking radius that occurs during the core collapse event - where the progenitor star once used to be very big in size, it is remarkably small afterwards the supernova. This is a similar effect to seeing an ice skater spinning on the spot with their arms extended and then bringing them in - as they do, their rotational velocity increases. This means that the resulting neutron star contains approximately one to two times the mass of the Sun squeezed into about a millionth of the size - with most neutron stars being around 20 kilometres in diameter - making neutron star matter the densest material in the known universe.

Intense magnetic fields on the surfaces of these tightly packed objects tend to be upwards of 10 billion times the strength of Earth’s magnetic field and can accelerate electrons up towards the star’s magnetic poles (as well as other regions) to produce luminous radiation. This emission generally detected most prominently at radio wavelengths, often creates a pulse-like effect as the rotation of the neutron star shifts the direction of this radiative beam periodically; creating a lighthouse-style effect which is why astronomers term them ‘pulsars’.

Often, neutron stars and pulsars are referred to as one of the final evolutionary phases of massive main-sequence stars, and this is true - once these giants have exhausted their fuel reserves and undergone their supernovae events, they have transitioned into new, compact remnants. 

But even neutron stars and pulsars evolve over time. For neutron stars, their surfaces cool down eventually, no longer emitting in the high-energy regime. In the case of pulsars, they lose angular velocity and their magnetic field levels subside, eventually falling below a threshold known as the ‘pulsar death line’ - where they are no longer capable of producing pulsed radio emissions due to the low magnetic field levels and slow spin periods. At this point, they just revert to becoming cooling neutron stars.   

For some of these now ‘dead’ pulsars, who reside in binary pairs, they can have their angular velocity spun up again through the accretion of materials from their companions, as they evolve (these are a sub-class known as millisecond pulsars).   

What makes J0901-4046 interesting is that it resides within the ‘pulsar graveyard zone’ - its spin is too slow to be doing what the other pulsars are doing - generating radio emissions. Unless of course, we are looking at an entirely new sub-class of neutron stars that have powerful magnetic fields but slow angular rotational velocities.

“The sensitivity that MeerKAT provides, combined with the sophisticated searching that was possible with MeerTRAP and an ability to make simultaneous images of the sky made this discovery possible. Even then it took an eagle eye to recognise it for something that was possibly a real source because it was so unusual looking!” said Dr Ian Heywood from the ThunderKAT team and the University of Oxford who collaborated on this study. 

Following the initial discovery and a review of the data, Dr Caleb set out to conduct follow-up observations to confirm the findings and the timing solution of this peculiar source, as well as other observations at different wavelengths, in the hopes that it might shed some light on this fascinating object.   

At first, they observed with the MeerKAT radio telescope and later used the CSIRO Murriyang Parkes telescope to obtain precise timing measurements confirming the periodicity of the pulses that they had observed at radio wavelengths, and then additionally, they followed up using the space-based x-ray Swift Observatory to look for any detections at these frequencies. Unfortunately, none were found in this high-energy band. 

However, the discovery of J0901-4046 does open up a range of new questions that has scientists excited - such as the possibility that many more of these types of objects exist across the Milky Way Galaxy which are not able to be seen from Earth due to their beams pointing away from our field of view.

Or maybe what we are finally observing is a missing link of the evolutionary phases of these compact objects, as they transition from magnetar to neutron star and pulsars or vice versa. 

There’s even discussion that these objects might be linked to a relatively new, and yet not-fully-understood phenomena in radio astronomy - Fast Radio Bursts - with some theories suggesting these are generated by magnetars. 

With this case now announced, further studies will hopefully find more of these objects, increasing the population that can be analysed by astrophysicists and revealing a little more about the outliers. Flipping the model on its head.