Discovery of a Double Profile Change Event in a Millisecond Pulsar

The fastest spinning measured objects in the Universe are millisecond pulsars (MSPs), which rotate on their axis hundreds of times every second. Of these, the fastest is spinning 716 times per second, which is faster than the average kitchen blender. Incredibly, these objects are tiny (relatively speaking) -- a sphere measuring only about 20 kilometres across, no bigger than a small town. Yet, inside this small volume, there’s about the equivalent mass of almost 1.5 Suns squeezed in, making them also incredibly dense.

MSPs are considered very rotationally stable over the long term, especially relative to the classical pulsar population (known as canonical pulsars, like the first pulsar ever discovered by Dame Professor Jocelyn Bell Burnell). The canonical pulsars don’t rotate as fast as MSPs, and many have exhibited ‘glitch events’ - where their rotation is suddenly jolted. In contrast, astronomers have observed only three glitches across the MSP population since their discovery 44 years ago. 

Pulsars beam radio waves from their magnetic poles like a lighthouse, so when combined with this rapid and stable rotation, astronomers can use these objects as precise cosmic clocks to conduct sensitive timing experiments. This includes searching for low-frequency gravitational waves and fundamental tests of physics, such as supporting Einstein’s theory of gravity.

As the pulsar’s beam rotates into view, we see the signal of the radio light. This gives us the “pulse” per rotation (and because they are rotating in a periodic manner, we get a train of pulses -- which is where the name ‘pulsars’ comes from). No two pulses are the same; each has its own shape, which reflects the random processes that occur in the complex pulsar magnetosphere, where these radio emissions are generated.

However, because MSPs spin hundreds of times per second, astronomers can record all these pulses and, over a few minutes to hours, collect hundreds of thousands or even millions. When these are averaged together to form one pulse, this is called the integrated pulse profile, as it reveals the signal shape of all the pulses combined.

Like rotational stability, the integrated profiles of MSPs have long been thought to be very stable over the long term. However, our views of this are now starting to shift.

In 2016, astronomers reported a profile change event -- where the stable integrated profile suddenly undergoes a rapid change, then recovers - for the millisecond pulsar PSR J1643-1224. On another MSP, PSR J1713+0747, two events were noticed in 2008 and 2016, and then suddenly, a significant event occurred in 2021, where the pulsar threw a tantrum.

Now, we’ve discovered not one, but two consecutive profile change events on the closest and brightest MSP to Earth.

PSR J0437-4715 (hereafter, ‘J0437’) is a really special MSP. Dr Simon Johnston of CSIRO discovered it in 1993, and it is the closest millisecond pulsar to Earth, at a distance of 511.93 +/- 0.35 light-years (one of the most precisely measured distances to any object outside the Solar System we know). Because of its proximity, it is also the brightest MSP. It has a white dwarf companion, and the pair orbit each other every 5.75 days.

Thanks to its brightness and proximity, astronomers have studied this MSP across the electromagnetic spectrum, mostly in radio waves and high-energy waves (X-rays and Gamma rays). This makes it a favourite among astronomers, as it lets us use it as a laboratory with extreme magnetic fields, extreme gravity, and extreme radiation (conditions we can never reproduce on Earth). These studies include looking for dark matter, learning about the material between the stars in the interstellar medium, developing a better understanding of what the interiors of neutron stars are made of (which we still don’t fully understand), pulsar timing applications, and of course, trying to understand how the magnetospheres of pulsars work.

Because of its location in the southern sky, J0437 is a high-priority target for southern hemisphere pulsar timing array experiments, such as the Parkes Pulsar Timing Array (PPTA) project in Australia and the MeerKAT Pulsar Timing Array (MPTA) project in South Africa. Both projects routinely observe J0437.

Pulsar timing array projects rely on the integrated profile to remain stable over the long term (these projects run for decades) because each new observation is compared against this stable template profile to measure its arrival time, which is then compared with its predicted arrival time. When a difference is measured in its arrival time, that difference encodes a wealth of information that can include effects intrinsic to the pulsar, effects of the pulsar signal propagating across the interstellar medium, effects of the solar wind, and even effects from our ionosphere. Importantly, these unmodelled shifts in arrival time (when correlated across many MSPs) also encode the effects these experiments are looking for -- gravitational waves generated by the inspiral of supermassive black holes as distant galaxies collide across the Universe. 

Therefore, when the profile morphology (i.e. the pulse shape) varies from the stable integrated profile, it introduces a new unmodelled effect that must be characterised and accounted for.

In 2021, a study using data from Murriyang (CSIRO’s Parkes radio telescope) reported that J0437 exhibited a discrete profile change event around the central peak of the integrated pulse profile that lasted a few months. Then things got quiet and returned to normal with this MSP.

Until now.

We observe J0437 (and other MSPs) at the PPTA using Murriyang about once every three weeks, with a special instrument called the Ultra-Wideband Low-frequency (UWL) receiver, developed by CSIRO. What makes this instrument so special is that it is currently the only one of its kind on any telescope in the world that is fully operational, and can observe astrophysical objects across a wide range of frequencies - from about 704 MHz up to 4032 MHz. Along with this wide frequency coverage, Murriyang also provides excellent polarisation coverage. Polarisation describes the orientation and behaviour of the electric field of light (in this case, radio waves) as it traverses from the pulsar to us. Both of these are highly advantageous for observing profile change events, because polarisation helps constrain the event's origin and the large bandwidth captures the frequency evolution.

What we discovered is fascinating. In our data, we observed a profile change event on J0437 in April 2022. Three regions of the integrated profile (which should be stable) suddenly changed, then began a long-term recovery back toward the original profile configuration. Importantly, this was not a global profile change; it was confined to these three specific regions of the profile, which we call the ‘Left Feature ’, ‘Main Feature ’, and ‘Central Feature’. Interestingly, we found that the Main Feature had an anti-correlated response to the surrounding Left and Central Features - that is, as the Main Feature region of the profile weakened, the surrounding two features strengthened. Over time, as the profile recovered, the role reversed: the Main Feature increased in power while the outer two features decreased.

But the biggest surprise was yet to come.

When analysing the April 2022 event, we noticed that the data before the April 2022 event onset (and stretching back to the end of 2018, when our dataset began) also showed the same variation pattern as the current event, but with a much earlier onset. We analysed this carefully, looked at even older data, and found another profile-change event on J0437 in November 2017. Remarkably, this earlier event also affected the same three phase locations and showed the same anti-correlated behaviour with similar temporal evolution.

This is the first time this behaviour has been reported on any MSP -- not one, but two consecutive profile change events, almost like a recurrent process.

To make sure what we were seeing was real and not an instrumental issue, we collaborated with colleagues from the MPTA who also observed J0437. They also observed the same April 2022 event and the same recovery tail from the 2017 event, occurring with the same three features and the same temporal signature.

Bingo! Two independent telescopes observing the same profile change behaviour confirms that what we were seeing was astrophysical, rather than a telescope-related issue. 

To determine the origin of profile change events in MSPs, we take into account several factors. This includes consideration of changes that happen localised to the star itself (for example, a starquake, a disturbance in the magnetosphere, or influence from a binary companion). We also consider changes that are induced as the pulsar’s signal crosses interstellar space (such as passing through a blob of magnetised plasma) and even when the signal crosses into the Solar System and is affected by the Sun’s activity (like the solar wind, or variations in Earth’s ionosphere). To be absolutely sure, we also run many tests to ensure our telescope and processes aren't creating something that isn't real.

In this study (as well as the PSR J1713+0747 case), we found that profile change events originate in the magnetospheres of millisecond pulsars. However, a major mystery remains -- we still do not know what triggers these events.

Pulsar magnetospheres are extremely complex environments, and we do not fully understand them. We know they are filled with a plasma of particles that co-rotate with the star out to a certain distance, and radio emissions are generated in this highly magnetised plasma; however, there is no consensus on how these emissions are generated. 

These radio emissions are highly polarised, and this polarisation tells us a lot about the magnetised environment as well as how the emission propagates through it. When we receive the pulsar signal, we also see its ‘polarisation profile’. We found that the polarisation profiles changed only in specific regions, and in a way related to the two modes of wave propagation (known as the orthogonal polarisation modes). The polarisation effects were the smoking gun that told us this was indeed magnetospheric in origin, as only three specific regions of the profile and corresponding polarisation effects changed, rather than the broader effect we would expect from other causes like the magnetised blob in the interstellar medium.

While we don’t fully understand what triggers these events, they now let us probe how the magnetosphere reacts to sudden changes and how it slowly recovers over time. These events let us look inside these complex environments and learn more about them.

The consecutive profile change events we have now reported on for J0437 join the significant 2021 event on PSR J1713+0747 (and its previous 2008 and 2016 events), the previous J0437 2015 event, as well as the 2015 event on PSR J1643-2124. 

These events are pulsar-independent and not correlated; however, a trend is beginning to emerge. What we once thought were stable integrated profiles are likely varying because we're observing them longer (i.e., our datasets are growing). Additionally, our instrumentation is becoming much more sensitive, so we can probe these events at much smaller scales, whereas they once likely existed below the noise floor.

In the last two weeks, two additional studies have also been published. The first, by University of Manchester PhD candidate Bhavnesh Bhat, looked for profile change events in the MeerKAT Pulsar Timing Array project dataset and found many cases. In addition to the findings of profile variations, Bhat’s paper also found a correlation with signal-to-noise. Another PhD candidate, We Li from Xingjiang Astronomical Observatory, reported a deeper dive on PSR J1547-5709, which also highlights that this MSP had a significant profile change event.

These findings are reshaping our perspective on the stability of millisecond pulsar profiles, and sensitive timing experiments -- such as pulsar timing array projects -- are now exploring methods to characterise, model, and mitigate these variations. That’s because these variations induce unmodelled noise that is much more dominant than the feeble gravitational wave signals that these experiments are searching for. 

Additionally, radio astronomers are about to turn on some very powerful new telescopes, including the SKA, DSA-1000 and the ngVLA. With higher sensitivity and as our datasets continue to grow, we will likely start to see an emerging population of these events, which will provide important insights into these complex environments. 

Each time we observe one of these events, another little piece of the puzzle on how millisecond pulsar magnetospheres work is revealed, and these powerful next-generation telescopes are about to give us many more pieces to work with.

We acknowledge the Wiradjuri people as the traditional land owners where Murriyang, the CSIRO Parkes radio telescope observatory, is located.

PSR J0437-4715 pulsar and white dwarf video animation credit: Credit: Carl Knox / OzGrav.