Pulsar Glitches after 30 Years

At 12:26 pm on the 9th of October 2019, give or take 12 minutes, radio pulses originating from a massive compact object known as a pulsar located in the constellation Vela jolted from the persistent, rhythmic and repetitive cycle that had been observed for over 30 years.

The ever so small change in the pulsar’s signal arrival time had been traveling for what scientists estimate to be roughly 9,700 years, sweeping past the Earth like it were a grain of sand on a beach, as a small breaking wave gently washed over it.

As it passed over an unusual looking telescope, on the outskirts of Canberra, Swinburne Ph.D. student Marcus Lower continued his observations – looking at radio signals and data streaming in from elsewhere in the sky. A massive object in the far reaches of the universe had suddenly undergone a dramatic, yet minute change – and not a single soul on Earth knew it had happened.

It was two months later, on the 10th December 2019 that something intrigued Marcus, who was reviewing the data that the telescope was producing. 

“I was at my desk in the graduate student offices at Swinburne, skimming through our quick-look results for the more than 400 pulsars we monitor with the Molonglo telescope when I noticed J0908 had glitched” said Marcus.

Whilst such a minuscule change in a niche field of astrophysics might seem insignificant, it’s these glitches that tell scientists about some of the most complex states of matter, answering questions that relate all the way back to Einstein’s famous 1916 paper: The Foundation of the General Theory of Relativity.

Using the Molonglo Observatory Synthesis Telescope (MOST), an Australian team of scientists has, for the first time, caught a glitch in a Pulsar (aptly named: PSR J0908-4913) that has been studied for over 30 years. The glitch has caused the Pulsar’s signal to arrive early indicating that something has caused the neutron star to start spinning faster and providing an opportunity to analyse what these dense remnant cores of former massive stars could be made from – a mystery in astrophysics that has been ongoing for decades.

“Finding new glitches is always exciting, but I was quite surprised after checking glitch catalogues to learn J0908 had never been observed to glitch. Particularly after noting it had been regularly monitored by the Parkes telescope for more than 20 years” he said.

To date, PSR J0908-4913 has not corrected itself back to its original spin slow-down rate - a phenomenon that interests scientists, as it could help describe the internal structure of neutron stars or how matter behaves under such extreme conditions. 

Returning to the original observed spin-down rate (recovery) has been observed in other pulsars who have also experienced glitches, but later returned to their original declining rates prior to the event occurring. Some pulsar recovery periods have occurred over days, others years. 

“There’s no evidence for any recovery so far. We’re still observing the pulsar to see if any recovery does occur, as that might tell us something about the frictionless superfluid inside the neutron star” continued Marcus.

The new findings, published in the December 2019 issue of the Research Notes of the AAS, was undertaken by Australian scientists from a range of institutions, including:

• Swinburne University of Technology
• CSIRO Astronomy and Space Science
• OzGrav: ARC Centre of Excellence for Gravitational-Wave Discovery
• ARC Centre of Excellence for All-sky Astrophysics
• The University of Sydney

The observational data were collected using the MOST.

The first time most people see the Molonglo Observatory Synthesis Telescope (MOST) always results in an odd reaction. Unlike the conventional image conjured when thinking about telescopes (the big radio dish or the dome housing an optical telescope), MOST is in the shape of a giant cross – with better holistic views seen from above, such as those found on Google Earth. The observatory is located approximately 40 km to the south-east of Australia’s capital, Canberra.

“When people first see MOST, they often mistake it for a giant irrigator!” said astronomer Dr. Chris Flynn. “It makes a tremendous impression for its size and scope – timing some of the Universe’s most accurate clocks and searching for bursts originating billions of years ago”.

The MOST is designed as two long, intersecting semi-cylinders (‘arms’), that resembles the half-pipe used by an adventurous skateboarder. One arm is aligned East-West and the other North-South. The arms are 1.6 km long each, crossing perpendicular in the middle. Each semi-cylinder is about 12 m in diameter. Whilst the North-South arm is no longer operational (but currently undergoing upgrades), the East-West arm forms the MOST, which features 7,744 circular antennas, allowing right-hand circular polarised radio emissions to be observed with the central frequency of 835 MHz, in a chunk 31.25 MHz wide.

The design of the Molonglo observatory originated from Australian radio astronomy pioneer Bernard Mills. Before designing the Molonglo Observatory, Mills built the Mills Cross Telescope out at Badgerys Creek, about 40 km west of Sydney. The Mills Cross Telescope also featured two intersecting arms of North-South, East-West alignment and operated at 85.5 MHz frequency. During its operation, the Mills Cross telescope surveyed over 2,000 discrete radio emission sources.

“Pulsars were just discovered as MOST was just being finished, which meant it suddenly and unexpectedly had the opportunity to search for and discover the richer pulsar skies of the Southern Hemisphere” said Dr. Flynn.

Learnings and success from the Mills Cross Telescope allowed Bernard Mills to refine the design and build of the pioneering work produced by the father of radio astronomy, Groter Reber, which lead to the construction of the Molonglo Observatory in 1960. For his contributions to radio astronomy, Mills was awarded the 2006 Grote Reber medal, and today a memorial to Reber – inclusive of some of his ashes – remains at the site.

Whilst the observatory is owned and operated by the University of Sydney, a major collaborative partner for the (UTMOST) project is provided by Swinburne University of Technology with a focus on probing the radio transient sky in real-time, monitoring pulsars and magnetars and searching for elusive and mysterious Fast Radio Bursts (FRBs).

As part of UTMOST, the North-West arm is also being upgraded to return operations to the full cross telescope and help localise the host galaxies from which FRBs originate.

Like the rest of the Australian radio astronomy community, Dr. Flynn - who also works regularly on-site at MOST, is looking forward to the science that the upgrade generates.

“We've started taking first observations with the close-to-finalised system, validating its performance. Major deployment of the new technology on the N-S arm is slated for the early new year. The new system will be 10 times more sensitive than the E-W arm, and we'll only need to outfit a small part of the N-S arm to match the existing system. With both arms very sensitive, Fast Radio Bursts will be so accurately localised in celestial position that we'll know which galaxy they come from, and we'll be able to tell their distances and the travel time for them to reach the Earth. This, in turn, will allow us to "weigh the Universe" by determining how much matter they have passed through to reach us” he said.

When massive stars explode in a supernova, most of the star's material is thrown off into space. However, the core is compressed during the event and this causes electrons and protons to be squeezed together, resulting in neutrons.

The post-supernova object is thus a compact, massive sphere, approximately 20 km across, known as a neutron star. On average, neutron stars have about 1.4 times the mass of our Sun squeezed into this small city-sized object making them extraordinarily dense.

Angular momentum is conserved during the core-collapse supernova event, and as a result – the neutron star that is created rotates at enormous speed. Due to the powerful magnetic fields and this rotation, energetic radio beams are accelerated from the poles and into space – much like a lighthouse beam sweeps across a dark ocean sky.

When these beams sweep past the Earth, observatories record these ‘pulses’ (hence the name of the object is a pulsar) and can use this data to precisely measure the signal time of arrival, along with a range of additional data – such as what lies between the Earth and the pulsar (known as the interstellar medium), the polarisation of the beam, how fast the pulsar rotates and by how much it is slowing down per second. The precision of these measurements is so great, it can extend as far as 18 decimal places.

Furthermore, due to the high population of binary stars in our Universe, some neutron stars actually form part of a binary system - where one of the pair will first become a pulsar through the supernova event, and the remaining star still remains in orbit with the newly formed compact object.

Interesting things begin to happen when the remaining original star is close enough to the newly formed pulsar. Material from the remaining star begins to fall and accrete on the newly formed pulsar, causing it to ‘spin-up’ from its original spin rate when it first formed. 

These pulsars, therefore, tend to be much older, but have higher rotational velocities - and are called millisecond pulsars (this class of pulsars are spinning on their axis with periods between 1 - 10 millisecond).

Because of this precision in timing, scientists can calculate (extremely well) the spin-down rate of a pulsar – how much it slows down per second due to the loss of energy throughout the system (conservation of energy). 

However, every now and then – the declining value will jump up in a glitch to a higher spin rate. This sudden change is normally recovered back to the original spin rate – but for some pulsars, this does not occur after the glitch.

It’s currently not known what actually causes glitches in pulsars, but scientists believe these can occur when there is a shift in the internal structure of the pulsar or the crust of the pulsar cracks. For this reason, glitches are excellent probes in understanding the mysterious internal structures of neutron stars, attempting to answer questions such as ‘how does matter behave under such extreme densities’.

PSR J0908-4913 is a pulsar (not of the millisecond kind) that was discovered 31 years ago in 1984 and has a spin period of 107 milliseconds, rotating on its axis about 10 times per second. Put another way, this translates to an equatorial rotational velocity of approximately 600 km/second (assuming the radius is a clean 10 km).

The pulsar is located approximately 9,700 light-years away and was formed about 100,000 years ago (both estimates are derived through noting the pulsar’s spin-down rate and dispersion measure value). Its motion across the sky is fairly fast too, creating a bow-shock behind it known as the Speedboat Nebula, a discovery made in 1998 using the CSIRO Australian Telescope Compact Array. 

“The “speedboat” name comes from the fact the pulsar is speeding through the Milky Way. Parts of the nebula get left behind, so it ends up looking like a speedboat racing across a lake” said Marcus.

The signal for PSR J0908-4913 (which is considered ‘bright’ in radio wavelengths) is not just a single pulse but also features a smaller stepped-peak (an inter-pulse) midway between the main pulse peak.

The glitch observed by Marcus and his team for PSR J0908-4913 is 203.6 +/- 1.2 𝙭 10-9 Hz (or a tiny change of 0.0000002036 revolutions per second), which might at first seem very small – but given the 31-year observations of this pulsar with its never-changing declining spin-down rate – this event indicates something special has happened on the star.

Previously, PSR J0908-4913 has been observed on a regular basis with the CSIRO Parkes Radio Telescope for over two decades and as part of the UTMOST timing program since 2015.

Glitches are not unique to millisecond pulsars, with a range of glitches already registered and studied in detail. Glitches also occur in non-millisecond pulsars, which rotate on their access at slower velocities. 

The University of Tasmania, using the Mount Pleasant radio telescope observed a regular pulsar, known as the Vela Pulsar (which is the same constellation as PSR J0908-4913) to also glitch and in 2019, an amateur astronomer (Steve Olney) on the outskirts of Sydney – caught the Vela Pulsar glitching again from his backyard radio astronomy observatory.

By analysing glitches in the time of arrival of pulsar signals, scientists can understand what is going on inside the extreme internal structures of a neutron star. Scientists believe that this is made of several regions including an inner and outer core and an inner and outer crust.

What these regions compromise of, is another question all together. At such extreme densities and gravity – neutrons start to behave with superfluid properties where two separate layers snap and then catch up with each other, whilst the inner core of the star could be made up of particles known as quarks – currently only observed through experimental results at particle accelerator facilities such as the Large Hadron Collider.

Whilst glitches can’t be predicted, in some pulsars, they do re-occur at semi-regular occurrences so there is opportunity for astrophysicists to be ready for the next one. 

“Some [pulsars], like the Vela pulsar seem to regularly have big glitches every 3 or so years, while others (like the Crab pulsar) glitch randomly. However, even the ones that glitch somewhat periodically aren’t perfect. For example, Vela was supposed to have a big glitch late last year (in 2019), but instead, it glitched about 6 months ahead of schedule” said Marcus.

Whilst glitches might not provide the final proof of what is occurring inside neutron stars, they do allow scientists to get a sense of what is happening on these extreme objects – something that would never be achievable in-situ due to interstellar distances (and the unimaginable risks of getting close to such objects).

With the upgrades underway to increase the sensitivity of MOST, a new era of discovery awaits with a higher efficiency value – the re-introduction of the North-South arm will allow more radio objects in space to be observed at faster turnarounds.

“After the upgrade, we'll be able to simultaneously time hundreds of pulsars every week - the key to finding glitches is regular and frequent timing. With the larger field of view the N-S arm will have, and increased sensitivity with the new technology, we'll be able to probe the lower amplitude and more common glitches” said Dr. Flynn.

Marcus added “we don’t actually know if small glitches are more common than big ones, or if we even see all the glitches a given pulsar undergoes. Those are questions our high cadence monitoring might provide an answer to”

With the upgrades to MOST coming into effect soon, astrophysicists like Marcus and Chris expect to find more pulsars, fine-tune details about their glitches and change the world’s knowledge-base of what we know about these mysterious, high-mass, rapidly rotating compact objects in the far reaches of our Galaxy.

Through these studies, we’ll learn more about what resides below their crushing surfaces and how they’re able to spin at such velocities – and yet, never tear themselves apart. In doing so, we’ll answer questions about gravity that have been asked for almost 100 years now – ever since Einstein penned his theory on it.

And as for the Molonglo telescope – we’ll be one step closer to achieving the vision Mills’ once established – surveying the southern skies to the greatest detail ever achieved.

Who knows what else we’ll find, glitching in the dark.