14 mins read 06 Aug 2022

55 Years of Pulsar Science

Today marks the 55th anniversary of the discovery of pulsars, made by Dame Prof. Jocelyn Bell Burnell. Since then, we’ve learned so much about these intriguing objects, whilst they’ve taught us so much about the Galaxy too. Rami Mandow takes a look at some of the exciting pulsar astronomy from Australian scientists that we’ve covered over the last few years.

Dame Prof. Jocelyn Bell Burnell, as a PhD student, and the two radio charts that indicated the first detection of pulsars on 6 August 1967, then follow up confirmation on 28 November 1967. Credit: CSIROpedia.

History of Discovery

On this day in 1967, then 24-year old PhD student Jocelyn Bell Burnell was reviewing the squiggly lines that tracked along the chart recordings, from the telescope she had helped build over the prior years - the Mullard Radio Astronomy Observatory just outside Cambridge.

As the pen recorder rolled over the chart paper, it would occasionally detect an odd deviation, producing small squiggles on the plot, though no one really took much note of it at the time. This was the routine for months, and to everyone, this was some background noise, some factor that just needed to be accounted for during a deeper analysis. 

Every now and then, Jocelyn would notice the little jiggle in the plot, and without any proper explanation at the time she would jokingly refer to it as “LGM” - little green men - referencing an extraterrestrial signal. But it did start to get her thinking. Was it local interference? Was the wiring of the telescope faulty? Could it be the recording device?

One day, something clicked. A moment that would change astrophysics forever. Jocelyn realised that this little bit of scruff, kept coming from a particular patch of sky, and she started to remind herself that she had seen something like this before. So she went back to previous recordings and found that this little re-occurring squiggle would sometimes appear, but when it did, it was fixed amongst the background of stars. In other words, it was coming from an astrophysical source.  

After speaking with her supervisor, Prof. Tony Hewish, she switched to a faster and better recording device, just to make sure it wasn't an anomaly associated with the equipment. And for some time, there was nothing - no funny little squiggle in the data, no scruff, no little green men. 

Dame Prof. Jocelyn Bell Burnell working on connecting the instrumentation at the Mullard Radio Astronomy Observatory just outside Cambridge. Credit: University of Cambridge.

Until 28 November 1967. The little jiggles of scruff returned, like a string of equally separated wave packages space exactly 1.3 seconds apart. Jocelyn had stumbled across a new type of astrophysical object - known as a pulsar - a term coined in 1968, the shortened analogue of “pulsating star”. 

Pulsars are one of the sub-varieties of neutron stars and are forged during the last dying moment of a massive star, as it undergoes a supernova event. And they are extreme, to say the least. During the supernova, the collapsing core of the progenitor star crushes matter down into a sphere no bigger than the size of a small city (about 20 kilometres across), forcing electrons and protons together to become neutrons (hence the ‘neutron star’). This equates to approximately 1.4 times the mass of our Sun being squeezed into this tiny space - so the stuff that makes up a pulsar, is the densest material in the Universe. 

With this much mass squeezed into a small space, their surface gravity is enormous. If you could live on the surface of a pulsar and wanted to leave in a rocket, your rocket would need to be moving at about half the speed of light to be able to climb out of the huge gravitational well. 

But it doesn’t stop there. These exotic objects also have extreme magnetic fields, equating to roughly a billion times that of Earth’s. And they’re rotating fairly fast - some spinning on their axis 10s of times per second, with the fastest recorded pulsar (known as a millisecond pulsar) rotating 716 times per second. For this particular beast, its equator is moving at ~24% of the speed of light. 

Most of the neutron stars we know of today are pulsars, and that’s because pulsars emit beams of electromagnetic energy (in radio wavelengths) from their magnetic poles as they rotate. The only reason why we see these pulsars is because that beam of radiation sweeps across Earth’s field of view - like a lighthouse beam - and our radio telescopes can pick it up. 

Pulse. Pulse. Pulse…… like a regular ticking clock, we can measure each pulse that arrives to Earth, keeping track of how fast the pulsar is spinning. And it is because of this clock-like nature that we can use these objects to conduct sensitive timing experiments testing some of nature's most extreme conditions - like what happens to space-time, matter, and energy in the presence of such a strong gravitational field. It would be impossible to replicate these conditions on Earth in a lab, so pulsars are one of the Universe’s most beneficial sources to conduct our remote sensing experiments. And it’s all thanks to Jocelyn Bell Burnell’s discovery, 55 years ago today. Since then, we’ve learned so much about the Universe, thanks to these exotic objects. 

For example. The first indirect evidence of gravitational waves came from a binary system that features a pulsar and a neutron star. In these kinds of systems, it is expected that the two objects would slowly get closer together, emitting gravitational waves in the process. By measuring the accurate ticking of the pulsar, astronomers were able to quantify this to a high degree of certainty and it was just as Einstein had predicted - the orbit was getting smaller by the exact amount expected from the emissions of these gravitational waves. This discovery went on to win a Nobel Prize. 

The first exoplanets ever discovered, 30 years ago this year, were also found around a pulsar. Once again the highly accurate nature of observing the periodic ticking of pulsars allowed astronomers to calculate the delay in the signal arrival time, finding that it matched the exact pattern that would be expected if there were not one, but two, small planets that were slightly tugging on the pulsar. 

Pulsars have also given us a chance to learn about all the stuff between us and the stars in our Milky Way Galaxy thanks to their radio beams of light they emit. As this light travels across the interstellar medium (ISM), an inhomogeneous, turbulent mixture of ionised, atomic and molecular gas (as well as some dust), the pulsar’s signal is affected. For example, as the pulsar signal encounters the magnetic fields of the ISM, its polarisation orientation can experience changes as it encounters a variety of magnetic fields along its path, and by the time it gets to Earth - these measured changes can tell us about the magnetic field of the entire Galaxy. 

Since their discovery by Bell Burnell, pulsar science has taken off - and Australian astronomers have been contributing to this field for decades. In fact, most pulsars we know about have been found using the Parkes radio telescope, owned and operated by Australia’s national science agency, CSIRO. But along with discovery and decades of science papers - we’ve captured some pulsar science stories from our local astronomy communities, that has added global value. 

Pulsars can do weird things

Artist illustration of a pulsar with radio beams emitted from its magnetic poles. Credit: Kevin Gill.

One of the most powerful properties of pulsars is their regular, clock-like ticking, due to its consistency. But when astronomers first started observing these ticks, they noticed that for every revolution the star makes, the period would increase by an extraordinarily small amount. This gave us another tool to use - if we knew exactly how much the period was increasing, then we could predict how fast the pulsar would be spinning in a day, year or decade from now. And so, astronomers devised experiments around this - to keep monitoring pulsars to see if they followed the predicted models. 

But they didn’t. 

Every now and then, some pulsars would momentarily speed up, in small but measurable little jolts. This effect is known as glitching and helps us figure out what is going on, in the interior of these dense objects. One pulsar stunned astronomers when it glitched for the first time after 30 years of observations. 

Along with radio waves, pulsars also emit in other parts of the electromagnetic spectrum - especially in the higher energy bands, such as gamma rays and X-rays. Space-based observatories (that look at the light in these bands) have found many pulsars that are bright and emitting across the Galaxy in these energy regimes. An interesting mystery that is currently being investigated is that there appears to be an excess of gamma rays emanating from the Milky Way’s Galactic centre. A number of models have been put forward to explain this, including the possibility of dark matter annihilating in this region, or the more favourable idea that this excess of high energy is coming from millisecond pulsars. 

Using Pulsars to Prove Einstein Was Right

Artistic impression of the double pulsar system. The orbital motion of these extremely dense neutron stars causes a number of relativistic effects, including the creation of ripples in space-time known as gravitational waves. Credit: Michael Kramer/MPIfR.

In the early 1900s, when Einstein released his General Theory of Relativity (GR), no one really knew neutron stars and pulsars existed. And since then, there has been many experiments and tests to support GR as a strong theory that describes gravity and how space-time functions in relation to the masses it possesses. 

But once again, thanks to the regular ticking of pulsars - GR experiments can be conducted with a high degree of accuracy, in environments that feature extreme gravitational fields. This is especially relevant when you have pulsars and a high mass companion, like another neutron star, pulsar, or white dwarf in a binary system. As both masses orbit a common centre, their gravitational influence affects the other in ways that can be observed, by reviewing the pulsar’s regular ticking. A perfect GR experimental lab that nature has given us access to. 

For example, there is a one and only kind of system that astronomers have discovered that features two pulsars - each ticking with at its own rotation, and each having a different orientation to our perspective view. Studies of this pulsar, over the course of 16 years and using the Parkes radio telescope, found that this system was agreeing with the laws set out by GR, to 99.99% accuracy. 

In another study, a pulsar - white dwarf binary was observed to show the effects of what happens when a massive object (like a neutron star) spins, and drags the fabric of space-time around with it - with the results once again pointing towards a high level of agreement with Einstein’s GR. 

Listening in on Galaxies Colliding

Artist rendition of two merging supermassive black holes. Credit: NASA Goddard Space Flight Centre.

Astronomers have taken Einstein’s GR a step further by using pulsars placed around the Galaxy to simulate a giant gravitational wave detector that has the ability to listen in on the rumbling background roar of all of the collisions and mergers of galaxies, in the Universe’s history. 

Much like their stellar counterparts, gravitational waves of this nature are formed when large masses are accelerated, but in this case - it’s the masses of supermassive black holes at the centres of galaxies, as they orbit and coalesce. This cosmic dance generates low-frequency gravitational waves that have wave periods in decades, and so to observe them - we need to use pulsars located across the galaxy. 

This is known as a pulsar timing array and there are now several teams around the world working on the first detection of this kind. 

And whilst pulsar timing arrays are looking for low-frequency gravitational waves, pulsars themselves (and on their own) can emit these waves if they have tiny deformations on their almost perfect spherical surfaces. These tiny ‘mountains’ only measure a few millimetres to centimetres in height, but it's enough to create a continuous hum of gravitational waves - something that current detectors are also searching for

Some Pulsars are Nasty

Artist illustration of a black widow millisecond pulsar and its low-mass companion. Due to the close proximity to the pulsar’s powerful radiation, the companion is slowly evaporated away. Credit: Guizhou Normal University.

Whilst you wouldn’t want to get anywhere near a pulsar (or neutron star) due its intense radiation, extreme magnetic field, and enormous gravity - there are some neutron stars that are just a bit less friendly than others. 

One category of pulsars are known as spider pulsars, and they come in two varieties. Black Widows and Redbacks. These are pulsars that feature a low-mass companion, which (unfortunately for the companion) is caught in an orbit around the pulsar at more than comfortable proximity. 

And thanks to the intense radiation emitted by the pulsar (including a relativistic wind of particles) the companion is slowly shredded over time. In other words, the pulsar is eating its companion - which is why they are named after Earth’s arachnids (which consume their counterparts after mating). 

Recently, one of these nasty pulsars was caught in the act of destroying its poor friend, which will eventually be evaporated into nothing, leaving the solitary pulsar to continue its life for several billion years. 

The other nasty kind of neutron star (which is related to pulsars) are the magnetars - named after their ultra-strong magnetic fields, which can reach as high as 1,000 trillion times that of the Earth’s. These objects are so intense, that if we placed one of them about 1,000 kilometres from Earth, the magnetic field would distort the electrons in the atoms in our bodies, dissolving life as we know it. Very bizarre types of magnetars have been spotted in the Milky Way using the Parkes radio telescope. 

We’re Still Learning So Much

Since the discovery of pulsars 55 years ago by Jocelyn Bell Burnell, the family of neutron stars has grown to include a large range of objects, and continually surprise scientists, enlightening our views of the cosmos, and potentially providing us with new, unexplored science. 

A review of the population of pulsars and neutron stars to date has shown that pulsars are usually fast-spinning objects, but there is a handful that rotate slowly. Up until recently, the record holder for the slowest rotation period was a pulsar which spun once on its axis every 23 seconds. 

So why don’t we see slower pulsars? It’s in the physics of emission generation which requires a rapidly rotating magnetic field to create the environment in which these radio emissions can form around the magnetic poles of these stars. So as the pulsars slow down over time, they end up blinking out and becoming regular neutron stars, slowly cooling over the age of the Universe. 

But two new cases have challenged this notion. 

Using the MWA telescope in Western Australia, astronomers found an object with a rotation period of 18 minutes, which challenges the science of pulsar emissions. And in another study, another object was observed at having rotation periods of 76 seconds. Both cases are well outside the record holders for the slowest pulsar.

Are we now looking at a new class of objects that have never been detected before? Time and further observations will tell, especially as our technology advances, and new instruments, like the SKA come online towards the end of this decade. 

In any case, 55 years of pulsar science has brought about an enormous contribution to astronomy and astrophysics, nuclear science, relativity and more. These remarkable, tiny objects - that pack so much power and punch - have reframed our position on the nature of the cosmos at large. 

And how far we have come,  all thanks to Dame Prof. Jocelyn Bell Burnell and her “scruff”

Video Credits: ICRAR

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site on which ASKAP and MWA is located. We also acknowledge the Wiradjuri people as the traditional owners of the Parkes radio telescope observatory.