The Very Bright Pulsar in a Galaxy, Not Too Far Away

We’re a little lucky that we have such weird and wonderful neighbours, celestially speaking. The Milky Way - a large spiral galaxy - is an excellent model for us to observe many varieties of stars, and at different evolutionary phases, but to test our theories, ideas and models, we want to observe what stars are doing in other galaxies as well. Thankfully, right on our doorstep, there are a number of smaller galaxies that we also can look at - in great detail, and across the entire electromagnetic spectrum.

If you’re lucky enough to live under dark skies in the southern hemisphere, you’d likely be familiar with the two, relatively bright clouds that keep us company, circling near the south celestial pole for most of the year. The bigger and closer of the two, the Large Magellanic Cloud (LMC), resides about 160,000 light-years away whilst its neighbour, the Small Magellanic Cloud is just a little further out, at 200,000 light-years.

We’ve been able to study our two neighbours in great detail, and from this, learning so much astronomical knowledge thanks to their close proximity (astronomically speaking) - such as how we can measure distances in space-time (in particular, using a type of star known as a Cepheid Variable), or how a supernova unfolds (thanks to the large supernova event that occurred in February of 1987). 

Now, Australian astronomers are learning more from our neighbourhood friends, announcing the discovery of one of the most luminous pulsars ever detected, located in the Large Magellanic Cloud. And to find it, they’ve employed a rather interesting technique, that might progress into finding more, unusual pulsars in the future. 

The new pulsar is known as PSR J0523-7125, and what makes this discovery fascinating is that it happens to be so very bright, yet located beyond the realms of our own Galaxy.

To make the discovery, the science team used the ASKAP radio telescope, which is owned and operated by Australia’s national science agency, CSIRO, and is located in central Western Australia. 

“This is surprising that it isn’t found before since it’s so bright,” said University of Sydney PhD student Yuanming Wang, who led the investigation that resulted in the identification of this object as a pulsar. 

The discovery was made by Sydney University undergraduate student Teresa Klinner-Teo, and followed up by Yuanming’s work, as part of the Variable and Slow Transients (VAST) survey project. Co-authors on the paper also include astronomers from CSIRO, OzGrav, Swinburne University of Technology, Western Sydney University, and Macquarie University, as well as an international cohort of collaborators. 

“The aim of VAST is to explore the unknown, finding rare objects that change on rapid timescales. These represent some of the most extreme phenomena in the Universe, from gamma-ray bursts to flaring stars,” said Professor Tara Murphy from the University of Sydney, who also leads the VAST project. 

Pulsars are rapidly rotating and highly magnetised neutron stars - the compact, dense remnants of former massive stars that have undergone a supernovae explosion. They’re tiny, relative to most things in space, about the size of a city (~20 kilometres in diameter), but they pack a lot of energy in their magnetic fields, their rotation, and of course, all the matter that is squeezed into such a small space (~1.4 solar masses crushed down).

As pulsars spin, they emit powerful beams of energy from their magnetic poles, and when these beams sweep past the Earth, we detect a ‘pulse’, very similar to how a ship located far out at sea catches a pulse of bright light when the beam of a lighthouse sweeps across its view. We measure these signals using dish antennas, such as the Parkes radio telescope (also known as Murriyang). 

Usually, pulsars are discovered through processes that identify a periodic signal (which has led to the discovery of over 3,000 pulsars so far). But PSR J0523-7125 was found using a continuum survey - a method that allows pulsars to be discovered, regardless of their periodicity, or any scattering of their signal by material that resides between us and the pulsar. 

“The traditional method is to capture the periodic pulses - this is a conclusive feature for a pulsar,” said Yuanming about how pulsars searches usually yield results. “So they want data at a high time resolution and frequency resolution to ‘draw’ the pulse shape”.

“For a continuum survey, however, we average data at time and frequency to generate a ‘sky map’. We cannot see the pulsar’s profile since they’ve been averaged, but we can see a bright source in its position in the continuum image. We then select the pulsar using the other method - polarisation”

One particular process of identifying pulsars in continuum surveys is by studying the polarisation of light captured during observations. Polarisation refers to the orientation of light that oscillates as it traverses along its path, and it comes in two types of flavours - linear, which moves up and down in a singular plane, and circular which spirals around like a helix.  If you’ve ever held a hose or skipping rope and moved your hand up and down, the waves that oscillate in the up-down motion as they traverse down the rope are linearly polarised. Repeat the process but instead move your arm in a circle, and the spiral oscillations that traverse down the rope, are circularly polarised. This is similar to how polarised light behaves especially when it encounters surfaces or magnetic fields. In the case of PSR J0523-7125, the circular polarisation was found to be very high (about 20%).

“Circular polarisation is normally related to highly magnetised objects, like pulsars and red dwarf stars. They are relatively rare. The majority of sources in the radio sky are active galactic nuclei and they exhibit very little or no circular polarisations,” said Yuanming. 

Nearly all of the 3,000 or so pulsars found so far, the majority of them have been within our own Milky Way Galaxy, with a handful found in both Magellanic Clouds. Beyond this, and out in other galaxies, we haven’t detected any radio pulsars - and this might be for a number of reasons, such as their signal being drowned out amongst the background noise or scattering within their own galaxies themselves. 

And as pulsars are related to stars, in our Galaxy, we tend to find them mostly around the Galactic plane, but there are a few that reside away from this region. These renegade runaways, likely ended up away from the plane thanks to the natal kick they received from the supernova explosion that birthed them. 

To measure the distances to pulsars, astronomers employ a rather interesting method - they record how the different frequencies of radio light from each pulsar is delayed as it arrives at radio telescopes, with lower frequencies delayed more than higher frequencies. This delay is caused by the number of electrons between us and the pulsar and is known as the Dispersion Measure (DM). 

By determining the value of the delay, and using this in conjunction with a model of the electron density distribution across the Galaxy (i.e., we have a model of how many electrons reside in most directions), astronomers can use this information to calculate the distance to the pulsar. This value, the electron density column, is expressed in units of parsec per cm^-3, or pc cm^-3.

Normally, in the direction that PSR J0523-7125 was found, the DM usually measures between 50 - 60 pc cm^-3 for radio sources that are associated with our Galaxy. However, in the case of this discovery, the DM was found to be 157.5 pc cm^-3, which is similar in value and distance to other pulsars discovered in the LMC. That is, it resides well outside our Galaxy. 

A number of pulsars have now been discovered in both Magellanic Clouds, as would be expected. These irregular galaxies have enormous star-formation factories, so they’re birthing new, massive stars and when these massive stars exhaust their fuel loads and explode, they can leave behind neutron stars and pulsars. This likely also occurs in other galaxies, but we don’t have the sensitivity to observe regular pulses from these tiny compact objects, at such great distances. Excitingly, a new phenomenon has recently emerged in radio astronomy, where short-duration pulses - known as fast radio bursts - have started to be detected from very far distances. However, we are still to confirm what is producing these events, so we’re not absolutely certain they are coming from pulsars. 

The discovery of PSR J0523-7125 was made using the ASKAP radio telescope, which features 36 radio dish antennas, each 12 metres in diameter, and spread like a giant array across the remote Murchison Radio-astronomy Observatory in Western Australia. The longest distance between any two of ASKAP’s dishes is approximately 6 kilometres, which allows the whole array to be turned into a giant virtual dish of this diameter, achieving an even higher degree of resolution in detail. 

Six separate observations were made using ASKAP, spanning from August 2019 through to January 2020, which output a number of candidate signals. To dig a little further and verify what scientists were hoping to achieve, some astronomical detective work (using a suite of other radio telescopes) was required.

Follow up observations of PSR J0523-7125 were made with both the MeerKAT instrument (located in South Africa) which detected pulses and CSIRO’s Parkes radio telescope, Murriyang, which also identified the object. Additional to these two instruments, CSIRO’s Australia Telescope Compact Array (another interferometer with six dishes, located near Narrabri, NSW) was also used to observe and detect the pulsar. 

Once the team knew what they were looking for, they jumped back into some old archival data, captured during ASKAP’s commissioning program, and even the Sydney University Molonglo Sky Survey (which uses a Mills Cross telescope, located outside Canberra) with both data sets confirming the candidate signal. 

Prior to confirming the pulsar, astronomers also looked across a range of wavelengths to confirm that any of their findings were not related to any other known or observable object in other frequencies, such as other stars. To do so, they tapped into a handful of multiwavelength observations that stretched across the entire electromagnetic spectrum, namely the archival data of space-based satellites such as GAIA (low UV - optical - low infrared), WISE (infrared), ROSAT (x-rays) and Fermi (gamma-rays). Across all these data sets, no multiwavelength counterpart signal was observed, which when combined with the high value of circular polarisation, absolutely confirmed that the discovery was indeed, a pulsar. 

“We are now pursuing more ‘timing’ observations using Parkes - to precisely measure its period (and any period variations). It will help us learn its age (young or old), and if it has a companion,” said Yuanming. 

This new method also opens up an opportunity to consider what future and much more powerful telescopes, like the SKA, will be able to achieve, and how this might change our perspective of the Universe.

“We can use this method with the future SKA to find more similar (unusual) pulsars - the improved sensitivity [of the SKA] would also help to find more distant pulsars, maybe even beyond the LMC,” she concluded. 

We acknowledge the traditional custodians of the lands upon which the following instruments are located: 

• ASKAP is located on the traditional lands of the Wajarri Yamatjii people
• MeerKAT is located on the traditional lands of the San and Khoi people in the Northern Cape Province of South Africa
• The Parkes radio telescope, Murriyang, is located on the traditional lands of the Wiradjuri people
• The Australia Telescope Compact Array is located on the traditional lands of the Gomeroi people