By The Division of Communications and Advancement

The motion of the planets through the night sky has captivated humanity since prehistoric times. The Great Saturn-Jupiter Conjunction of 2020 was particularly special. This remarkable astronomical event, which brought these two gas giants closer together than they had been for nearly four centuries, offered an enticing spectacle for stargazers worldwide. Professional astronomers, with telescopes capable of seeing well beyond the relative “backyard” of our solar system, also couldn’t resist the temptation to look. 

Among those eager to witness this event was the South African Radio Astronomy Observatory (SARAO), with its MeerKAT radio telescope. Bright moving targets such as planets are notoriously difficult to image accurately with radio interferometers, so the challenge of processing the data fell to the world experts of the Rhodes Centre for Radio Astronomy Techniques & Technologies (RATT) team, working together with the Radio Astronomy Research Group (RARG) at SARAO, and a number of international collaborators from as far away as France, the UK, and Australia.  In particular, this has required RATT and RARG to make a “movie” of the observation – which is very unusual, since radio astronomers normally concentrate on making deep, static, still images of the sky. The spectacular results of this “movie-making” can be seen here (or here on YouTube).

RATT was established in 2012 around the eponymous SKA (SARChI) Research Chair awarded to Rhodes University’s Distinguished Professor Oleg Smirnov. Prof Smirnov also leads the RARG group at SARAO. Last year, RATT won the prestigious NRF Science Team Award.

Jupiter, with its 10-hour rotation period and intriguing "radiation belts”, has long been a fascinating target for radio observations (in fact, some of the pioneering radio astronomy work at Rhodes University was focused on Jupiter). These belts, comprised of charged particles trapped within the planet's magnetic field, create mesmerising visuals when observed in the radio spectrum. However, during the Great Conjunction, MeerKAT's observations unveiled more than just Jupiter's magnetic mysteries.

The photobombers

As MeerKAT observed the Conjunction, an interesting twist occurred when two Fanaroff-Riley Type II (FRII) radio galaxies made an appearance. Such galaxies host supermassive black holes at their centres, which emit jets of plasma that blow out massive radio lobes in the surrounding intergalactic medium, giving these objects their distinctive double-lobed, barbell-like appearance. These "photobombers" added an extra visual treat to the already captivating event, as well as providing a fitting connection to a famous South African. The Fanaroff-Riley classification of radio galaxies, dating back to 1974 and still in use today, was the brainchild of Dr Bernard Fanaroff and his PhD supervisor at the time, Dr Julia Riley. Dr Fanaroff subsequently became the first director of SKA South Africa – the precursor to SARAO – and played a crucial role in conceiving and building MeerKAT.

FRIIs are not uncommon in the Universe, and MeerKAT’s exquisite sensitivity seems to reveal more of them everywhere it looks. “Having two FRII’s join the Great Conjunction was thus more of a pleasing coincidence than a true surprise. The true surprise was still in store!” said Prof Smirnov and explained how a completely unanticipated transient radio source suddenly switched on in the vicinity of Saturn, and lasted for about 45 minutes, before winking out again. 

Such transient sources are very rare, and almost always point to something exciting going on. Over the next few months, MeerKAT returned to the same location on the sky (the two planets having long vacated the scene) to try to get to the bottom of things. The source winked, and winked again. Eventually, MeerKAT detected pulsed emission coincident with the winking, indicating that it was produced by a pulsar. This newfound pulsar was named a PARROT, short for ‘pulsar with anomalous refraction recurring on odd timescales’.

“Of course, we loved this acronym,” said Prof Smirnov. “The Rat & Parrot has been a favourite watering hole for generations of Rhodes University staff and students. In fact, one of the technical breakthroughs that made this discovery possible (Smirnov & Tasse 2015) started over drinks at the Rat & Parrot. In a way, it feels like we are paying homage to that.” 

Pulsars are the remnants of massive stars that have ended their life in a supernova explosion. As the outer layers of the star blow away, its core collapses into an incredibly dense neutron star, which retains most of the angular momentum and magnetic field of its progenitor. What remains is a  very rapidly rotating (with periods of milliseconds to seconds), extremely magnetised object. If its magnetic field axis is tilted with respect to its spin axis, it creates two beams of powerful radio emission as it swings around – like a cosmic lighthouse. “If the Earth happens to be in the path of such a beam, then every time the beam sweeps through – every 1.6 seconds, in the case of the PARROT – we observe a distinct pulse of emission that has given pulsars their name,” explained Prof Smirnov. 

In a further pleasing piece of symmetry, the Great Conjunction movie showcases another object with a tilted, rotating magnetic field – Jupiter itself. Though much slower (10-hour period), bigger and far less dense and magnetised than a neutron star, Jupiter provides a perfect visual illustration of a rotating magnetosphere – which is exactly the mechanism that drives its radiation belts. 

A peculiar pulsar

Thousands of pulsars have been discovered, but what sets the PARROT apart is its remarkable behaviour. The radio signal from the PARROT occasionally experiences amplification by factors of ten or more within minutes. During the Great Conjunction, this amplification reached nearly a factor of 100. “Pulsars, and other very compact radio-emitting objects, often exhibit variations due to scintillation in the solar wind and interstellar medium. Radio scintillation is not unlike the optical ‘twinkling’ in the atmosphere that we can see by eye when we look up at the stars. However, no amount of twinkling will brighten a star by a factor of ten or more, and the same goes for scintillation! Such extreme amplification behaviour, known as ‘lensing,’ on such short timescales is virtually unprecedented,” said Prof Smirnov. The exact cause of this phenomenon remains a topic for future research, with potential explanations ranging from unusual structures in the solar wind to dense plasma in the pulsar's environment.

The discovery of the PARROT pulsar during MeerKAT's observation of the Great Conjunction has opened up new avenues of research and presented intriguing questions for astronomers. This discovery was detailed in a paper titled "The RATT PARROT: Serendipitous Discovery of a Peculiarly Scintillating Pulsar in MeerKAT Imaging Observations of the Great Saturn-Jupiter Conjunction of 2020," led by Prof Smirnov and published by the Monthly Notices of the Royal Astronomical Society. It will also be presented at this week’s MeerKAT@5 conference, which celebrates 5 years of MeerKAT operations. 

Finding PARROTs

The discovery of the PARROT highlights MeerKAT's capabilities and its potential for uncovering more such transients and variables in the cosmos. As astronomers delve deeper into understanding the nature of this peculiar pulsar and the mechanisms behind its extraordinary radio amplifications, the scientific community eagerly anticipates further revelations and insights from MeerKAT's observations.

The Great Saturn-Jupiter Conjunction of 2020, though a celestial event steeped in history, continues to surprise and inspire astronomers. The serendipitous discovery of the PARROT pulsar during MeerKAT's observation of this event adds a fascinating chapter to our understanding of the cosmos, reminding us that the Universe always has more secrets waiting to be unveiled.