A new dawn in radio transient surveys

20 November 2019

Observers from the ThunderKAT project, co-led by Professor Rob Fender at Oxford University and Professor Patrick Woudt at the University of Cape Town, have discovered the first of what promises to be a bumper harvest of variable and transient radio sources in images from the MeerKAT radio telescope in South Africa. ThunderKAT team member Laura Driessen, a PhD student from the University of Manchester, was looking at the hundreds of background radio sources which are picked up during regular observations of a well-studied black hole and found that a source away from the black hole was varying dramatically and erratically.

A MeerKAT first

This source rapidly brightened by more than a factor of three over a period of about three weeks. This is the first new transient source discovered with MeerKAT and is likely the tip of an iceberg of so-called ‘commensal’ transient events to be discovered with the telescope. Using an assortment of telescopes around the globe, the researchers determined that the source of the flare is a binary system where the two objects orbit each other approximately every 22 days. While the cause of the flaring and the exact nature of the stars that make up the system (named MKT J170456.2-482100, the first source to have the new ‘MKT’ designation) is still uncertain, it is possibly associated with an active corona – the hot tenuous outer atmosphere – of the brighter star. Such systems can have massive star spots, which can power energetic flares at radio and X-ray wavelengths. The orbital period of the binary was determined using optical observations with the Southern African Large Telescope (SALT). Fortuitously, the star is sufficiently bright to have also been monitored by optical telescopes for the last 18 years and is seen to vary in brightness every three weeks, matching the orbital period.

‘This source was discovered just a couple of weeks after I joined the ThunderKAT team,’ comments Laura. ‘It was amazing that the first MeerKAT images I worked on had such an interesting source in them. Once we found out that the radio flares coincided with a star, we discovered that the star emits across almost the entire electromagnetic spectrum from X-ray to UV to radio wavelengths. The unique combination of variation and multi-wavelength behaviour meant that I had to consider a wide range of possible exotic explanations.’

A new discovery phase

Professor Fender, Head of Astrophysics at Oxford and who also has a visiting professorship at Cape Town, was delighted with the discovery: ‘Variable and transient radio emission is associated with some of the most extreme and exotic phenomena in the universe. For over a decade we have been predicting a population of such objects should be found as unusual “background” source in images taken for other purposes. Laura’s discovery of this object in a study of a single imaging field suggests we are, finally, entering a new discovery phase of large numbers of radio transients.’

Professor Woudt, Head of Astronomy at the University of Cape Town added: ‘Since the inauguration in July 2018 of the South African MeerKAT radio telescope, the ThunderKAT project on MeerKAT has been monitoring parts of the southern skies to study the variable radio emission from known compact binary stars, such as accreting black holes. The excellent sensitivity and the wide field of view of the MeerKAT telescope, combined with the repeat ThunderKAT observations of various parts of the southern skies, allows us to search the skies for new celestial phenomena that exhibit variable or short-lived radio emission.’

Rob Adam, Director of the South African Radio Astronomy Observatory (SARAO) added: ‘Once again we see the potential of the MeerKAT telescope in finding interesting and possibly new astrophysical phenomena, as well as the power of the multi-wavelength approach to the analysis of observations.’

Dr David Buckley from the South African Astronomical Observatory, who leads the SALT transient follow-up programme, concludes: ‘This is a perfect example of where coordinated observations across different wavelengths were combined to give a holistic view of a newly discovered object. This study was one of the first to involve coordination between two of South Africa’s major astronomy facilities and shows the way for future such work.’

Read the article.
MeerKAT telescope: www.ska.ac.za/science-engineering/meerkat/about-meerkat/
SALT telescope: www.salt.ac.za
© SARAO South African Radio Astronomy Observatory