News Release

South Africa's new radio telescope reveals giant outbursts from binary star system

Peer-Reviewed Publication

University of Southampton

An international team of astronomers have reported the first scientific results from the Karoo Array Telescope (KAT-7) in South Africa, the pathfinder radio telescope for the $3 billion global Square Kilometre Array (SKA) project.

The results appear in the latest issue of the prestigious international astronomical journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Using the seven-dish KAT-7 telescope and the 26 m radio telescope at the Hartebeesthoek Radio Astronomy Observatory (HartRAO), astronomers have observed a neutron star system known as Circinus X-1 as it fires energetic matter from its core into the surrounding system in extensive, compact `jets' that flare brightly, details of which are visible only in radio waves.

Circinus X-1 is an X-ray binary (or two-star system) where one of the companion stars is a high-density, compact neutron star (a neutron star is an extremely dense and compact remnant of an exploded star and only 20km in diameter.) The two stars orbit each other every 16.5 days in an elliptical orbit. When the two stars are at their closest the gravity of the dense neutron star pulls material from the companion star. A powerful jet of material then blasts out from the system.

During the time astronomers, including a team from the University of Southampton, observed Circinus X-1 (13 December 2011 to 16 January 2012) the system flared twice at levels among the highest observed in recent years. KAT-7 was able to catch both of these flares and follow them as they progressed. This is the first time that the system has been observed in such detail during the full flare cycle.

"One way of explaining what is happening is that the compact neutron star gobbles up parts of its companion star and then fires much of this matter back out again," explains Dr Richard Armstrong, an SKA SA Fellow at the University of Cape Town and lead author of the paper. "The dramatic radio flares happen when the matter Circinus X-1 has violently ejected slows down as it smashes into the surrounding medium."

Professor Rob Fender, Head of the Astronomy Research Group at the University of Southampton, says: "Circinus X-1 continues to reveal new aspects of its behaviour, and is arguably the best laboratory for relativistic jet astrophysics in the southern hemisphere. It is furthermore an excellent control to the large population of jets associated with accreting black holes."

Dr Armstrong adds: "These types of observations are crucial for understanding the processes of both accretion of matter onto extremely dense systems, such as neutron stars and black holes of both about the sun's mass, and also the so-called supermassive variety we now know to be at the centre of most galaxies."

KAT-7 is the world's first radio telescope array consisting of composite antenna structures. It is the test array for MeerKAT, a much larger radio array, which is itself in turn a precursor for the dish-based component of the SKA.

The MNRAS study was carried out as part of the development for the ThunderKAT project on MeerKAT, which will find many more of these types of systems in the galaxy and search for new types of radio systems that change rapidly with time.

Professor Fender, who is co-leader of the MeerKAT project, adds: "This project will test the extremes of physics, density, temperature, pressure, velocity, gravitational and magnetic fields, and are beyond anything achievable in any laboratory on Earth. It provides a unique glimpse of the laws of physics operating in extraordinary regimes. Nearly all such events are associated with transient radio emission. By studying radio bursts from these phenomena, we can pinpoint the sources of explosive events, probe relativistic accretion and understand the budget of kinetic feedback by such events in the ambient medium."


Notes for editors:

1. A copy of the scientific paper 'A return to strong radio flaring by Circinus X-1 observed with the Karoo Array Telescope test array KAT-7 (Armstrong et al, 2013)' published in Monthly Notices of the Royal Astronomical Society is available from Media Relations on request.

2. Images available -

  • Image of Circinus X-1 showing the core source flaring

    Caption - The bright region in the middle of the KAT-7 radio images, observed at 1822 MHz (with 256 MHz bandwidth), shows Circinus X-1. Also shown are several supernova remnants - the large loop mid bottom, and the smaller loop mid left, as well as several other radio sources.

  • An artist's impression of the Circinus X-1 system showing the binary (double) star system.

    Caption - The small white sphere is a neutron star – an extremely dense and compact remnant of an exploded star, only about 20 km in diameter. The red sphere represents an ordinary star – the companion star in this system. These two stars orbit each other every 16.6 days in an elliptical orbit. When the two stars are at their closest, the gravity of the dense neutron star pulls material from its companion star. An accretion disk (the wavy blue lines) forms around the neutron star, containing the matter that is sucked from the ordinary star. Powerful jets of material (the orange rays) then blasts out from the neutron star at almost the speed of light, causing powerful flares in radio frequencies.

  • High res images of KAT-7 are downloadable at:

3. The University of Southampton is a leading UK teaching and research institution with a global reputation for leading-edge research and scholarship across a wide range of subjects in engineering, science, social sciences, health and humanities.

With over 23,000 students, around 5000 staff, and an annual turnover well in excess of £435 million, the University of Southampton is acknowledged as one of the country's top institutions for engineering, computer science and medicine. We combine academic excellence with an innovative and entrepreneurial approach to research, supporting a culture that engages and challenges students and staff in their pursuit of learning.

The University is also home to a number of world-leading research centres including the Institute of Sound and Vibration Research, the Optoelectronics Research Centre, the Institute for Life Sciences, the Web Science Trust and Doctoral training Centre, the Centre for the Developmental Origins of Health and Disease, the Southampton Statistical Sciences Research Institute and is a partner of the National Oceanography Centre at the Southampton waterfront campus.

For more information:

Peter Franklin, Media Relations, University of Southampton, Tel 023 8059 5457, email

Marina Joubert , SKA South Africa Project Office, email: /

Dr Richard Armstrong, email:

Follow us on twitter:

Like us on Facebook:

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.