Engineering News

South Africa’s first radio telescope array, the seven-dish KAT-7 located in the Karoo, which was originally intended to be mainly an engineering prototype for the much larger MeerKAT array, has now established itself as an effective scientific instrument. The first scientific paper produced from research using KAT-7 was released online on Thursday by the UK journal Monthly Notices of the Royal Astronomical Society (which is one of the oldest and most prestigious scientific journals in the world, and has been in continual publication for some 186 years).

A team of local and overseas astronomers used KAT-7 in parallel with the 26-m-diameter dish at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) west of Pretoria to observe the Circinus X-1 binary star system. The two instruments observed Circinus X-1 at the same time but at different frequencies.

This binary star system is composed of a star similar to our sun and a neutron star (which is the extremely compact – about 20 km in diameter – but incredibly dense remnant of a massive star that exploded into a supernova). They orbit each other in an elliptical orbit, with each orbit taking 16.5 days.

When they are closest together, the neutron star drags material off its companion. This material forms an accretion disc around the neutron star. Some of this material subsequently hits the surface of the neutron star, producing X-rays and, as a result, Circinus X-1 is classified as an X-ray binary. In fact, it is one of the most luminous X-ray binaries known.

In sharp contrast, other material is accelerated away from the neutron star, at close to the speed of light, in two very powerful, compact and long jets (one from each pole). These jets, in turn, trigger powerful radio wavelength flares when they collide with gas clouds surrounding the binary system. It was these flares that the KAT-7 and HartRAO instruments were used to study.

“These types of observations help us to understand how matter is accreted onto extremely dense systems, such as neutron stars and black holes,” explained University of Cape Town Square Kilometre Array (SKA) fellow Dr Richard Armstrong. “They also shed light on how neutron stars are able to generate these powerful outflows and associated radio bursts.”

KAT-7 caught two of these flares during its observations of Circinus X-1 and recorded their development. It was the first time this system had been subjected to detailed observation during more than one flare cycle.

“It bodes well for the delivery of our 64-dish MeerKAT telescope, currently under construction in the Karoo, and for our ability to play a key role in building and commissioning thousands of SKA antennas over the next ten years,” enthused Science and Technology Minister Derek Hanekom.