SKA SA test telescope reveals binary star system

South Africa's test telescope reveals secrets of a binary star system and proves the country is able to deliver first-class scientific instruments.

South Africa’s Karoo Array Telescope (KAT-7) has produced its first scientific paper, marking its move from an engineering test-bed for the Square Kilometre Array (SKA) to a functioning scientific instrument.

The seven-dish telescope in the Karoo was "really intended as an engineering test-bed to refine the design and systems for the MeerKAT telescope that we are working on now, but we are absolutely delighted that it has turned out to be a top quality science instrument, capable of producing significant science", said Professor Justin Jonas of Rhodes University, who is also the associate director for science and engineering at the SKA South Africa.

The 64-dish MeerKAT – which SKA South Africa director Bernie Fanaroff once described as "meer KAT", Afrikaans for “more KAT-7” – is the country’s SKA precursor telescope. It will be incorporated into the giant SKA, which will comprise thousands of antenna sprawling across Africa and Australia.

"We plan to continue using KAT-7 to do science until at least 2015 when part of the MeerKAT telescope will become available to researchers," Jonas said.

Science and Technology Minister Derek Hanekom – who, at his department’s budget vote on Thursday announced that the paper on radio jets from binary star systems had been published in science journal Monthly Notices of the Royal Astronomy Society – was more effusive: "This is a significant milestone for South Africa's SKA project, proving that our engineers are able to deliver a cutting-edge scientific instrument, and that our scientists are able to use it for frontier science.

"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 10 years."

Binary star systems
The paper is informed by data collected from KAT-7 and South Africa’s 26m telescope at Hartebeesthoek Radio Astronomy Observatory (HartRAO) on a binary star system known as Circinus X-1.

A binary star system involves two stars which orbit each other. In the case of Circinus X-1, one of these stars is a neutron star, while the other is an ordinary star; slightly cooler than our sun, explained lead author on the paper, Dr Richard Armstrong, an SKA South Africa fellow at the University of Cape Town.

A neutron star – which is very dense – is formed when a star goes supernova and collapses in on itself. The average neutron star weighs anywhere between one and a half to three solar masses (a solar mass is the mass of our sun), but is only a fews tens of kilometres in radius.

"When at their closest, the neutron star devours matter from the other star," said Armstrong. When this happens, the neutron star will sometimes eject much of that matter at speeds close to the speed of light in thin 'jets', which cause radio emissions when they encounter anything in their path.

“This is one of the unresolved mysteries of this whole puzzle. It might seem that when matter is sucked toward a neutron star or a black hole, it could broadly be expected to stay there,” said Armstrong, "but instead a huge amount of matter and energy is ejected away in the form of radio jets."

The big question is how exactly this process occurs, because it is very difficult to observe directly. "We've been able to show that the more extreme radio flare events happen when the neutron star is probably accreting at a lower rate."

"Accretion" is when the neutron star "gobbles" matter from the other star.

When asked why this observation was important, Armstrong noted that this was the first time that more than a full cycle of this X-ray binary star system had been observed at the high time resolution made possible by HartRAO and KAT-7.

"It’s rare to be able to observe Circinus X-1 through its full orbit. Usually we'd have to make do with several snapshots."

Written by: Sarah Wild

Picture credit: Reuters

  • This article was published on Mail & Guardian Online.

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