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

[Sam Rametse]

http://www.ska.ac.za/releases/20130516.php

The first scientific paper <http://arxiv.org/abs/1305.3399>  based on
observations performed with South Africa's new KAT-7 radio telescope, has
been accepted for publication by the prestigious journal Monthly Notices of
the Royal Astronomy Society. 

"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," says Derek
Hanekom, South Africa's Minister of Science and Technology. "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."

Using the new KAT-7 telescope in the Karoo and the existing 26 m radio
telescope at the Hartebeesthoek Radio Astronomy Observatory (HartRAO), South
African and international astronomers have observed a neutron star system
known as Circinus X-1 as it fires energetic matter from its core in
extensive, compact jets that flare brightly. The details of the flares 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, only about 20 km
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 that KAT-7 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 these flares and follow them
as they progressed. This is the first time that the system has been observed
in such detail during multiple flare cycles.

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

At the same time Circinus X-1 was being observed at HartRAO at two higher
frequencies as part of a long-term study of this object. "The flares are
much stronger at the higher frequencies and by combining the three sets of
measurements, we could study how each flare evolved as time progressed and
investigate details of the turbulent interactions of the jet," adds HartRAO
Emeritus Astronomer Dr George Nicolson, a pioneer of radio astronomy in
South Africa.

"These types of observations help us to understand how matter is accreted
onto extremely dense systems, such as neutron stars and black holes,"
Armstrong says. "They also shed light on how neutron stars are able to
generate these powerful outflows and associated radio bursts."

"KAT-7 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," says Professor Justin
Jonas of Rhodes University, who is also the associate director for science
and engineering at the SKA South Africa Project Office. "We plan to continue
using KAT-7 to do science until at least 2015 when part of the 64-dish
MeerKAT telescope will become available to researchers."

Scientists from the SKA Project in South Africa and local and international
universities worked together on both the observations and the analysis. This
work is 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.

The two leaders of the ThunderKAT project, Professor Rob Fender of the
University of Southampton and Professor Patrick Woudt of the University of
Cape Town, explain that the ThunderKAT project searches for all types of
radio bursts and flashes in KAT-7 and MeerKAT data on timescales from
seconds to years. Finding and studying the systems that produce these
outbursts will allow us to test the extremes of physics, and are beyond
anything achievable in any laboratory on Earth. "These systems provide a
unique glimpse of the laws of physics operating in extraordinary regimes",
Woudt says, "and nearly all such events are associated with transient radio
emission."

The ThunderKAT project is already well under way, and besides these
observations, has made targeted observations of other exciting systems
including the flaring black hole candidate Swift J1745.1-2624, the diffuse
radio structure around the black-hole binary GRS 1915+105, as well as a
system that is very close to our Sun, the brown dwarf binary WISE 1049-5319.

 

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