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X-ray outburst observed by school children
Posted: 20 April 2011

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The study of an unusual double outburst of powerful radiation from million-degree hot gas in a X-ray binary star system has been assisted by five schools, four in the UK, collecting data with the two-metre Faulkes Telescope North in Hawaii. Their findings, combined with observations from a multitude of other observatories across the electromagnetic spectrum, were presented this week at the Royal Astronomical Society’s National Astronomy Meeting in Llandudno, Wales.

The Faulkes Telescopes are a pair of two-metre telescopes, one in Hawaii and the other in Australia, which are part of the Las Cumbres Observatory Global Telescope network and provide remote access to the night sky for schools and amateurs all around the globe. Four schools in the UK – St Brigid’s school in Denbigh, St David’s College in Cardiff, Paulet High School in Burton-on-Trent and The Kingsley School in Leamington Spa – as well as Czacki High School in Warsaw, Poland, used Faulkes Telescope North to collect data from the X-ray binary IGR J00291+5934 when it went into outburst in September 2008.

This X-ray binary system is a rare breed, incorporating a dense, spinning neutron star known as a pulsar and an ordinary Sun-like star that orbit one another. The gravity of the dense pulsar tears away material from the normal star, which forms an accretion disc of hot gas around the pulsar. Only 12 such systems are known, and every so often ‘00291’, as it is known for short, goes into outburst. In September 2008 the outburst increased its luminosity in X-rays by a factor of a thousand, and at visible wavelengths by a hundred times.

But then something odd happened. As the Faulkes Telescope, along with the Keck Telescope, the Isaac Newton Telescope, the Westerbork Synthesis Radio Telescope, the infrared-observing 1.3-metre PAIRITEL telescope, and the orbiting, X-ray detecting Swift, XMM-Newton and RXTE satellites, watched the outburst, they saw it suddenly fade back into obscurity after 20 days. This was extremely odd behaviour – outbursts usually last for months or years. Then, without warning, it brightened again ten days later. What was going on?

An artist’s impression of an X-ray binary, with a pulsar, an accretion disc and a normal star. Image: NASA/Goddard Space Flight Center.

The outbursts are thought to be driven by the accretion disc emptying as it dumps its mass onto the surface of the pulsar, causing a localised explosion, says Fraser Lewis of the University of Glamorgan, who led the research with Dr David Russell from the University of Amsterdam. The time between outbursts tells us of the size of the accretion disc and how long is needed to replenish it. But perhaps, rather than being evidence for an amazingly rapid turnaround and two separate outbursts, what Lewis’ team and the schoolchildren saw was one single interrupted outburst. They turned to Dr Jacob Hartman of the US Naval Laboratory in Washington for assistance, who suggested that a kind of ‘propellor’ effect, where infalling material is turned around and ejected from the system, brought a momentary halt to the outburst. Then whatever mechanism causes the propellor effect switches off and the outburst resumes, although the nature of this mechanism is currently a mystery.

“There are still many things we don’t understand,” says Lewis, but the outburst at least provided the opportunity for schoolchildren to become involved in cutting edge astrophysics research.

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