Issued by: Dr Jacqueline Mitton RAS Public Relations Officer Phone: Cambridge ((0)1223) 564914 FAX: Cambridge ((0)1223) 572892 E-mail: jmitton@ast.cam.ac.uk Stellar cataclysm under X-ray surveillance after amateur tip-off An outburst of a unique cataclysmic variable star is currently under surveillance by a powerful X-ray telescopes in space following a 'tip-off' by amateur observers, who spotted when the star's visual brightness started to rise in February. The star, known as GK Persei (or GK Per for short), brightens by a factor of 15 or so every three or four years, but the time of these outbursts is not exactly predictable. For this reason professional astronomers rely on the world wide network of amateur astronomers, looking at the sky with binoculars or small telescopes, to tell them when such outbursts start. In 1995 Dr Julian Osborne of the University of Leicester planned a campaign of observations of GK Per using NASA's large X-ray astronomy satellite called the X-ray Timing Explorer (XTE) which was then still to be launched. Eventually launched on December 30th, the XTE contains the largest area of X-ray detectors ever flown. Part of its mission is rapid follow-up of new events in the sky, such as the outbursts of cataclysmic variables. Because the outbursts of GK Per are not predictable, Dr Osborne relied on the amateur astronomers around the world to tell him when the outburst was starting. Observers belonging to organisations such as the AAVSO (American Association of Variable Star Observers) and the BAA (British Astronomical Association) make frequent brightness estimates of cataclysmic variables. Having been asked to pay special attention to GK Per, they were ready to make an announcement as soon as they saw it brightening. Eventually, after an early warning a few days before, the AAVSO announced in February that the outburst had started. Normally 13th magnitude, GK Per brightened to magnitude 10.3 (still much fainter than any star visible to the naked eye). The outburst of GK Per is still continuing today, with amateur astronomers watching it each night. As soon as he was alerted, Dr Osborne was able to ask for the XTE to be pointed at GK Per, which was done within 19 hours and, on February 27, a major campaign of observations using X-ray astronomy satellites was started. Since then there have been four more XTE observations of GK Per during this outburst. Dr Osborne has also made an observation of GK Per using the Japanese X-ray astronomy satellite ASCA in collaboration with colleagues from the USA and Japan. GK Per consists of a pair of stars orbiting each other at a separation of 2 million miles once every 2 days. One of the stars is a magnetic white dwarf, a dead star. Having burnt all its nuclear fuel it has collapsed down to the size of Earth although it is almost as massive as the Sun. The other star is a normal star but gas from its surface is being pulled onto the white dwarf because the two companions are so close. The gas spirals in towards the white dwarf, forming a disk as it does so. The heat given off makes the disk shine brighter than either of the two stars. Cataclysmic variables like GK Per undergo outbursts every now and again. During an outburst, not only does the disk get much brighter, but much more gas is dumped onto the white dwarf. Because this falls straight onto the white dwarf at very high speed from this disk, it gets tremendously hot (about 100 million degrees Celsius) and emits X-rays as a result. Because GK Per is much larger than typical cataclysmic variables, its outbursts last about 2 months, much longer than is usual. The relatively slow rise and fall of GK Per means that it is the only magnetic cataclysmic variable that gives astronomers sufficient time to study the changing behaviour through the outbursts. Such studies aim to describe the behaviour of gas at extreme conditions of temperatures, magnetic field and density that cannot be reached on Earth. The current outburst is a minor 'dwarf nova' outburst, but in 1901 GK Per became very much brighter when the gas dumped on the surface of the white dwarf star exploded like a huge nuclear bomb. That type of event is called a 'classical nova'. Stars like GK Per are believed to undergo classical nova outbursts every 10,000 years although GK Per may be unique as it is thought the 1901 explosion may have been the first by this pair of stars. The explosion blew off a huge shell of gas at speeds of several hundred kilometres per second. In powerful telescopes, the glowing remnant of this cataclysmic explosion is still visible today, almost a hundred years later. The picture, taken in 1993 by the astronomers Dr Tim O'Brien and Dr Andrew Slavin of Liverpool John Moores University using the UK's 4.2-metre William Herschel Telescope on La Palma, shows this ever-expanding gigantic shell of gas around GK Per. The bright object in the centre of the shell is light from the disk of material spiralling down onto the surface of the white dwarf star. Contact: Dr Julian P Osborne, X-ray Astronomy Group, Dept of Physics and Astronomy, University of Leicester, Leicester LE1 7RH Tel (0116) 2523598; Fax (0116) 2523311; e-mail julo@star.le.ac.uk Images The 1993 optical image of GK Per referred to in the text is available on the World Wide Web (address - http://www.livjm.ac.uk/astro/pics1.html#GKPer). Credit: Tim O'Brien and Andrew Slavin, Liverpool John Moores University. For a colour print contact Dr Osborne (see above). A line drawing illustrating the nature the class of magnetic cataclysmic variable to which GK Per belongs (known as 'intermediate polars') is available from the World Wide Web address: http://www.star.le.ac.uk:80/~julo/. For a hard copy, contact Dr Osborne (see above).