AMATEURS HELP PROFESSIONAL ASTRONOMERS WITH X-RAY OBSERVATIONS OF MAGNETIC STAR On February 27 of this year a major campaign of observations using X-ray astronomy satellites was started on the cataclysmic variable GK Per by Dr Julian Osborne of the University of Leicester. GK Per 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. 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, it is dead, having burnt all its nuclear fuel it has collapsed down to the size of the earth although it is almost as massive as the sun. The other star is a normal star, but because of its close proximity to the white dwarf gas is pulled from its surface towards the collapsed star. 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 gas falls straight onto the white dwarf at very high speed from this disk, it gets tremendously hot (about 100 million degrees C) and so emits X-rays. Because GK Per is much larger than typical cataclysmic variables, its outbursts last much longer (about 2 months) than is usual. This 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 temperatures, magnetic fields and densities that cannot be reached on earth. Last year Dr Osborne planned a campaign of observations of GK Per using a then still-to-be-launched large NASA X-ray astronomy satellite called the "X-ray Timing Explorer". Eventually launched on December 30, the XTE contains the largest area of X-ray detectors ever flown. Part of its mission is to rapidly follow-up 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, and having been asked to pay special attention to GK Per 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 that the outburst had started. Dr Osborne was then able to ask the XTE to point at GK Per, which they did within 19 hours. Since then there have been 4 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. The outburst of GK Per is still continuing today, with amateur astronomers watching it each night. This outburst has been 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. This type of outburst is called a `classical nova'. Classical nova events are believed to take place every 10,000 years although GK Per may be unique as it is thought the 1901 explosion may have been the first for this object. The explosion blew off a huge shell of gas at speeds of several hundred kilometres per second. The glowing remnant of this cataclysmic explosion is still visible today, almost a hundred years later, using powerful telescopes. The picture, taken in 1993 by the astronomers Dr Tim O'Brien and Dr Andrew Slavin from 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. Contacts 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 email julo@star.le.ac.uk Images The picture referred to in the text is available on the world wide web (address - http://www.livjm.ac.uk/astro/pics1.html#GKPer). I can supply colour prints if required. A line drawing of an Intermediate Polar (the class of magnetic cataclysmic variable of which GK Per is a member) is available from the web address http://www.star.le.ac.uk:80/~julo/. Again, I can supply hard copies if needed.