7 – 18 August 2000
Media release
From Jacqueline Mitton (Meeting Press Officer)
phone: +44 (0)1223 564914
Phone contact 7 – 16 August [Meeting Press Room]
+44 (0)161 275 7832
+44 (0)161 275 9458
+44 (0)161 275 9499
Mobile phone 07770 386133
Date released: 14 August 2000
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Dr Janet A. Mattei
American Association of Variable Star Observers
Phone: (+1) 617 354 0484
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Triggered by phone calls from amateur astronomers about a flaring white dwarf star, scientists quickly repositioned two NASA satellites to observe the event from start to finish in unprecedented detail. Full analysis of the observations made of this event in October 1996 have recently been completed. They combined the ‘backyard’ optical data obtained by amateurs with extreme ultraviolet and X-ray data from orbiting satellites to reveal the nature of the flow of gas from a small red star onto its shrunken, dying companion.
The results were presented at the General Assembly of the International Astronomical Union in Manchester on 12 August by Dr Janet Mattei, director of the American Association of Variable Star Observers (AAVSO). Co-authors of the work are Dr Christopher W. Mauche of the of the Lawrence Livermore National Laboratory who led observations with the Extreme Ultraviolet Explorer (EUVE) and Dr Peter J. Wheatley of the University of Leicester (UK), who led the observations made with the Rossi X-ray Timing Explorer (RXTE).
"For years, amateur astronomers have tipped off professionals to bursting star systems and other cosmic events" said Mattei. "This time we nipped the action in the bud and notified scientists working on two different satellites. The result was an in-depth, multi-wavelength observation of an entire event, not just the start or a tail end."
The outburst took place in SS Cygni, a close binary star system in the constellation Cygnus (the Swan). SS Cygni contains a red dwarf star and white dwarf. A red dwarf is a star a little cooler than the Sun. The white dwarf was once as large as the Sun but subsequently ran out of nuclear fuel, blew its outer shell into space and collapsed to form a white-hot ember. The dense white dwarf, with its strong gravitational attraction, pulls a stream of gas from its companion star. This transferred gas collects in a disk, called an accretion disk, around the white dwarf star.
The dramatic brightening that occurs is the result of an instability in the disk, which forces the disk material to rain down onto the surface of the white dwarf. This causes a titanic release of energy equivalent to billions of atomic bombs exploding every second. Such stellar explosions, which often occur without warning and rarely last more than one or two weeks, serve as flood-lights that brighten a dim star system for scientists to study.
The SS Cygni outburst was discovered in its very earliest stages by observers in Kansas and California and confirmed by a Hawaiian observer, all amateur astronomers and members of the AAVSO. They called Mattei and she called Mauche and Wheatley, who respectively arranged for the EUVE and RXTE to turn to the event. This was within 12 hours of the initial optical observation, still before the outburst was producing the high-energy radiation that the EUVE and RXTE observe.
The optical, EUVE and RXTE observations show that a white dwarf outburst starts with the radiation of visible light, which comes from the outer part of the accretion disk. The radiation changes its character about a day later, and X-rays are observed. Extreme ultraviolet radiation is detected when the gas flow reaches the white dwarf. The dramatic switch at the beginning of the outburst from X-ray to extreme ultraviolet emission is the result of a drop in the temperature of the boundary layer between the accretion disk and the white dwarf from one hundred million degrees to a hundred thousand degrees Kelvin (Celsius). The increased density around the boundary allows the region to cool.
"This transition has never been observed before, and was only detected due to the superb response by the staff of the EUVE and RXTE to the observations by the amateur astronomers" says Mattei. "A similar transition occurs in other accreting systems like SS Cygni and to observe it one needs fast response and multi-satellites, observing at different wavebands.
In gratitude for the successful SS Cygni observations and to give new opportunities to amateur astronomers, the EUVE director granted the AAVSO nearly three days’ worth of observation time on EUVE for an object of their choice.
The AAVSO was founded in 1911 at Harvard College Observatory to coordinate variable star observations made largely by amateur astronomers and became an independent non-profit organization in 1954. Today, AAVSO has members in over 40 countries and maintains the world’s largest computer-readable variable star data archive with nearly 10 million observations, growing by almost half a million yearly. For more information, refer to the AAVSO web site