RXTE Discoveries

Amateur & Professional Astronomers Team Up to Capture Bursting Star in Action - June 2000

by Christopher Wanjek

Rochester, N.Y. -- 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.

The observations, which combine backyard optical telescope data with orbiting extreme-ultraviolet and X-ray data, reveal the nature of the flow of gas from a small red star onto its shrunken, dying companion. Dr. Janet A. Mattei, director of the American Association of Variable Star Observers (AAVSO) -- a non-profit, amateur astronomy organization -- presents the results of this unique observation today at the American Astronomical Society meeting in Rochester, N.Y.

Also presenting with Mattei are Dr. Christopher W. Mauche of the Lawrence Livermore National Laboratory and Dr. Peter J. Wheatley of the University of Leicester. Drs. Mauche and Wheatley led observations with the Extreme Ultraviolet Explorer (EUVE) and the Rossi X-ray Timing Explorer (RXTE) respectively.

"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 tail end or an afterglow."

The outburst took place in SS Cygni, a close binary star system in the constellation of Cygnus (The Swan) that contains a red dwarf star and a 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 fuel, blew its outer shell into space, and collapsed to form a white-hot ember. The dense white dwarf, with its strong gravitational potential, pulls a stream of gas off of its companion star. This transferred gas collects in a disk, called an accretion disk, around the white dwarf.

The dramatic brightening by many orders of magnitude in this system is the result of an instability in the disk, which forces the disk material to drain down onto the surface of the white dwarf. This causes a titanic energy release equivalent to that of 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 from the initial optical observation, still before the outburst had reached the high-energy wavebands that EUVE and RXTE observe.

The optical, EUVE and RXTE observations show that a white dwarf outburst starts in the optical band in the outer part of the accretion disk, moves to X-ray wavelengths about a day later, and then becomes detectable in the extreme ultraviolet as 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 the dramatic drop of 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. The increased density around the boundary allows the region to cool.

"This transition has never been observed before, and was only detected due to superb response by the staff of the EUVE and RXTE to the triggering observations by amateur astronomers" says Mattei. "A similar transition may occur in other accreting systems, such as black holes."

In gratitude for the successful SS Cygni observations and to give new opportunities to amateur astronomers, the EUVE director granted the AAVSO nearly three day's worth of observation time on EUVE to observe an object of their choice. Ground-based variable star observers have also successfully collaborated on the alphabet soup of NASA and European Space Agency missions, such as HEAO-1 and 2, IUE, Voyager, ASTRO-1 and 2, ROSAT, HIPPARCOS, ISO, ORFEUS, CGRO, HST and FUSE.

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 over 10 million observations, and growing by almost half a million yearly. The AAVSO headquarter is in Cambridge, Massachusetts.

For more information, refer to the AAVSO website at http://www.aavso.org. The talk, entitled "EUVE TOO Helps Reveal Stages of Optical, X-ray, and EUV Emission Observed in SS Cygni Outburst," will be held during session 13 at the Rochester meeting, from 10:00-11:30 a.m. in the Highland B/J meeting room.


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