Text only version of this page
Slide 1 of 2
Contributed by: Peter Wheatley, University of Leicester, UK; Christopher Mauche, Lawrence Livermore National Laboratory; and Janet Mattei, American Association of Variable Star Observers (AAVSO: representing the 146 observers who contributed 785 optical observations).
Based on paper: Wheatley, P. J., Mauche, C. W., & Mattei, J. A. 2000, in Proceedings of the Brian Warner Symposium, in press. (Copies available at http://www.star.le.ac.uk/~pjw/papers/).
The figure shows the best multi-wavelength coverage ever obtained of a dwarf nova outburst. It is the result of a close collaboration between amateur optical observers, professional astronomers, and the EUVE and RXTE teams.
The outburst of SS Cyg (the brightest dwarf nova) was discovered in its very earliest stages by observers in the mainland USA and confirmed by a Hawaiian observer of the American Association of Variable Star Observers. A superb response by all those involved resulted in Target of Opportunity (TOO) observations by EUVE and RXTE within twelve hours of the discovery of the optical outburst - even before the outburst had reached the high-energy wavebands.
Dwarf novae are binary stars in which gas flows from a normal red dwarf star to an extremely dense white dwarf. To conserve angular momentum, this gas must orbit the white dwarf in an accretion disk before falling onto the degenerate star. The optical flux of dwarf novae is predominately from the outer disk, while the inner disk and the boundary layer between the disk and the surface of the white dwarf is responsible for the high-energy emission. The dramatic outbursts of dwarf novae are a result of an instability in the rate of mass transfer through the disk.
The optical, EUVE, and RXTE observations show that the outburst starts in the optical band and then about a day later moves to the X-ray and then the extreme ultraviolet (EUV) bands as the gas flow reaches the white dwarf. The dramatic switch at the beginning of the outburst from X-ray to EUV emission is the result of the boundary layer becoming optically thick as the mass transfer rate onto the white dwarf increases dramatically.This transition has never been observed before, and was only detected due to the rapid response of both EUVE and RXTE. A detailed study of these data will reveal the nature of the gas flow through the disk, and help us understand why it is unstable.