Multiwavelength Observations of SS Cyg (Abstract)

Volume 28 number 2 (2000)

Janet A. Mattei

Christopher Mauche

Peter J. Wheatley

Abstract

(Abstract only) AAVSO observers have been participating in exciting multiwavelength observations of the outbursts of cataclysmic variables with several satellites such as the Extreme Ultraviolet Explorer (EUVE) and the Rossi X-ray Timing Explorer (RXTE). Through very fruitful collaborations among AAVSO observers, professional astronomers, and the EUVE and RXTE planning teams, these satellites have been pointed at the observing targets within hours of being alerted to the start of outbursts by AAVSO observers. Through simultaneous observations, excellent multiwavelength data have been obtained in the optical, extreme ultraviolet (EUV), and X-ray wavelengths in order to map the physical changes in the accretion disk and to understand better the outburst phenomenon. We describe the recent unprecedented results obtained from the simultaneous observations by AAVSO observers, EUVE, and RXTE during the outbursts of two bright dwarf novae, SS Cyg and U Gem. Multiwavelength coverage of the October 1996 outburst of SS Cygis the best ever obtained of a dwarf nova outburst. The multiwavelength observations show that the outburst starts in the optical band, and about a day later moves to the X-ray band as the gas flow in the accretion disk reaches the white dwarf. About twelve hours later there is a dramatic switch from X-ray to EUV emission. This transition had never been observed before and proved that the X-ray and EUV emissions arise from the same site, probably the boundary between the accretion disk and the white dwarf, and that the transition is the result of the boundary layer becoming optically thick as the mass transfer rate onto the white dwarf increases dramatically (Wheatley, P. J., Mauche, C. W., and Mattei, J. A. 2000, in Proceedings of the Brian Warner Symposium (held in Keele, UK, April 1999), in press). In the U Gem outburst of November 1997, the behavior of U Gem is very different from SS Cyg in that Xrays are brighter during outburst and are not quenched. These results may be due to the fact that U Gem is different from SS Cyg in several ways; its orbital period is shorter (4.4 hours) than SS Cyg's (6.6 hours) and the inclination of its oribit is high, almost edgeon to our line of sight. This viewing perspective may be the key to the apparently different X-ray behavior. We gratefully acknowledge the dedicated efforts and the observations of hundreds of variable star observers around the world. Their observations were vital to the success of these multiwavelength observations.