Recent Science Highlights

1 July 1995

(following information provided by Dr. Antonella Fruscione)

Christopher Mauche (LLNL), John Raymond (CfA), Janet Mattei (AAVSO), and Jay Holberg (LPL/UA) report the recent observation by EUVE of the dwarf nova VW Hydri in outburst and the subsequent quiescent period during the interval 1995 July 8 through 18.

• This pre-approved target-of-opportunity EUVE observation began less than 4 hours after an alert from the AAVSO (American Association of Variable Star Observers) that the source had gone into outburst. The source produced a count rate of about 0.2 cnts/s in the Deep Survey photometer for approximately the first 4 hours of the observation and then rose rapidly to about 4 cnts/s, approximately 0.5 days after the rise of the optical flux. Over the next day, the count rate declined slowly to its value before the outburst.
• In addition to EUVE, the Voyager spacecraft was observing this source simultaneously, and should produce spectroscopy in the waveband from 912 to 1150 Å to complement the EUV spectroscopy, which for this source extends from 80 to 350 Å.
• Simultaneous optical, far-UV, and EUV measurements are important to the study of dwarf novae because these bandpasses measure the flux produced by, respectively, the outer disk, the inner disk, and the boundary layer between the disk and the surface of the white dwarf. These measurements place important constraints on the mechanism responsible for dwarf nova outbursts, which is thought to be an instability in the thermal state of disk material due to the sharp increase of the opacity and specific heat of gas due to the partial ionization of hydrogen at temperature of about 6,000 degrees.