EXTREME-ULTRAVIOLET EXPLORER OBSERVATIONS OF THE MAGNETIC CATACLYSMICVARIABLE RE 1938-461

The magnetic cataclysmic variable RE 1938-461 was observed by the Extr eme Ultraviolet Explorer (EUVE) Deep Survey instrument on 1992 July 8- 9 during in-orbit calibration. It was detected in the Lexan/boron (65- 190 angstrom) band, with a quiescent count rate of 0.0062 +/- 0.0017 s -1, and was not detected in the aluminum/carbon (160-360 angstrom) ban d. The Lexan/boron count rate is lower than the corresponding ROSAT wi de-field camera Lexan/boron count rate. This is consistent with the fa ct that the source was in a low state during an optical observation pe rformed just after the EUVE observation, whereas it was in an optical high state during the ROSAT observation. The quiescent count rates are consistent with a virtual cessation of accretion. Two transient event s lasting approximately 1 hr occurred during the Lexan/boron pointing, the second at a count rate of 0.050 +/- 0.006 s-1. This appears to be the first detection of an EUV transient during the low state of a mag netic cataclysmic variable. We propose two possible explanations for t he transient events. (1) They may have been caused by intermittent acc retion of dense filaments of matter, formed by an instability at the s econd star surface, which impact and heat the white dwarf photosphere, causing it to emit EUV radiation. For an assumed blackbody temperatur e of 20-50 eV and an interstellar column density of 10(20) cm-2, the l uminosity of the second transient is approximately 10(32) (d/300 pc)2 ergs s-1, and the fractional emitting area of the white dwarf is appro ximately 6 x 10(-5) (d/300 pc)2 for a 0.7 M. white dwarf. (2) Given th e high rotation rate of the secondary and the possibility that the sec ondary is magnetically active, it is possible that these transients ar e the EUV component of magnetic flares. For an assumed thermal plasma with a temperature of 10(6)-10(8) K and an interstellar column density of 10(20) cm-2, the luminosity of the second transient is approximate ly 10(33) (d/300 pc)2 ergs s-1 and the emission measure is approximate ly 10(55)-10(56) (d/300 pc)2 cm-3. We note that a magnetic flare or a coronal mass ejection may be the source of the intermittent accretion enhancements of the first scenario.