EXTREME-ULTRAVIOLET EXPLORER OBSERVATIONS OF THE MAGNETIC CATACLYSMIC VARIABLE RE 1938-461

The magnetic cataclysmic variable RE 1938-461 was observed by the Extreme 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) band. The Lexan/boron count rate is lower than the corresponding ROSAT wide-field camera Lexan/boron count rate. This is consistent with the fact that the source was in a low state during an optical observation performed 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 events 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 magnetic cataclysmic variable. We propose two possible explanations for the transient events. (1) They may have been caused by intermittent accretion of dense filaments of matter, formed by an instability at the second star surface, which impact and heat the white dwarf photosphere, causing it to emit EUV radiation. For an assumed blackbody temperature of 20-50 eV and an interstellar column density of 10(20) cm-2, the luminosity 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 approximately 6 x 10(-5) (d/300 pc)2 for a 0.7 M. white dwarf. (2) Given the high rotation rate of the secondary and the possibility that the secondary is magnetically active, it is possible that these transients are 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 approximately 10(33) (d/300 pc)2 ergs s-1 and the emission measure is approximately 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.