EUVE Observations of the Magnetic Cataclysmic Variable RE1938-461

The magnetic cataclysmic variable RE1938-461 was observed by the \euve\ Deep Survey instrument on July 8--9, 1992, during in-orbit calibration. It was detected in the Lexan/boron band (centered at 100 Angstroms) with a quiescent count rate of 0.0062+/- 0.0017 s(-1) and was not detected in the aluminum/carbon band (centered at 200 Angstroms). In addition, two transient events lasting ~ 1 hr occurred during the Lexan/boron pointing, the second at a count rate of 0.050+/- 0.006 s(-1) . This is appears to be the first detection of an EUV burst during the low state of a magnetic cataclysmic variable. In quiescence, RE1938-461 was observed with lower count rates than the corresponding \rosat\ Wide Field Camera count rates. 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. The two bursts may have been caused by intermittent accretion of dense filaments of matter, formed by an instability at the secondary 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 burst is inferred to be of the order of 10(32) (d/300 pc)(2) erg s(-1) . Alternatively, given the strong magnetic field of the primary, it is possible that these bursts are the EUV component of close binary flares resulting from magnetic reconnection. This work was supported by NASA grant NAGW-1290 and NASA contracts NAS5-30180 and NAS5-29298.