EUVE J0425.6−5714: A Newly Discovered AM Herculis Star

Abstract

ABSTRACTWe detected a new AM Her star serendipitously in a 25 day observation with the EUVE satellite. A coherent period of 85.82 minutes is present in the EUVE Deep Survey imager light curve of this source. A spectroscopic optical identification is made with a 19th magnitude blue star that has H and He emission lines, and broad cyclotron humps typical of a magnetic cataclysmic variable. A lower limit to the polar magnetic field of 50 MG is estimated from the spacing of the cyclotron harmonics. EUVE J0425.6−5714 is also detected in archival ROSAT HRI observations spanning 2 months, and its stable and highly structured light curve permits us to fit a coherent ephemeris linking the ROSAT and EUVE data over a 1.3 yr gap. The derived period is 85.82107 ± 0.00020 minutes, and the ephemeris should be accurate to 0.1 cycles until the year 2005. A narrow but partial X‐ray eclipse suggests that this object belongs to the group of AM Her stars whose viewing geometry is such that the accretion stream periodically occults the soft X‐ray emitting accretion spot on the surface of the white dwarf. A nondetection of hard X‐rays from ASCA observations that are contemporaneous with the ROSAT HRI shows that the soft X‐rays must dominate by at least an order of magnitude, which is consistent with a known trend among AM Her stars with large magnetic fields. This object should not be confused with the Seyfert galaxy 1H 0419−577 (=LB 1727), another X‐ray/EUV source that lies only 3.′95 away and that was the principal target of these monitoring observations.