Asynchronous Rotation in By Cam: It’S Got a Good Beat and You Can Dance to it (Very Slowly)

Abstract

Since its discovery, BY Cam was known to be an atypical AM Her type CV (Remillard et al. 1986). Observations showed the orbital and white-dwarf spin periods differ by about 1% (Silber et al. 1992). As the white dwarf rotates relative to the secondary, the accretion stream encounters different magnetic geometries, and hence follows different accretion paths to the white dwarf. The magnetic geometry seen by the accretion stream repeats at the beat period (1/Pbeat = 1/Pspin − 1/Porb), which is 7 to 20 d (the range is due to the uncertainty in the measured periods). The shape of the light curve changes from night to night, which can be explained by the changing geometry of the highly-beamed cyclotron emission. Even if the asynchronous rotation is the underlying cause of the changing light curve, the light curve could differ at similar beat phases if the threading process is chaotic. Previous attempts to study the beat period using IUE (Zucker et al. 1995) and optical data (Mason & Chanmugam 1992) have been inconclusive.