Two-second variability in AM Herculis binaries

Abstract

AN UMa and the newly identified AM Her star E1405--451 both show excesses of optical variability in the broad frequency range from 0.4 to 0.8 Hz (2.5--1.25 s). The optical fluxes from AN UMa and E1405--451 must vary by 2.4% and 1.2% rms, respectively, in order to produce the level of noise observed in the power spectra. The noise is not produced during any one section of time or orbital phase but is approximately a constant proportion of the net optical light from E1405--451. The feature is possibly bimodal in AN UMa with a minimum of power at 0.65 Hz. Time series observations of AM Her made during both its ''bright'' and ''faint'' phases show that no such noise feature is present to less than 0.25% and 1.3% rms of the stellar light. The predictions of a model for this phenomenon, recently advanced by Langer, Chanmugam, and Shaviv and based on a thermal instability in the height of a standoff accretion shock, are tested with inconclusive results.