Spectroscopy of the intermediate polar FO Aquarii

Abstract

Analysis of 1000, 1-min spectra of FO Aqr reveals a peculiar absorption S-wave feature, varying with the orbital cycle, in the He I λ 4471 and Balmer lines. The absorption S-wave is blueshifted by 200 km s–1 from the line centre. The He II λ 4686 line shows a more usual emission S-wave which is not blueshifted. We suggest that the absorption feature results from material thrown out of the plane of the binary, possibly connected with the impact of the mass-transfer stream with the disc. We also draw parallels between this observation and the recent discovery of anomalous absorption in the globular cluster X-ray source, AC211. Because of the S-waves present in all lines we are unable to measure the orbital velocity of the white dwarf. The emission lines show a further radial-velocity variation with the spin cycle, phased with maximum blueshift at photometric maximum. This result supports the model for optical modulations in intermediate polars in which the variations are caused by the varying visibility of the accretion curtain as it sweeps round with the spin cycle. We exclude the alternative model of reprocessing by the disc. producing the optical modulations. Our data also indicate that the spin-modulated emission arises predominantly at one magnetic pole of the white dwarf. In addition we detect variations of the He II λ 4686 line with three orbital side-bands of the spin period. Thus our data display the full range of periods predicted by Warner.