A survey for QPOs in AM Herculis stars and a detailed study of the QPOs in AN Ursae Majoris

• Published in: Monthly notices of the Royal Astronomical Society Vol. 260; no. 1; pp. 209 - 220
• Main Authors: Ramseyer, Tod F., Robinson, Edward L., Zhang, Erho, Wood, Janet H., Stiening, Rae F.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford University Press 01.01.1993; Blackwell Science
• Subjects: accretion; accretion discs; Astronomy; binaries: close; cataclysmic variables; Earth, ocean, space; Exact sciences and technology; novae; Novae, dwarf novae, recurrent novae, and other cataclysmic (eruptive) variables; Stars; stars: individual: AN UMa; stars: individual: VW Pup; stars: oscillations; Variable and peculiar stars (including novae); Photometric observation; Four color photometry; Cataclysmic variable star; Oscillation; AM Herculis binary; Nova; Close binary; Light curve; Accretion disk; Quasi periodic variation
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/260.1.209

We have searched for quasi-periodic oscillations with frequencies between 0.1 and 2.4 Hz in the light curves of nine AM Her stars using simultaneous multi-colour photometry. Oscillations were not seen in BY Cam ( = H 0538 + 608), EF Eri, AM Her, DP Leo ( = E 1114 + 182), ST LMi ( = CW 1103 + 254), MR Ser ( = PG 1550 + 191) or QQ Vul ( = E 2003 + 225), but were detected in AN UMa and VV Pup. We were unable to find any systematic difference between those systems with and those without quasi-periodic oscillations. When averaged over the orbital period, the amplitude of the oscillations in AN UMa is largest in the B bandpass ( ~ 3.5 per cent). The colour of the oscillations varies systematically with orbital phase, however, and the amplitude is at least as high in the U band as in the B band during the main hump in the light curve. The average coherence time as measured with autocorrelation techniques is 37 ± 2 s for strong oscillations in the B band. We show that the accretion column is divided into 2–45 regions, each of which is oscillating simultaneously but independently. Our observations are consistent with the shock-oscillation model.