A VLT-ULTRACAM study of the fast optical quasi-periodic oscillations in the polar V834 Centauri

¹ LUTH, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: martine.mouchet@obspm.fr
² CEA Saclay, DSM/Irfu/Service d’Astrophysique, 91191 Gif-sur-Yvette, France
³ CEA-DAM-DIF, 91297 Arpajon, France
⁴ LERMA, Observatoire de Paris, PSL Research Univ., CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75005 Paris, France
⁵ South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa
⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁷ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
⁸ Instituto de Astrofisica de Canarias, 38205 La Laguna, Tenerife, Spain

Received: 29 November 2016
Accepted: 19 December 2016

Abstract

Quasi-periodic oscillations (QPOs) of a few seconds have been detected in some polars, the synchronised subclass of cataclysmic systems containing a strongly magnetised white dwarf which accretes matter from a red dwarf companion. The QPOs are thought to be related to instabilities of a shock formed in the accretion column, close to the white dwarf photosphere above the impact region. We present optical observations of the polar V834 Centauri performed with the fast ULTRACAM camera mounted on the ESO-VLT simultaneously in three filters (u′, He ii λ4686, r′) to study these oscillations and characterise their properties along the orbit when the column is seen at different viewing angles. Fast Fourier transforms and wavelet analysis have been performed and the mean frequency, rms amplitude, and coherence of the QPOs are derived; a detailed inspection of individual pulses has also been performed. The observations confirm the probable ubiquity of the QPOs for this source at all epochs when the source is in a high state, with observed mean amplitude of 2.1% (r′), 1.5% (He ii), and 0.6% (u′). The QPOs are present in the r′ filter at all phases of the orbital cycle, with a higher relative amplitude around the maximum of the light curve. They are also detected in the He ii and u′ filters but at a lower level. Trains of oscillations are clearly observed in the r′ light curve and can be mimicked by a superposition of damped sinusoids with various parameters. The QPO energy distribution is comparable to that of the cyclotron flux, consistent for the r′ and He ii filters but requiring a significant dilution in the u′ filter. New 1D hydrodynamical simulations of shock instabilities, adapted to the physical parameters of V834 Cen, can account for the optical QPO amplitude and X-ray upper limit assuming a cross section of the accretion column in the range ~(4 − 5) × 10¹⁴ cm². However, the predicted frequency is larger than the observed one by an order of magnitude. This shortcoming indicates that the QPO generation is more complex than that produced in a homogeneous column and calls for a more realistic 3D treatment of the accretion flow in future modelling.