Fast quasi-periodic oscillations in the eclipsing polar VV Puppis from VLT and XMM-Newton observations

We present high time resolution optical photometric data of the polar VV Puppis obtained simultaneously in three filters (u', HeII $\lambda$4686, r') with the ULTRACAM camera mounted at the ESO-VLT telescope. An analysis of a long 50 ks XMM-Newton observation of the source, retrieved from the database, is also provided. Quasi-periodic oscillations (QPOs) are clearly detected in the optical during the source bright phase intervals when the accreting pole is visible, confirming the association of the QPOs with the basis of the accretion column. QPOs are detected in the three filters at a mean frequency of $\sim$ 0.7 Hz with a similar amplitude $\sim$ 1\%. Mean orbitally-averaged power spectra during the bright phase show a rather broad excess with a quality factor Q= $\nu$/$\Delta \nu$ = 5-7 but smaller data segments commonly show a much higher coherency with Q up to 30. The XMM (0.5--10 keV) observation provides the first accurate estimation of the hard X-ray component with a high kT $\sim$ 40 keV temperature and confirms the high EUV-soft/hard ratio in the range of (4--15) for VV Pup. The detailed X-ray orbital light curve displays a short $\Delta \phi \simeq 0.05$ ingress into self-eclipse of the active pole, indicative of a accretion shock height of $\sim$ 75 km. No significant X-ray QPOs are detected with an amplitude upper limit of $\sim$30\% in the range (0.1--5) Hz. Detailed hydrodynamical numerical simulations of the post-shock accretion region with parameters consistent with VV Pup demonstrate that the expected frequencies from radiative instability are identical for X-rays and optical regime at values $\nu$ $\sim$ (40--70) Hz, more than one order magnitude higher than observed. This confirms previous statements suggesting that present instability models are unable to explain the full QPO characteristics within the parameters commonly known for polars.
Comment: 11 pages, 10 figures, accepted for publication in A&A