Abstract
We present high time resolution optical photometric data of the polar VV Puppis obtained simultaneously in three filters (u′, HeII λ4686, 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 ∼0.7 Hz with a similar amplitude ∼1%. Mean orbitally-averaged power spectra during the bright phase show a rather broad excess with a quality factor Q = ν/Δν = 5−7 but smaller data segments commonly show a much higher coherency with Q up to 30. The X-ray Multi-mirror Mission XMM (0.5–10 keV) observation provides the first accurate estimation of the hard X-ray component with a high kT ∼ 40 keV temperature and confirms the high extreme ultraviolet (EUV)-soft/hard ratio in the range of 4−15 for VV Pup. The detailed X-ray orbital light curve displays a short Δϕ ≃ 0.05 ingress into self-eclipse of the active pole, indicative of an accretion shock height of ∼75 km. No significant X-ray QPOs are detected with an amplitude upper limit of ∼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 ν ∼ 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.
Highlights
The star VV Pup was recognised very early on as a periodic variable star from the analysis of photographic plates taken at the defunct Union Observatory (Johannesburg, South Africa)
We report on new fast optical photometric observations of VV Pup, which were obtained at the Very Large Telescope (VLT) in order to better characterise the fast quasi-periodic oscillations and to compare their properties with what was expected from the standard accretion model and from improved magnetohydrodynamic (MHD) numerical simulations
– The Quasi-periodic oscillations (QPOs) are always solely observed during the bright phase, indicating that the QPO region is eclipsed and likely linked to the bottom of the accretion column
Summary
The star VV Pup was recognised very early on as a periodic variable star from the analysis of photographic plates taken at the defunct Union Observatory (Johannesburg, South Africa). 1D numerical hydrodynamic simulations for a large sample of polar physical parameters have shown that the dominant frequencies of the shock oscillations are generally an order of magnitude greater than the typical QPO detected frequencies (Busschaert et al 2015; Van Box Som et al 2018a). These inconsistencies show that the standard column model is not complete and that there is a lack of a fundamental physical ingredient. We present the first detailed VV Pup X-ray light curve and spectrum during a high state, which was obtained from an extended XMM-Newton observation
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