UU Aqr from high to low state

In this paper we present Physical Parameter Eclipse Mapping (PPEM) of UBVRI eclipse light curves of UU Aqr from high to low states. We used a simple, pure hydrogen LTE model to derive the temperature and surface density distribution in the accretion disc. The reconstructed effective temperatures in the disc range between 9000 K and 15000 K in the inner part of the disc and below 7000 K in the outer parts. In the higher states it shows a more or less prominent bright spot with T_eff between about 7000 K and 8000 K. The inner part of the disc (R < 0.3R) is_L1 optically thick at all times, while the outer parts of the disc up to the disc edge (0.51 ± 0.04R_L1 in the high state and 0.40 ± 0.03R_L1 in the low state) deviate from a simple black body spectrum indicating that either the outer disc is optically thin or it shows a temperature inversion in the vertical direction. While during high state the disc is variable, it appears rather stable in low state. The variation during high state affects the size of the optically thick part of the disc, the white dwarf or boundary layer temperature and the uneclipsed component (originating in a disc chromosphere and/or cool disc wind), while the actual size of the disc remains constant. The difference between high and low state is expressed as a change in disc size that also affects the size of the optically thick part of the disc and the presence of the bright spot. Using the PPEM method we retrieve a distance for UU Aqr of 207±10 pc, compatible with previous estimates.     

Accretion stream mapping of HU Aquarii

We present a new mapping algorithm, the Accretion Stream Mapping (ASM), which uses the full phase-coverage of a light curve to derive spatially resolved intensity distributions along the accretion stream in magnetic cataclysmic variables of AM Herculis type (polars). The surface of the accretion stream is approximated as a 12-sided (duodecadon-shaped) tube. After successfully testing this method on artificial data we applied it to emission-line light curves of H β , H γ and He  ii λ 4686 of the bright eclipsing polar HU Aqr. We find hydrogen and helium line emission bright in the threading region of the stream where the stream couples on to magnetic field lines. It is particularly interesting that the stream is bright on the irradiated side facing the white dwarf, which highlights the interplay of collisional and radiative excitation/ionization.     

The magnetic SW Sextantis star RX J1643.7+3402

We present time-resolved spectroscopy and circular spectropolarimetry of the SW Sex star RX J1643.7+3402. We find significant polarization levels exhibiting a variability at a period of  19.38 ± 0.39  min. In addition, emission-line flaring is found predominantly at twice the polarimetric period. These two findings are strong evidences in favour of the presence of a magnetic white dwarf in the system. We interpret the measured periodicities in the context of our magnetic accretion model for SW Sex stars. In contrast with LS Pegasi – the first SW Sex star discovered to have modulated circular polarization – the polarization in RX J1643.7+3402 is suggested to vary at  2(ω−Ω)  , while the emission lines flare at  (ω−Ω)  . However, a  2ω/ω  interpretation cannot be ruled out. Together with LS Peg and V795 Her, RX J1643.7+3402 is the third SW Sex star known to exhibit modulated circular polarization.     

Indirect imaging of the accretion stream in eclipsing polars – III. HU Aquarii low state

We apply our technique for indirect imaging of the accretion stream to the polar HU Aqr, using eclipse profiles observed when the system was in a low-accretion state. The eclipse profile is different from that in the high state, and more variable from cycle to cycle. We find that the stream maps are brightest near the white dwarf and there is no significant brightening in the threading region. In the low state the stream threads on to the magnetic field closer to the L₁ point than in the high state, with a footpoint of the accreting field line at high latitude. We then produce maps of the accretion region from polarimetry using Stokes imaging. These show that the majority of the accretion occurs near the equator. The difference between the maps may be explained if most of the stream material is not emitting significantly in the low state. If so, neither the stream eclipse mapping nor Doppler tomography techniques will trace the bulk of the accretion flow between the two stars.     

Indirect imaging of the accretion stream in eclipsing polars – II. HU Aquarii

We apply our technique for indirect imaging of the accretion stream to the polar HU Aquarii, using eclipse profiles observed when the system was in a high accretion state. The accretion stream is relatively luminous, contributing as much as the accretion region on the white dwarf, or more, to the overall system brightness. We model the eclipse profiles using a model stream consisting of a ballistic trajectory from the L1 point followed by a magnetically channelled trajectory that follows a dipole field line out of the orbital plane. We perform model fits using two geometries: a stream that accretes on to both footpoints of the field line, and a stream that accretes only on to the footpoint of the field line above the orbital plane. The stream images indicate that the distribution of emission along the stream is not a simple function of the radial distance from the white dwarf. The stream is redirected by the magnetic field of the white dwarf at a distance 1.0–1.3×10¹⁰ cm from the white dwarf; this implies a mass transfer rate in the range 8–76×10¹⁶ g s⁻¹. The absorption dips in the light curve indicate that the magnetically entrained part of the stream moves from 42° to 48° from the line of centres over the three nights of observation. This is in close agreement with the results of the one-footpoint models, suggesting that this is the more appropriate geometry for these data. The stream images show that, in almost all sections of the stream, the flux peaks in B and is successively fainter in U , V and R .     

The changing X-ray light curves of the intermediate polar FO Aquarii

Comparison of five X-ray observations of the intermediate polar FO Aqr reveals that the morphology of the X-ray light curve changes considerably with time. In particular, power spectra of the 1988 Ginga   and 1993 ASCA   data reveal strong sideband and orbital variations, whereas the 1990 Ginga   observation does not. This suggests that the amount of stream-fed accretion varies with epoch, and the system was accreting predominantly from a disc in 1990.   In contrast to other intermediate polars, the X-ray spin-pulse profiles show significant variations between observations, ranging from relatively sinusoidal to sawtooth-shaped at medium energies. During the 1988 and 1990 observations a notch is visible at spin phase zero, due to the presence of an interpulse at phase 0.85, which is absent during the other observations. At lower energies a narrow pulse of emission is seen at spin phase 0.2.   We interpret the pulse profile from the 1990 Ginga   observation using a model for accretion from a disc on to a dipolar magnetic field, the axis of which is offset from the white dwarf centre by ∼ 0.15 white dwarf radii. In order to account for the later occurrence of the hardness-ratio maximum in 1988 and 1993, we suggest that the accretion-rate profile changes so that accretion is favoured along the field lines which trail the magnetic pole. This also accounts for the disappearance of the interpulse and notch in 1993.     

Doppler signatures of Hα flares in AE Aquarii

We present high-speed spectrophotometric observations of the Hα emission in the exotic cataclysmic variable AE Aqr. Over 10 000 spectra with a sampling rate of ∼ 6 s were obtained. The well-known 16.5- and 33-s oscillations were not detected in either the continuum or the line, nor were any quasi-periodic oscillations detected. The Hα line shows remarkably complex behaviour, with changes in flux of up to a factor of 3 in a few hours. The line does not act as a single entity — the instantaneous line profile is highly asymmetric and consists of a superposition of emission components that are localized in velocity. We find that during flares the Hα emission is delayed and decays more slowly than the continuum. The Hα variations are not linearly correlated with the continuum variations. Examination of the trailed spectrograms and Doppler tomograms reveals no signatures of an accretion disc, but rather supports the idea that mass is being ejected by a propeller mechanism.     

V348 Puppis: a new SW Sex star in the period gap

We present time-resolved optical spectroscopy and photometry of the nova-like cataclysmic variable V348 Puppis. The system displays the same spectroscopic behaviour as SW Sex stars, so we classify V348 Pup as a new member of the class. V348 Pup is the second SW Sex system (the first is V795 Herculis) which lies in the period gap. The spectra exhibit enhanced He  ii λ 4686 emission, reminiscent of magnetic cataclysmic variables. The study of this emission line gives a primary velocity semi-amplitude of     . We have also derived the system parameters, obtaining:     ,         ,     and     . The spectroscopic behaviour of V348 Pup is very similar to that of V795 Her, with the exception that V348 Pup shows deep eclipses. We have computed the '0.5-absorption' spectrum of both systems, obtaining spectra that resemble the absorption spectrum of a B0 V star. We propose that absorption in SW Sex systems can be produced by a vertically extended atmosphere which forms where the gas stream re-impacts the system, either at the accretion disc or at the magnetosphere of the white dwarf (assuming a magnetic scenario).     

A normal and superoutburst study of the eclipsing SU UMa star: DV Ursae Majoris

We report on time-resolved photometry carried out during the 1995 short outburst and the 1997 long outburst in the eclipsing dwarf nova DV UMa. The revised orbital period is 0.0858526172 (67) d. We detected gigantic superhumps with an amplitude of ∼0.6 mag in the mid-phase of the 1997 outburst, revealing the SU UMa nature of DV UMa. The superhump period is 0.0887 (4) d. The superhumps became less clear during the late phase of the superoutburst, and we found two possible periods of 0.0885 (15) and 0.0764 (15). During both outbursts, the eclipse was wide and shallow near the maximum, and then became narrower and deeper, which is qualitatively well explained by the current disc instability theory.     

Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables – I. Observations of VW Hyi

From archived and recent high-speed photometry of VW Hyi we find dwarf nova oscillations (DNOs) occasionally present throughout outburst, evolving from a 14.06-s period at maximum to >40 s near the end of outburst. A relatively slow increase of period is followed by a rapid increase and a subsequent decrease.
Quasi-periodic oscillations (QPOs) are seen at periods of hundreds of seconds. For the first time, the evolution of a QPO period is seen, increasing steadily during the final decline of an outburst. The occasional presence of two DNOs, separated in frequency by the QPO frequency, suggests reprocessing of the rotating DNO beam by a 'wall' rotating progradely in the disc at the QPO period.     

Simultaneous UBVRI observations of the cataclysmic variable AE Aquarii: Temperatures and masses of fireballs

We report simultaneous multicolour observations in 5 bands (UBVRI) of the flickering variability of the cataclysmic variable AE Aqr. Our aim is to estimate the parameters (colours, temperature, size) of the fireballs that produce the optical flares. The observed rise times of the optical flares are in the interval 220‐440 s. We estimate the dereddened colours of the fireballs as (U ‐B)₀∼0.8‐1.4, (B ‐V)₀∼0.03‐0.24, and (V ‐I)₀∼0.26‐0.78. We find for the fireballs temperatures of 10000‐25000 K, masses of (7‐90)x10¹⁹ g, and sizes of (3‐7)x10⁹ cm (using a distance of d = 86 pc). These values refer to the peak of the flares observed in the UBVRI bands. The data are available upon request from the authors (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)