D. Steeghs

Multi-epoch Doppler tomography and polarimetry of QQ Vul ⋆

ABSTRA C T We present multi-epoch high-resolution spectroscopy and photoelectric polarimetry of the long-period polar (AM Herculis star) QQ Vul. The blue emission lines show several distinct components, the sharpest of which can unequivocally be assigned to the illuminated hemisphere of the secondary star and used to trace its orbital motion. This narrow emission line can be used in combination with Na i absorption lines from the photosphere of the companion to build a stable long-term ephemeris for the star: inferior conjunction of the companion occurs at HJDa 244 8446:4710O5UaE 0: 154 520 11O11U: The polarization curves are dissimilar at different epochs, thus supporting the idea of fundamental changes of the accretion geometry, e.g., between one- and two-pole accretion modes. The linear polarization pulses display a random scatter by 0.2 phase units and are not suitable for the determination of the binary period. The polarization data suggest that the magnetic (dipolar) axis has a colatitude of 238, an azimuth of 2508, and an orbital inclination between 508 and 708. Doppler images of blue emission and red absorption lines show a clear separation between the illuminated and non-illuminated hemispheres of the secondary star. The absorption lines on their own can be used to determine the mass ratio of the binary by Doppler tomography with an accuracy of 15‐20 per cent. The narrow emission lines of different atomic species show remarkably different radial velocity amplitudes: Ka 85‐130 km s 21 : Emission lines from the most highly ionized species, He ii, originate closest to the inner Lagrangian point L1. We can discern two kinematic components within the accretion stream; one is associated with the ballistic part, and the other with the magnetically threaded part of the stream. The location of the emission component associated with the ballistic accretion stream appears displaced between different epochs. Whether this displacement indicates a dislocation of the ballistic stream, e.g. by a magnetic drag, or emission from the magnetically threaded part of the stream with near-ballistic velocities, remains unsolved.

Emission-line oscillations in the dwarf nova V2051 Ophiuchi

We have detected coherent oscillations, at multiple frequencies, in the line and continuum emission of the eclipsing dwarf nova V2051 Ophiuchi using the 10m Keck II telescope. Our own novel data acquisition system allowed us to obtain very fast spectroscopy using a continuous readout of the CCD on the LRIS spectrograph. This is the first time that dwarf nova oscillations are detected and resolved in the emission lines. The accretion disc is highly asymmetric with a stronger contribution from the blue-shifted side of the disc during our observations. The disc extends from close to the white dwarf out to the outer regions of the primary Roche lobe. Continuum oscillations at 56.12s and its first harmonic at 28.06 s are most likely to originate on the surface of a spinning white dwarf with the fundamental period corresponding to the spin period. Balmer and Helium emission lines oscillate with a period of 29.77s at a mean amplitude of 1.9%. The line kinematics as well as the eclipse constraints indicate an origin in the accretion disc at a radius of 12 R_wd. The amplitude of the emission line oscillation modulates (0-4%) at a period of 488s, corresponding to the Kepler period at R=12 R_wd. This modulation is due to the beating between the white dwarf spin and the orbital motion in the disc. The observed emission line oscillations cannot be explained by a truncated disc as in the intermediate polars. The observations suggest a non-axisymmetric bulge in the disc, orbiting at 12R_wd, is required. The close correspondence between the location of the oscillations and the circularisation radius of the system suggests that stream overflow effects may be of relevance

On the observability of spiral structures in cataclysmic variable accretion discs

We use the grid of hydrodynamic accretion disc calculations of Stehle to construct orbital phase-dependent emission-line profiles of thin discs carrying spiral density waves. The observational signatures of spiral waves are explored to establish the feasibility of detecting spiral waves in cataclysmic variable discs using prominent emission lines in the visible range of the spectrum. For high Mach number accretion discs (Mvcs≃ 15 – 30), we find that the spiral shock arms are so tightly wound that they leave few obvious fingerprints in the emission lines. Only a minor variation of the double peak separation in the line profile at a level of ∼8 per cent is produced. For accretion discs in outburst (M≃ 5 – 20) however, the lines are dominated by the emission from an m=2 spiral pattern in the disc. We show that reliable Doppler tomograms of spiral shock patterns can be reconstructed provided that a signal-to-noise ratio of at least 15, a wavelength resolution of ∼80xa0kmxa0s−1 and a time resolution of ∼50 spectra per binary orbit are achieved. We confirm that the observed spiral pattern in the disc of IP Pegasi can be reproduced by tidal density waves in the accretion disc and demands the presence of a large, hot disc, at least in the early outburst stages.

Spiral shocks in the accretion disc of IP Peg during outburst maximum

In response to our recent discovery of spiral arms in the accretion disc of IP Peg during rise to outburst, we have obtained time-resolved spectrophotometry of IP Peg during outburst maximum. In particular, indirect imaging of He II 4686, using Doppler tomography, shows a two-arm spiral pattern on the disc image, which confirms repeatability over different outbursts. The jump in He II intensity (a factor of more than 2) and in velocity (∼ 200–300 km s-1) clarifies the shock nature of the spiral structure. The He II shocks show an azimuthal extent of ∼ 90 °, a shallow power-law emissivity ∼ V-1, an upper limit of 30 ° in opening angle, and a flux contribution of 15 per cent of the total disc emission. We discuss the results in view of recent simulations of accretion discs which show that spiral shocks can be raised in the accretion disc by the secondary star.

Eclipse maps of spiral shocks in the accretion disc of IP Pegasi in outburst

Eclipse light curves of the dwarf nova IP Peg during the 1996 November outburst are analysed with eclipse mapping techniques to constrain the location and investigate the spatial structure of the spiral shocks observed in the Doppler tomograms of Harlaftis et al. Eclipse maps in the blue continuum and in the Cxa0iii+Nxa0iiiλ4650 emission line show two asymmetric arcs of ∼90° in azimuth, extending from the intermediate to the outer disc regions R≃(0.2–0.6)RL1, where RL1 is the distance from the disc centre to the inner Lagrangian point], which are interpreted as being the spiral shocks seen in the Doppler tomograms. The Hexa0iiλ4686 eclipse map also shows two asymmetric arcs diluted by a central brightness source. The central source probably corresponds to the low-velocity component seen in the Doppler tomogram, and is understood in terms of gas outflow in a wind emanating from the inner parts of the disc. We estimate that the spirals contribute about 16 and 30xa0per cent of the total line flux, respectively, for the Hexa0ii and Cxa0iii+Nxa0iii lines. Comparison of the Doppler and eclipse maps reveals that the Keplerian velocities derived from the radial position of the shocks are systematically larger than those inferred from the Doppler tomography, indicating that the gas in the spiral shocks has sub-Keplerian velocities. We undertake simulations with the aim of investigating the effect of artefacts on the image reconstruction of the spiral structures.

SPIRAL STRUCTURE IN THE ACCRETION DISC OF THE BINARY IP PEGASI

We have found the first convincing evidence for spiral structure in the accretion disc of a close binary. The eclipsing dwarf nova binary IP Peg, observed during the end phase of a rise to outburst, shows strong Balmer and Helium emission lines in its spectra, with asymmetric double peaked velocity profiles produced in the accretion disc around the white dwarf. To reveal the two armed spiral on the accretion disc, we de-project the observed emission line profiles onto a Doppler coordinate frame, a technique known as Doppler tomography. The two armed spiral structure we see in the Doppler tomograms is expected to form when the disc becomes sufficiently large in outburst so that the tides induced by the secondary star can excite waves in the outer disc. Such spiral waves have been predicted in studies of tidal effects in discs and are fundamental in understanding the angular momentum budget of accretion discs.

Hot spiral structure in the accretion disk of the dwarf nova IP Pegasi

We present observational evidence of high-temperature spiral structure on the accretion disk of the dwarf nova IP Peg during outburst. The Doppler tomogram of ionized helium He II 4686 shows a two-arm spiral pattern on the disk image. The spiral shocks are generated by the tides raised by the secondary star on the disk and shows that a new mechanism may be important in transporting angular momentum out of the disk in addition to viscosity.