Balmer absorption variability and the disc wind of V592 Cassiopeiae

We present results from time-series optical spectroscopy of the low-inclination, nova-like cataclysmic variable, V592 Cas. The data span the wavelength range from ∼4000 to 5000  Å, and include Balmer lines (Hβ to Hε) as well as He i and He ii . The Balmer lines are generally characterized by shallow absorption troughs with superimposed narrower central emission components. The absorption troughs are variable on time-scales of at least ∼20 min, but the fluctuations are asymmetric such that they are more dominant on the blueward side of the profile out to ∼−2000 km s⁻¹. Fourier analysis reveals modulation periods for emission radial velocities and absorption changes of ∼0.114 and 0.103 d; we support earlier suggestions that the latter is likely to be 1-day alias of the former, which is essentially the system orbital period. However, there is a phase lag of ∼0.3 between absorption and emission changes. The Balmer absorption changes are compared to the predicted behaviour caused by the (stream overflowing) SW Sex phenomenon. There are several discrepancies in this scenario, including the highly asymmetric nature of blueward changes, the velocity amplitudes, and the phase relation to the emission changes. The role of a disc wind in V592 Cas is also discussed; in particular, with this data set, we cannot rule in or rule out the possibility that axisymmetry of the outflow is broken because it is seated on a warped/tilted inner disc, which is implied by previous evidence for negative superhumps in this system.     

Observations of the SW Sextantis star UU Aquarii

We present 14 nights of medium resolution (1–2 Å) spectroscopy of the eclipsing cataclysmic variable UU Aquarii obtained during a high accretion state in 1995 August–October. UU Aqr appears to be an SW Sextantis (SW Sex) star, as noted by Baptista, Steiner & Horne, and we discuss its spectroscopic behaviour in the context of the SW Sex phenomenon. Emission-line equivalent width curves, Doppler tomography, and line profile simulation provide evidence for the presence of a bright spot at the impact site of the accretion stream with the edge of the disc, and a non-axisymmetric, vertically and azimuthally extended absorbing structure in the disc. The absorption has maximum depth in the emission lines around orbital phase 0.8, but is present from φ≈0.4 to φ≈0.95. An origin is explored for this absorbing structure (as well as for the other spectroscopic behaviour of UU Aqr) in terms of the explosive impact of the accretion stream with the disc.     

X-ray and ultraviolet observations of the dwarf nova VW Hyi in quiescence

We present an analysis of X-ray and ultraviolet (UV) data of the dwarf nova VW Hyi that were obtained with XMM–Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles on to the white dwarf. The plasma has a continuous temperature distribution that is well described by a power law or a cooling flow model with a maximum temperature of 6–8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of  8 × 10³⁰ erg s^-1  , which is only 20 per cent of the disc luminosity. The rate of accretion on to the white dwarf is  5 × 10⁻¹² M_⊙ yr⁻¹  , about half of the rate in the disc. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km s⁻¹, which is close to the rotation velocity of the white dwarf but is significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux, which is likely to be due to the rotation of the boundary layer. The X-ray and the UV flux show strong variability on a time-scale of ∼1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ∼100 s. The correlation indicates that the variable UV flux is emitted near the transition region between the disc and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ∼100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disc is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.     

Investigating the structure of the accretion disc in WZ Sge from multiwaveband time-resolved spectroscopic observations – II

We present our second paper describing multiwaveband time-resolved spectroscopy of WZ Sge. We analyse the evolution of both optical and IR emission lines throughout the orbital period and find evidence, in the Balmer lines, for an optically thin accretion disc and an optically thick hotspot. Optical and IR emission lines are used to compute radial velocity curves. Fits to our radial velocity measurements give an internally inconsistent set of values for K ₁, γ and the phase of red-to-blue crossing. We present a probable explanation for these discrepancies, and provide evidence for similar behaviour in other short orbital period dwarf novae. Selected optical and IR spectra are measured to determine the accretion disc radii. Values for the disc radii are found to be strongly dependent on the assumed WD mass and binary orbital inclination. However, the separation of the peaks in the optical emission line (i.e., an indication of the outer disc radius) has been found to be constant during all phases of the supercycle period over the last 40 years.     

Investigating the structure of the accretion disc in WZ Sge from multiwaveband time-resolved spectroscopic observations – I

We present the first of two papers describing an in-depth study of multiwaveband phase-resolved spectroscopy of the unusual dwarf nova WZ Sge. In this paper we present an extensive set of Doppler maps of WZ Sge covering optical and infrared emission lines, and describe a new technique for studying the accretion discs of cataclysmic variables using ratioed Doppler maps. Applying the ratioed Doppler map technique to our WZ Sge data shows that the radial temperature profile of the disc is unlike that predicted for a steady state α disc. Time-averaged spectra of the accretion disc line flux (with the bright spot contribution removed) show evidence in the shapes of the line profiles for the presence of shear broadening in a quiescent non-turbulent accretion disc. From the positions of the bright spots in the Doppler maps of different lines, we conclude that the bright spot region is elongated along the ballistic stream, and that the density of the outer disc is low. The velocity of the outer edge of the accretion disc measured from the H α line is found to be 723±23 km s⁻¹. Assuming that the accretion disc reaches to the 3:1 tidal resonance radius, we derive a value for the primary star mass of 0.82 M_⊙. We discuss the implications of our results on the present theories of WZ Sge type dwarf nova outbursts.     

On the absence of jet features in the optical spectra of cataclysmic variables

Blue- and redshifted hydrogen and helium satellite recombination lines have recently been discovered in the optical spectra of at least two supersoft X-ray sources (SSSs), RX J0513−069 and RX J0019.8+2156, and, tentatively, also in one short-period cataclysmic variable star (CV), the recurrent nova T Pyx. These features are thought to provide evidence for the presence of highly collimated jets in these systems. No similar spectral signatures have been detected in the spectra of other short-period CVs, despite a wealth of existing optical data on these systems. Here, we ask if this apparent absence of 'jet lines' in the spectra of most CVs already implies the absence of jets of the kind that appear to be present in the SSSs and perhaps T Pyx, or whether the current lack of jet detection in CVs can still be ascribed to observational difficulties.
To answer this question, we derive a simple, approximate scaling relation between the expected equivalent widths (EWs) of the observed jet lines in both types of systems and the accretion rate through the disc, EW(line)∝˙M^4/3_acc. We use this relation to predict the strength of jet lines in the spectra of 'ordinary' CVs, i.e. systems characterized by somewhat lower accretion rates than T Pyx. Making the assumption that the features seen in T Pyx are indeed jet lines, and using this system as a reference point, we find that, if jets are present in many CVs, they may be expected to produce optical satellite recombination lines with EWs of a few hundredths to a few tenths of an angstrom in suitably selected systems. A similar prediction is obtained if the SSS RX J0513−069 is used as a reference point. Such EWs are small enough to account for the non-detection of jet features in CVs to date, but large enough to allow them to be detected in data of sufficiently high quality, if they exist.     

The effective temperature distribution of steady-state, mass-losing accretion discs

Mass loss appears to be a common phenomenon among astrophysical accretion disc systems. An outflow emanating from an accretion disc can act as a sink for mass, angular momentum and energy, and can therefore alter the dissipation rates and effective temperatures across the disc. Here, the radial distributions of dissipation rate and effective temperature across a Keplerian, steady-state, mass-losing accretion disc are derived, using a simple, parametric approach that is sufficiently general to be applicable to many types of dynamical disc–wind models.
Effective temperature distributions for mass-losing accretion discs in cataclysmic variables are shown explicitly, with parameters chosen to describe both radiation-driven and centrifugally driven outflows. For realistic wind mass-loss rates of a few per cent, only centrifugally driven outflows – particularly those in which mass loss is concentrated in the inner disc – are likely to alter the effective temperature distribution of the disc significantly. Accretion discs that drive such outflows could produce spectra and eclipse light curves that are noticeably different from those produced by standard, conservative discs.     

The stream impact region in the disc of WZ Sge

We report the observation of new features in the spectrum of the cataclysmic variable WZ Sge. The disc eclipse is seen as a well-defined structure in the Hα line. From phases 0.25 to 0.5 an absorption feature of the same shape as the emission S wave is seen in this line, but redshifted by ∼ 200 km s⁻¹. Two possible interpretations of this feature are given, both of which imply that it originates at the impact point of the stream on the disc edge. In addition, evidence is found for substructure in the velocity map of the spot. Emission from line-emitting post-shock material, extending to about 60° downstream from the continuum hotspot, is seen in the Doppler map in the form of a tail extending from the hotspot. A theoretical estimate shows that such a tail is to be expected as a consequence of the post-impact hydrodynamics of the stream. A new determination of the system parameters is made. They agree with those of Gilliland et al.; in particular, the data support a high primary mass. The variation of Hα surface brightness with distance from the primary is flatter, in the inner regions, than the r ^−1.5 dependence found by Horne for other cataclysmic variables in quiescence.     

The boundary layer of VW Hydri in quiescence

In this short paper, we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the second component commonly identified in the far-UV as the 'accretion belt'. We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario. We consider detailed multiwavelength observations and in particular the recent Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of VW Hyi in quiescence, which confirm the presence of a second component (the 'accretion belt') in the far-UV spectrum. The temperature  (≈ 50 000 K)  and rotational velocity  (≈ 3000 km s ⁻¹)  of this second component are entirely consistent with the optically thick region  (τ≈ 1)  located just at the outer edge of optically thin boundary layer in the simulations of Popham. This second component contributes about 20 per cent of the far-UV flux. Using geometrical assumptions and taking into account the X-ray luminosity, we find that the total boundary layer luminosity sums up to   L _BL= (0.53 ± 0.25) L _disc  , while the theory (Kluźniak) predicts, for the rotation rate of VW Hyi's white dwarf,   L _BL≈ (0.76 ± 0.03) L _disc  . About one-fifth of the boundary layer energy is emitted in the X-ray and the remaining is emitted in the UV. This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by Pandel, Córdova & Howell, from which we deduce here that the viscosity in the boundary layer region must be of the order of  ν≈ 10¹³–10¹⁴ cm² s ⁻¹  , depending on the white dwarf mass and the size of the boundary layer.