The outburst properties of AM CVn stars

We briefly summarise the observational properties of ultra‐compact binaries called AM CVn stars. We analyse their outbursts originating from the thermal‐viscous instability in helium accretion disc. We present our preliminary results in applying the model of Dwarf Novae outbursts to helium discs. We can calculate models of outbursts of reasonable amplitude of 2 mag with a constant α parameter throughout the calculation. Setting the mass transfer rate close to its upper critical value produces model lightcurves that resemble short superoutbursts (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the maximal value of the turbulent α ‐parameter in accretion discs

In this short paper we show that making turbulence two‐rather than three‐dimensional may increase the effective turbulent viscosity by about 40 %. Dimensionless hydrodynamical viscosity parameters up to α_max = 0.25 M_t² may be obtained in this approach, which are in better agreement with the observational data on non‐stationary accretion than the values obtained in numerical simulations. However, the α ‐parameter values known from observations are still several times higher (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Substantial stream–disc overflow found in three-dimensional SPH simulations of cataclysmic variables

We study numerically the interaction of the infalling gas stream and the rim of the accretion disc in cataclysmic variables. The simulations were performed with a smoothed particle hydrodynamics scheme with high spatial resolution. Parameters of the systems AM CVn, OY Car, DQ Her, U Gem and IP Peg were used for the simulations. The simulations cover a wide range of orbital periods, mass ratios and mass transfer rates, as well as different thermal states of the accretion disc. The main result of this study is that the accretion stream is not stopped at the impact region (the bright spot at the outer rim of the disc). In fact, after undergoing the shock interaction, most of the matter is deflected vertically and flows in a more or less diffuse stream to inner parts of the disc, hitting the disc surface close to the circularization radius at orbital phase 0.5. This is a common feature in all systems for all simulated parameters. This stream overflow can cause the X-ray absorption dips observed in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs) around orbital phase 0.7, if the inclination is at least 65°. Under certain circumstances, namely a sudden increase of the mass transfer rate from the secondary or a rather small disc, parts of the overflowing stream bounce off the disc surface after hitting it at orbital phase ≈0.5. Another absorption region can be expected around orbital phase 0.2.
In our simulations most of the infalling matter reaches the inner disc very quickly. This must alter the evolution of the quiescent disc and the outburst behaviour considerably compared with purely viscous transport of the material through the disc from the outer rim, and therefore should be taken into account in dwarf nova outburst cycle calculations. To our knowledge, the consequences of such a massive stream overflow for the dwarf nova outburst cycle have not been considered yet.     

SPH simulations of negative (nodal) superhumps: a parametric study

Negative superhumps in cataclysmic variable systems result when the accretion disc is tilted with respect to the orbital plane. The line of nodes of the tilted disc precesses slowly in the retrograde direction, resulting in a photometric signal with a period slightly less than the orbital period. We use the method of smoothed particle hydrodynamics to simulate a series of models of differing mass ratio and effective viscosity to determine the retrograde precession period and superhump period deficit  ɛ₋  as a function of system mass ratio q . We tabulate our results and present fits to both  ɛ₋  and  ɛ₊  versus q , as well as compare the numerical results with those compiled from the literature of negative superhump observations. One surprising result is that while we find negative superhumps most clearly in simulations with an accretion stream present, we also find evidence for negative superhumps in simulations in which we shut off the mass transfer stream completely, indicating that the origin of the photometric signal is more complicated than previously believed.     

Atlas of tilted accretion discs and source to negative superhumps

Using smoothed particle hydrodynamics, we numerically simulate steady-state accretion discs for cataclysmic variable dwarf novae systems that have a secondary-to-primary mass ratio  0.35 ≤ q ≤ 0.55  . After these accretion discs have come to quasi-equilibrium, we rotate each disc out of the orbital plane by  δ= (1, 2, 3, 4, 5 or 20)°  to induce negative superhumps. For accretion discs tilted  5°  , we generate light curves and associated Fourier transforms for an atlas on negative superhumps and retrograde precession. Our simulation results suggest that accretion discs need to be tilted more than 3° for negative superhumps to be statistically significant. We also show that if the disc is tilted enough such that the gas stream strikes a disc face, then a dense cooling ring is generated near the radius of impact.
In addition to the atlas, we study these artificially tilted accretion discs to find the source to negative superhumps. Our results suggest that the source is additional light from innermost disc annuli, and this additional light waxes and wanes with the amount of gas stream overflow received as the secondary orbits. The nodes, where the gas stream transitions from flowing over to under the disc rim (and vice versa), precess in the retrograde direction.     

Non-linear bending waves in Keplerian accretion discs

The non-linear dynamics of a warped accretion disc is investigated in the important case of a thin Keplerian disc with negligible viscosity and self-gravity. A one-dimensional evolutionary equation is formally derived that describes the primary non-linear and dispersive effects on propagating bending waves other than parametric instabilities. It has the form of a derivative non-linear Schrödinger (DNLS) equation with coefficients that are obtained explicitly for a particular model of a disc. The properties of this equation are analysed in some detail and illustrative numerical solutions are presented. The non-linear and dispersive effects both depend on the compressibility of the gas through its adiabatic index Γ. In the physically realistic case Γ < 3, non-linearity does not lead to the steepening of bending waves but instead enhances their linear dispersion. In the opposite case Γ > 3, non-linearity leads to wave steepening and solitary waves are supported. The effects of a small effective viscosity, which may suppress parametric instabilities, are also considered. This analysis may provide a useful point of comparison between theory and numerical simulations of warped accretion discs.     

Multi‐epoch time‐resolved photometry of the eclipsing polar CSS081231:071126+440405

The eclipsing polar CSS081231:071126+440405 turned bright (V_max ∼ 14.5) in late 2008 and was subsequently observed intensively with small and medium‐sized telescopes. A homogeneous analysis of this comprehensive dataset comprising 109 eclipse epochs is presented and a linear ephemeris covering the five years of observations, about 24000 orbital cycles, is derived. Formally this sets rather tight constraints on the mass of a hypothetical circumbinary planet, M_pl ≤ 2 M_Jup. This preliminary result needs consolidation by long‐term monitoring of the source. The eclipse lasts 433.08 ± 0.65 s, and the orbital inclination is found to be i = 79.3°–83.7°. The centre of the bright phase displays accretion‐rate dependent azimuthal shifts. No accretion geometry is found that explains all observational constraints, suggesting a complex accretion geometry with possible pole switches and a likely non‐dipolar field geometry. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The standstill luminosity in Z Cam systems

We consider accretion discs in close binary systems. We show that the heating of a disc at the impact point of the accretion stream contributes significantly to the local energy budget at its outer edge. As a result, the thermal balance relation between local accretion rate and surface density (the 'S-curve') changes; the critical mass transfer rate above which no dwarf nova outbursts occur can be up to 40 per cent smaller than without impact heating. Standstills in Z Cam systems thus occur at smaller mass transfer rates than otherwise expected, and are fainter than the peak luminosity during the dwarf nova phase as a result.     

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.     

Gravitational instability of discs with dissipative coronae around supermassive black holes

We study the dynamical structure of a self-gravitating disc with coronae around a supermassive black hole. Assuming that the magnetorotational instability responsible for generating the turbulent stresses inside the disc is also the source for a magnetically dominated corona, a fraction of the power released when the disc matter accretes is transported to and dissipated in the corona. This has a major effect on the structure of the disc and its gravitational (in)stability according to our analytical and self-consistent solutions. We determine the radius where the disc crosses the inner radius of gravitational instability and forms the first stars. Not only the location of this radius which may extend to very large distances from the central black hole, but also the mass of the first stars highly depends on the input parameters, notably the viscosity coefficient, the mass of the central object and the accretion rate. For accretion discs around quasi-stellar objects (QSOs) and the Galactic Centre, we determine the self-gravitating radius and the mass of the first clumps. Comparing the cases with a corona and without a corona for typical discs around QSOs or the Galactic Centre, when the viscosity coefficient is around 0.3, we show that the self-gravitating radius decreases by a factor of approximately 2, but the mass of the fragments increases with more or less the same factor. The existence of a corona implies a more gravitationally unstable disc according to our results. The effect of a corona on the instability of the disc is more effective when the viscosity coefficient increases.     

Angular momentum transport by thermal emission in black hole accretion disks

We calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disk, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disk material. While thermal emission can not drive accretion on its own, we show that along with disk heating it does nonetheless result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity‐driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi‐color disk is ∼0.4r_s/r, where r_s is the Schwarzschild radius of the black hole. We briefly highlight the key similarties between this effect and the closely related Poynting‐Robertson effect (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relativistic Fe Kα features in the XMM‐Newton spectrum of the intermediate Seyfert galaxy 4U 1344‐60

We present the analysis of optical and X‐ray XMM‐Newton data of the source 4U 1344‐60. On the basis of the optical data we propose to classify 4U 1344‐60 as a Seyfert 1.5 galaxy and we measured a redshift value z = 0.012 ± 0.001. The observed X‐ray spectrum is complex. The continuum emission can be described as a power law obscured by two neutral absorption components. 4U 1344‐60 exhibits a broad and skewed iron line at 6.4 keV most likely originated in a few gravitational radius of an accretion disc. The analysis also reveals the presence of two narrow emission line‐like features at ∼4.9 keV and ∼5.3 keV. Assuming that hot spots on the surface of the accretion disc, orbiting very close to the black hole is responsible of these emission lines, the accretion disc would present an inclination of 20° and the active regions would be located in the 6–10 R _g radius range. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tilted accretion discs in cataclysmic variables: tidal instabilities and superhumps

We investigate the growth of tidal instabilities in accretion discs in a binary star potential, using three-dimensional numerical simulations. As expected from analytic work, the disc is prone to an eccentric instability provided that it is large enough to extend to the 3:1 resonance. The eccentric disc leads to positive superhumps in the light curve. It has been proposed that negative superhumps might arise from a tilted disc, but we find no evidence that the companion gravitational tilt instability can grow fast enough in a fluid disc to create a measurable inclination. The origin of negative superhumps in the light curves of cataclysmic variables remains a puzzle.     

On the interpretation of the multicolour disc model for black hole candidates

We present a critical analysis of the usual interpretation of the multicolour disc model parameters for black hole candidates in terms of the inner radius and temperature of the accretion disc. Using a self-consistent model for the radiative transfer and the vertical temperature structure in a Shakura–Sunyaev disc, we simulate the observed disc spectra, taking into account Doppler blurring and gravitational redshift, and fit them with multicolour models. We show not only that such a model systematically underestimates the value of the inner-disc radius, but that when the accretion rate and/or the energy dissipated in the corona are allowed to change, the inner edge of the disc, as inferred from the multicolour model, appears to move even when it is in fact fixed at the innermost stable orbit.     

Doppler tomography of Algols

The technique of Doppler tomography has been influential in the study of mass transfer in Algol‐type interacting binaries. The Algols contain a hot blue dwarf star with a magnetically‐active late‐type companion. In the close Algols, the gas stream flows directly into the photosphere of the blue mass‐gaining star because it does not have enough room to avoid impact with that star. Doppler tomograms of the Algols have been produced from over 2500 time‐resolved spectra at wavelengths corresponding to Hα, Hβ, He I (6678 Å), Si II (6371 Å) and Si IV (1394 ° A). These tomograms display images of accretion structures that include a gas stream, accretion annulus, accretion disk, stream‐star impact region, and occasionally a source of chromospheric emission associated with the cool, mass‐losing companion. Some Algol systems alternate between streamlike and disk‐like states, and provide direct evidence of active mass transfer within the Algols. This work produced the very first images of the gas stream for the entire class of interacting binaries, and demonstrated that the Algols are far more active than formerly believed, with variability on time scales of weeks to months. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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 10-m 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 have been detected and resolved in the emission lines. The accretion disc is highly asymmetric with a stronger contribution from the blueshifted 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 56.12 s 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 29.77 s at a mean amplitude of 1.9 per cent. The line kinematics and the eclipse constraints indicate an origin in the accretion disc at a radius of 12±2 R _wd. The amplitude of the emission-line oscillation modulates (0–4 per cent) at a period of 488 s, corresponding to the Kepler period at R =12 R _wd. This modulation is caused by 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 12 R _wd, is required. The close correspondence between the location of the oscillations and the circularization radius of the system suggests that stream overflow effects may be of relevance.