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Register a new userSOAR observations of the high-viscosity accretion disc of the dwarf nova V4140 Sagitarii in quiescence and in outburst
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We report the analysis of 22 B-band light curves of the dwarf nova V4140 Sgr obtained with SOI/SOAR during two nights along the decline of a superoutburst in 2006 Sep 12-24 and in quiescence over 50 days following the superoutburst. Three-dimensional eclipse mapping of the outburst light curves indicates that the accretion disc is elliptical (eccentricity e=0.13) and that superhump maximum occurs when the mass donor star is aligned with the bulge of the elliptical disc. The accretion disc is geometrically thin both in outburst and in quiescence; it fills the primary Roche lobe in outburst and shrinks to about half this size in quiescence. The stability of the eclipse shape, width and depth along quiescence and the derived disc surface brightness distribution indicate that the quiescent accretion disc is in a high-viscosity, steady-state. Flickering mapping of the quiescent data reveal that the low-frequency flickering arises from an azimuthally-extended stream-disc impact region at disc rim and from the innermost disc region, whereas the high-frequency flickering originates in the accretion disc. Assuming the disc-related flickering to be caused by fluctuations in the energy dissipation rate induced by magneto-hydrodynamic turbulence (Gertseema & Achterberg 1992), we find that the quiescent disc viscosity parameter is large (alpha ~ 0.2-0.4) at all radii. The high-viscosity quiescent disc and the inferred low disc temperatures in superoutburst are inconsistent with expectations of the disc-instability model, and lead to the conclusion that the outbursts of V4140 Sgr are powered by mass transfer bursts from its donor star.
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In this paper we analyzed the behavior of the unusual dwarf nova EM Cyg using the data obtained in April-October, 2007 in Vyhorlat observatory (Slovak Republic) and in September, 2006 in Crimean Astrophysical Observatory (Ukraine). During our observations EM Cyg has shown outbursts in every 15-40 days. Because on the light curves of EM Cyg the partial eclipse of an accretion disc is observed we applied the eclipse mapping technique to reconstruct the temperature distribution in eclipsed parts of the disc. Calculations of the accretion rate in the system were made for the quiescent and the outburst states of activity for different distances.
We present an analysis of X-ray and UV data obtained with the XMM-Newton Observatory of the long period dwarf nova RU Peg. RU Peg contains a massive white dwarf, possibly the hottest white dwarf in a dwarf nova, it has a low inclination, thus optimally exposing its X-ray emitting boundary layer, and has an excellent trigonometric parallax distance. We modeled the X-ray data using XSPEC assuming a multi-temperature plasma emission model built from the MEKAL code. We obtained a maximum temperature of 31.7 keV, based on the EPIC MOS1, 2 and pn data, indicating that RU Peg has an X-ray spectrum harder than most dwarf novae, except U Gem. This result is consistent with and indirectly confirms the large mass of the white dwarf in RU Peg. The X-ray luminosity we computed corresponds to a boundary layer luminosity for a mass accretion rate of 2.E-11 Msun/yr (assuming Mwd=1.3Msun), in agreement with an expected quiescent accretion rate. The modeling of the O VIII emission line at 19A as observed by the RGS implies a projected stellar rotational velocity of 695 km/s, i.e. the line is emitted from material rotating at about 936-1245 km/s (for i about 34-48deg) or about 1/6 of the Keplerian speed; this velocity is much larger than the rotation speed of the white dwarf inferred from the FUSE spectrum. Cross-correlation analysis yielded an undelayed component and a delayed component of 116 +/- 17 sec where the X-ray variations/fluctuations lagged the UV variations. This indicates that the UV fluctuations in the inner disk are propagated into the X-ray emitting region in about 116 sec. The undelayed component may be related to irradiation effects.
We present an analysis of X-ray and ultra-violet 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 onto 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*10^30 erg/s, which is only 20 per cent of the disk luminosity. The rate of accretion onto the white dwarf is 5*10^-12 solar masses per year, about half of the rate in the disk. 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, which is close the rotation velocity of the white dwarf but significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux that is likely due to the rotation of the boundary layer. The X-ray and the ultra-violet 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 ultra-violet flux is emitted near the transition region between the disk 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 disk is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.
We report the analysis of time-series of infrared $JHK_s$ photometry of the dwarf nova V2051 Oph in quiescence with eclipse mapping techniques to investigate structures and the spectrum of its accretion disc. The light curves after removal of the ellipsoidal variations caused by the mass-donor star show a double-wave modulation signalling the presence of two asymmetric light sources in the accretion disc. Eclipse maps reveal two spiral arms on top of the disc emission, one at $R_1= 0.28pm 0.02 ,R_mathrm{L1}$ and the other at $R_2= 0.42pm 0.02 ,R_mathrm{L1}$ (where $R_mathrm{L1}$ is the distance from disc centre to the inner Lagrangian point), which are seen face-on at binary phases consistent with the maxima of the double-wave modulation. The wide open angle inferred for the spiral arms ($theta_s= 21^o pm 4^o$) suggests the quiescent accretion disc of V2051 Oph has high viscosity. The accretion disc is hot and optically thin in its inner regions ($T_mathrm{gas}sim 10-12 times 10^3,K$ and surface densities $sim 10^{-3}-10^{-2},g,cm^{-2}$), and becomes cool and opaque in its outer regions.
Dwarf nova outbursts result from enhanced mass transport through the accretion disc of a cataclysmic variable system. We assess the question of whether these outbursts are caused by an enhanced mass transfer from the late-type main sequence star onto the white dwarf (so-called mass transfer instability model, MTI) or by a thermal instability in the accretion disc (disc instability model, DIM). We compute non-LTE models and spectra of accretion discs in quiescence and outburst and construct spectral time sequences for discs over a complete outburst cycle. We then compare our spectra to published optical spectroscopy of the dwarf nova SS Cygni. In particular, we investigate the hydrogen and helium line profiles that are turning from emission into absorption during the rise to outburst. The evolution of the hydrogen and helium line profiles during the rise to outburst and decline clearly favour the disc-instability model. Our spectral model sequences allow us to distinguish inside-out and outside-in moving heating waves in the disc of SS Cygni, which can be related to symmetric and asymmetric outburst light curves, respectively.
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