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Register a new userEclipse mapping of the flickering sources in the dwarf nova V2051 Ophiuchi
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We report on the eclipse mapping analysis of an ensemble of light curves of the dwarf nova V2051 Oph with the aim to study the spatial distribution of its steady-light and flickering sources. The data are combined to derive the orbital dependency of the steady-light and the flickering components at two different brightness levels, named the faint and bright states. The differences in brightness are caused by long-term variations in the mass transfer rate from the secondary star. Eclipse maps of the steady-light show enhanced emission along the ballistic stream trajectory, in a clear evidence of gas stream overflow. We identify two different and independent sources of flickering in V2051 Oph. Low-frequency flickering arises in the overflowing gas stream and is associated to the mass transfer process. It maximum emission occurs at the position of closest approach of the gas stream to the white dwarf, and its spatial distribution changes in response to variations in mass transfer rate. High-frequency flickering originates in the accretion disk, showing a radial distribution similar to that of the steady-light maps and no evidence of emission from the hot spot, gas stream or white dwarf. This disk flickering component has a relative amplitude of about 3 per cent of the steady disk light, independent of disk radius and brightness state. If the disk flickering is caused by fluctuations in the energy dissipation rate induced by MHD turbulence, its relative amplitude lead to a viscosity parameter alpha= 0.1-0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the disk instability model [abridged].
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We report results of the eclipse mapping analysis of an ensemble of light curves of HT Cas. The fast response of the white dwarf to the increase in mass transfer rate, the expansion rate of the accretion disc at the same time, and the relative amplitude of the high-frequency flickering indicate that the quiescent disc of HT Has has high viscosity, alpha ~ 0.3-0.7. This is in marked disagreement with the disc-instability model and implies that the outbursts of HT Cas are caused by bursts of enhanced mass-transfer rate from its donor star.
Although flickering is one of the fundamental signatures of accretion, it is also the most poorly understood aspect of the accretion processes. A promising step towards a better undestanding of flickering consists in using the eclipse mapping method to probe the surface distribution of the flickering sources. We report on the analysis of light curves of the dwarf nova and strong flicker V2051 Ophiuchi with eclipse mapping techniques to produce the first maps of the flickering brightness distribution in an accretion disc.
We report on the investigation of the spatial distribution of the flickering sources in the dwarf nova V2051 Oph with eclipse mapping techniques. Low-frequency flickering originates in the gas stream and is related to the mass transfer process, whereas high-frequency flickering arises in the accretion disk and is probably connected to magneto-hydrodynamic turbulence.
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.
We report on high-speed eclipse photometry of the dwarf nova V2051 Oph while it was in a low brightness state, at B ~ 16.2 mag. In comparison to the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, ~4 and ~5. Flickering activity is mostly suppressed and the lightcurve shows the eclipse of a compact white dwarf at disc centre which contributes ~60 per cent of the total light at 3900--4300 A. We use measurements of contact phases in the eclipse lightcurve to derive the binary geometry and to estimate masses and relevant dimensions. We find a mass ratio of q= 0.19+/-0.03 and an inclination of i= 83+/-2 degrees. The masses of the component stars are M_1 = 0.78+/-0.06 M_dot and M_2 = 0.15+/-0.03 M_dot. Our photometric model predicts K_1 = 83+/-12 km/s and K_2= 435+/-11 km/s. The predicted value of K_1 is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made (Watts et al. 1986). The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultra-short period dwarf novae. V2051 Oph is probably a relatively young system, whose secondary star had not enough time to evolve out of thermal equilibrium.
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