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Register a new userTime-resolved flickering mapping of V2051 Ophiuchi
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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.
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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].
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 analyzed 29 pairs of time series in B and V bands of the recurrent nova RS Oph. The observations were carried out in 2008-2017 with duration 0.6 - 3.6 hours, with time resolution 0.5 - 3.3 min. We scanned digitally each series by data windows with various sizes Theta and derived two of the simplest fractal parameters for every Theta - standard deviation D and structural deviation S. Using the local minima of the structural function log S = f_S(log Theta) we unveiled 80 time structures, 42 in B band and 38 in V band, with time sizes 10-120 min. About 3/4 of the time sizes belong to the interval 10-40 min and about 1/4 lie in the interval 60-120 min. The respective cycles per day are 144-36 c/d and 24-15 c/d. On logarithmic scale, the distribution of the time sizes shows maximums at about 10, 21, 36 and 74 min. The 10 min flickering is poorly detectable in our series and we found the most widespread time structures (in about 1/5 of the cases) have time sizes about 21 min (about 69 c/d). Using the deviation function log D = f_D(log Theta) we estimated the relative cumulative energy (including the energy of the shorter structures in it), associated with the detected structure sizes, to be in the interval of the relative fluxes 2-11 %. The energies correlate weakly with the logarithms of the structure sizes, with correlation coefficients 0.60 and 0.57, under slope coefficients 0.04 and 0.03 in B and V band, respectively. The distributions of the energies occur bimodal, with maximums about 4% and 6% in B band, as well as about 3% and 5% in V band. The left and right modes of the distributions may be associated with the structure sizes 10 - 21 min and 37 - 74 min, respectively.
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.
We report observations of the flickering variability of the recurrent nova RS Oph at quiescence on the basis of simultaneous observations in 5 bands (UBVRI). RS Oph has flickering source with (U-B)_0=-0.62 pm 0.07, (B-V)_0=0.15 pm 0.10, (V-R)_0=0.25 pm 0.05. We find for the flickering source a temperature T_fl = 9500 pm 500 K, and luminosity L_fl = 50 - 150 L_sun (using a distance of d=1.6kpc). We also find that on a (U-B) vs (B-V) diagram the flickering of the symbiotic stars differs from that of the cataclysmic variables. The possible source of the flickering is discussed. The data are available upon request from the authors and on the web www.astro.bas.bg/~rz/RSOph.UBVRI.2010.MNRAS.tar.gz.
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