Eclipse Mapping of the Flickering Sources in the Dwarf Nova V2051 Ophiuchi

Abstract

We report an 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 dependence of the steady-light and the flickering components at two different brightness levels, named the and states. The differences in brightness are caused by long-term variations in the mass transfer rate from the secondary star. The white dwarf is hardly affected by the long-term changes; its flux increases by only 10% from the faint to the bright state, whereas the disk flux raises by a factor of 2. Eclipse maps of the steady light show asymmetric brightness distributions with enhanced emission along the ballistic stream trajectory, clear evidence of gas stream overflow. Comparison between the steady-light maps of the faint and bright states suggests that the quiescent disk responds to changes in mass transfer rate in a homologous way. The ability to separate the orbital dependence of the low- and high-frequency flickering components allows us to identify the existence of two different and independent sources of flickering in V2051 Oph. The low-frequency flickering arises mainly in the overflowing gas stream and is associated with 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 the mass transfer rate. The 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% 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 magnetohydrodynamic turbulence, its relative amplitude leads to a viscosity parameter αcool 0.1–0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the disk-instability model.