RESUMO
Accretion in white-dwarf binary systems can occur through discs, accretion columns or a combination of these, depending on the magnetic field of the white dwarf. Recent high-quality X-ray observations with the XMM-Newton and Chandra observatories have significantly advanced our understanding of the physics of the accretion process, and place severe tests on our existing models. There have been some surprises, such as the strong dependence of atmospheric heating on accretion rate. However, we believe that we are now confident that we understand in general the physical processes in the accretion region, although some complicating factors, such as absorption, remain. We also discuss new developments in ultra-short-period white-dwarf binary systems.
RESUMO
We present results of an experiment performed in Lake Baikal at a depth of approximately 1 km. The photomultipliers of an underwater neutrino telescope under construction at this site were illuminated by a distant laser. The experiment not only provided a useful cross-check of the time calibration of the detector but also allowed us to determine inherent optical parameters of the water in a way that was complementary to standard methods. In 1997 we measured an absorption length of 22 m and an asymptotic attenuation length of 18 m. The effective scattering length was measured as 480 m. By use of (cos theta) = 0.95 (0.90) for the average scattering angle, this length corresponds to a geometric scattering length of 24 (48) m.
