Rapid Optical Fluctuations in the Black Hole Binary V4641 Sagittarii

We report on unprecedented short-term variations detected in the optical fluxfrom the black hole binary system, V4641 Sgr. Amplitudes of the opticalfluctuations were larger at longer time scales, and surprisingly reached ~60%around a period of ~10 min. The power spectra of fluctuations are characterizedby a power law. It is the first case in black hole binaries that the opticalemission was revealed to show short-term and large-amplitude variations givenby such a power spectrum. The optical emission from black hole binaries isgenerally dominated by the emission from the outer portion of an accretiondisc. The rapid optical fluctuations however indicate that the emission from aninner accretion region significantly contributes to the optical flux. In thiscase, cyclo-synchrotron emission associated with various scales of magneticflares is the most promising mechanism for the violently variable opticalemission.Comment: 5 pages, 2 figures, accepted for publication in PAS