Long-term optical activity of the microquasar V4641 Sgr

Abstract

We present an analysis of the optical activity of the microquasar V4641 Sgr using the visual and photographic data. We analyze four smaller (echo) outbursts that followed the main outburst (1999). Their mean recurrence time TC is 377 days, with a trend of a decrease. We interpret the characteristic features of the recent activity of V4641 Sgr (the narrow outbursts separated by a long quiescence, a clear trend in the evolution of their TC) as the thermal instability of the accretion disk operating in dwarf novae and soft X-ray transients. We argue that the luminosity of four echo outbursts is too high to be explained by the thermal emission of the accretion disk. We interpret them as due to the thermal instability, in which the outburst gives rise to a jet radiating also in the optical. This supports the finding by Uemura et al., PASJ 54, L79 (2002). The pre-outburst observations (1964–1967) reveal ongoing activity of the system. It displays low-amplitude fluctuations on the timescale of several weeks, independent on the orbital phase. In addition, a larger brightening which lasted for several tens of days and occurred from the level of brightness higher than in other years can be resolved.