Optical Observation of the 2003 Outburst of a Black Hole X-Ray Binary, V4641 Sagittarii

M. Uemura, T. Kato, R. Ishioka, A. Imada, D. Nogami, et al., “Optical Observation of the 2003 Outburst of a Black Hole X-Ray Binary, V4641 Sagittarii,” PASJ, vol. 56, pp. 823-829, 2004.

We report on the results of our optical photometric observations of a black hole X-ray binary, V4641 Sgr during an outburst in 2003 August. During the outburst, we detected several properties having noteworthy similarities to those observed during an outburst in 2002 May; the outburst lasted about 6 days, during which it exhibited flares and rapid fluctuations, having timescales of 102-4s. In complicated profiles of light curves, we identified several recurring patterns, which were also observed during the 2002 outburst. First, the object frequently experienced sudden temporary fadings, which we call “dips”, appearing just after the states characterized by strong short-term fluctuations. Second, we detected optical flashes, which are characterized by a brightening by ˜ 0.5 mag within ˜ 50 s. Based on the similarity of the outbursts in 2002 and 2003, we conclude that they have the same nature. This is the first time that repetitive outburst phenomena have been confirmed in V4641 Sgr. The object probably has an outburst interval that can be as small as 1-2 years.