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.


Optical Variations of XTE J1859+226 during the 1999 Outburst

M. Uemura, T. Kato, E. Pavlenko, S. Shugarov, M. Mitskevich, et al., “Optical Variations of XTE J1859+226 during the 1999 Outburst,” PASJ, vol. 56, p. 147, 2004.

We report on our optical photometric observations of a black hole X-ray transient, XTE J1859 + 226 (= V406 Vul). We detected an optical reflare associated with an X-ray reflare about 50d after the outburst maximum. The X-ray delay of the reflare was shorter than 2.3d. We detected coherent modulations having a period of 0.38385 ± 0.00073 d. The modulation appeared within 7d after the outburst maximum. We propose two obvious interpretations for the nature of the modulations, that is, superhumps or orbital-period variations caused by an irradiated secondary star or an outer disk. The asymmetric profile of the modulations suggests the superhump scenario. The early appearance of superhumps indicates that the tidal dissipation had already grown before the reflare. The growing tidal dissipation is hence less likely to induce the reflare. In conjunction with the short X-ray delay of the reflare, the reflare may be triggered by the X-ray irradiation onto an outer cold disk. The appearance of humps may be earlier than the eccentricity growth time expected from the tidal instability model. It may imply a mass ratio larger than what could be inferred from the reported orbital period.


Outburst and Post-Outburst Active Phase of the Black Hole X-Ray Binary V4641 Sagittarii in 2002

M. Uemura, T. Kato, R. Ishioka, K. Tanabe, K. Torii, et al., “Outburst and Post-Outburst Active Phase of the Black Hole X-Ray Binary V4641 Sagittarii in 2002,” PASJ, vol. 56, p. 61, 2004.

The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May, which was detected at X-ray, optical, and radio wavelengths. The outburst lasted for only 6 days, but the object remained active for the next several months. Here, we report on the detailed properties of light curves during the outburst and the post-outburst active phase. We reveal that rapid optical variations of ˜ 100 s became more prominent when a thermal flare weakened and the optical spectrum flattened in the Ic, Rc, and V-band regions. In conjunction with the flat spectrum in the radio range, this strongly indicates that the origin of rapid variations is not thermal emission, but synchrotron emission. Just after the outburst, we detected repeated flares at optical and X-ray wavelengths. The optical and X-ray light curves exhibited a strong correlation with the X-rays lagging by about 7min. The X-ray lag can be understood in terms of a hot region propagating into the inner region of the accretion flow. The short X-ray lag, however, requires modifications of this simple scenario to account for the short propagation time. We also detected high-amplitude rapid variations 50 days after the outburst, which we call optical flashes. During the most prominent optical flash, the object brightened by 1.2mag only within 30s. The released energy indicates that the emission source should be at the innermost region of the accretion flow.