[] V4641 Sgr active



As noticed by Dr. Yamaoka in vsnet-alert 10580, the black hole X-ray binary, V4641 Sgr is currently active in X-rays. The above figures show the light curves of our observation (the red points) and the model of the ellipsoidal modulation (the green lines) (Upper panel: 7 - 17 Oct, lower panel: 16 - 26 Aug). On 14 and 16 Oct, the object was ~0.1-0.15 mag brighter than its quiescent level, although it was on the quiescent ellipsoidal modulation on 9 Oct and in August. The object is, thus, active also in the optical regime.

On 15 Oct, it was rather close to its quiescent level, as can be seen in the upper figure. At that time, the object was in the secondary minimum of the ellipsoidal modulation. In that phase, the secondary star of V4641 Sgr can hide the active region in the accretion disk around the black hole. The relatively low level on 15 Oct may be due to the partial eclipse of the central active region.