Optical and near-infrared follow-up observations of V4641 Sgr

ATel #5836; M. Uemura, Y. Moritani, R. Itoh, H. Akitaya (Hiroshima Univ.), A. Arai, K. Morihana, S. Honda (Univ. Hyogo), and K. Matsumoto (Osaka Kyoiku Univ.)
on 30 Jan 2014; 23:39 UT
Credential Certification: Makoto Uemura (uemuram@hiroshima-u.ac.jp)

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We performed optical imaging of the black hole X-ray binary, V4641 Sgr which is now active in X-rays (ATel #5803).

The near-infrared imaging was performed with 2-m "Nayuta" telescope in Hyogo. The object was: J = 12.45, 12.54, and 12.57, H = 12.26, 12.31, and 12.33, Ks= 12.21, 12.25, and 12.26, on 24.89, 26.89, and 28.89 (UT) Jan. 2014, respectively. The magnitudes of comparison stars are from 2MASS.

Rc and Ic-band images were taken with the 50-cm telescope in Osaka, in which the object was Rc=12.8 on 27.89 (UT) Jan. 2014 and Rc=13.13 and Ic=12.91 on 28.88 (UT) Jan. 2014. The magnitude of comparison stars are from the AAVSO sequence (Henden 2014).

The V, Rc, and Ic-band images were obtained with the 1.5-m "Kanata" telescope in Hiroshima. A preliminary analysis estimated the magnitudes of the object: V=13.52, Rc=13.15, and Ic=13.03 on 28.89 (UT) Jan. 2014. The V and Ic magnitudes of comparison stars are from MacDonald, et al. (2014), and the Rc magnitudes from USNO B1.0.

It is known that the object shows ellipsoidal modulations with a large amplitude of ~0.4 mag in quiescence. MacDonald, et al. (2014) have recently reported optical and NIR light curves in quiescence. According to them, our observation times correspond to the orbital phase of 0.64 (24 Mar), 0.35 (26 Mar), 0.70 (27 Mar), and 0.06 (28 Mar). Our results suggest that the object is now in the active state in the optical, as well as in X-rays. It is not in a major outburst, such as those observed in 1999, 2002, 2003, and 2004 (Uemura+02,04,and 05).