Swift/BAT light-curve (15−50 keV) with a bin size equal to 10 d. Tick marks on the segments located above the light-curve denote the times of the Chandra pointing observation (light blue), as well as the optical/IR photometric data from the ground-based telescopes ROTSEIIId (black), IAC80 (dark blue) and TCS (dark red). The optical spectroscopic observations come from OSN (dark red), NOT (blue) and RTT150 (light red; see also Table 2).