TITLE: GCN CIRCULAR NUMBER: 21393 SUBJECT: GRB 170728B: BOOTES-2/TELMA optical detection during the extended gamma-ray phase DATE: 17/07/30 17:00:39 GMT FROM: Alberto Castro-Tirado at Inst.de Astro. de Andalucia A. J. Castro-Tirado (IAA-CSIC Granada & ISA-UMA Málaga), A. González-Rodríguez, Y. Hu, R. Cunniffe, J. C. Tello, B.-B. Zhang (IAA-CSIC Granada), A. Castellón, I. Carrasco (UMA Málaga), M. Jelínek (ASU-CAS Ondrejov), S. Guziy (Nikolaev Univ.), S. B. Pandey (ARIES Nainital), S. Jeong (SKKU Seoul), M. D. Caballero-García (ASU-CAS Prague), M. Wildi (Vermes Obs.) and P. Kubánek (IP-ASCR Prague), on behalf of a larger collaboration, report: Following the detection of GRB 170728B by Swift (Cenko et al. GCNC 21371), the TELMA 0.6m robotic telescope at the BOOTES-2 astronomical station at IHSM/UMA-CSIC in Málaga (Spain) pointed to the GRB location 26 sec after the notice time and 40 sec after trigger time, i.e. at 23:03:59 UT on July 28. The first exposures (3 sec, unfiltered) were taken simultaneously to the extended gamma-ray emission phase (Stanbro et al. GCNC 21383, Ukwatta et al. GCNC 21384), confirming the optical transient within the Swift/XRT error box (D'Elia et al., GCNC 21376) being coincident with the optical afterglow position (D'Avanzo et al. GCN 21373) with magnitudes in the range reported by Lipunov et al. (GCNC et al. 21391) starting 2 sec after. A detailed analysis is ongoing. This message can be quoted. [GCN OPS NOTE(31jul17): Per author's request, the date was corrected from "July 2" to "July 28"]