TITLE: GCN CIRCULAR NUMBER: 30837 SUBJECT: GRB 210912A: Tautenburg observations DATE: 21/09/13 16:25:04 GMT FROM: Ana Nicuesa at TLS Tautenburg <ana@tls-tautenburg.de> A. Nicuesa Guelbenzu, S. Klose, S. Melnikov, B. Stecklum, C. Hoegner (all TLS Tautenburg) report: We observed the field of GRB 210912A (Troja et al., GCN 30820) with the Tautenburg 1.34m Schmidt telescope equipped with the TAUKAM 6kx6k CCD camera and the Sloan r-band filter. Observations were performed on September 12, at a midtime of 19.33 UT, 22.17 UT, and 02:36 UT (Sep 13). The optical transient (Hu et al., GCN 30823; Pankov et al., GCN 30825; Kuin & Troja, GCN 30832; Kann et al. GCN 30833) is detected with the following preliminary AB magnitudes: r_1 = 21.95 +- 0.19, r_2 = 22.17 +- 0.19, r_3 = 22.53 +- 0.19. Magnitudes are calibrated against the Pan-STARRS catalogue. A comparison with the previous observations suggests that around 0.6 days the decay index of the light curve has increased from a rather modest value of alpha = 0.4 to alpha about 1. Further observations are required in order to provide a more precise decay index.