TITLE: GCN CIRCULAR NUMBER: 13613 SUBJECT: GRB 120714B: GROND/FORS2 detection of a supernova DATE: 12/08/09 18:34:32 GMT FROM: Sylvio Klose at TLS Tautenburg S. Klose (TLS), J. Greiner (MPE), J. Fynbo (DARK), A. Nicuesa Guelbenzu, S. Schmidl (both TLS), A. Rau (MPE), T. Kruehler (DARK), report on behalf of a larger collaboration: GROND has been following the afterglow of GRB 120714B (Saxton et al., GCN 13471) at z=0.398 (Fynbo et al., GCN 13477) since July 15 (Nicuesa Guelbenzu et al., GCN 13478). The light of the optical transient has been fading in all GROND optical bands during the first 1.5 days, was then flattening in r',i',z', and finally rising at about 3 days after the trigger (while it is flattening in g'). The data suggested that this behavior can be interpreted as an upcoming supernova component with a an early rise similar to the SN associated with GRB 011121 at z=0.36 (Bloom et al. 2002, ApJ 672, 45; Garnavich et al. 2003, ApJ 582, 924; Greiner et al. 2003, ApJ 599, 1223; Price et al. 2002, ApJ 572, 51). Spectroscopy of the optical transient with VLT/FORS2 was performed on August 1/2, 18.3 days after the burst. Observations were done with the 300V grism, covering the wavelength range from 445 to 865 nm, and lasted 7200 sec. The spectrum reveals very broad features in the continuum with a prominent peak around 6500 A and broad lines of Si II and Ca II, in agreement with a broad-lined SN dominating the light of the optical transient. At present a decision between type Ib and Ic cannot be performed due to an uncomfortable presence of atmospheric features in the spectrum. We thank the staff at ESO Paranal and ESO Garching for their excellent support and for performing the spectroscopy. GROND observations are continuing.