GCN Circular 17200
A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), C.C. Thoene (IAA-CSIC), J. Gorosabel (IAA-CSIC, UPV-EHU), N.R. Tanvir (U. Leicester), J.P.U. Fynbo (DARK/NBI), P. Pesev (GRANTECAN), G. Gomez Velarde (GRANTECAN), and D. Perez Valladares (GRANTECAN) report on behalf of a larger collaboration: Photometric observations of the afterglow of GRB 141220A (Cummings et al. GCN 17196, Gorosabel et al. GCN 17197, de Ugarte Postigo et al. GCN 17198) were obtained by OSIRIS/GTC starting 20.6 min after the GRB onset. The afterglow was detected at r' = 18.11+/-0.05 mag in the first image and had decayed by 0.6+/-0.1 mag 38.7 min after the burst. Using our photometric data we derive a decay slope of alpha=0.96+/-0.11 (where F_nu ~ t^-alpha). These results are consistent with the observations reported by Guidorzi et al. (GCN 17199). Our photometry was performed using SDSS stars as reference. We encourage further observations and in particular spectroscopy of the afterglow, in particular to check for possible variability of the FeII* features between 5400 and 6100 A.