TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2602 SUBJECT: GRB 040511, very fast decay? DATE: 04/05/20 22:07:46 GMT FROM: David Bersier at STScI D. Bersier, J. M. Castro Cerón, J. Rhoads, A. Fruchter (STScI), A. Levan (U. of Leicester), J. Gorosabel (IAA-CSIC), C. Kouveliotou (NASA/MSFC, NSSTC), S. Patel (USRA, NSSTC), M. Merrill (NOAO), J. Bally, J. Walawender, (U. Colorado), and B. Reipurth (Hawaii) report on behalf of a larger collaboration: We have observed the field of GRB 040511 (GCN Dullighan et al. 2004, GCN 2588) with the CTIO 4m telescope on the nights of May 11 and May 12. Our R-band photometry of the candidate afterglow (Fox et al. 2004, GCN 2597) shows that this source decayed by 2.5 mag between 0.79d and 1.60d after the burst. Assuming that the decay follows a power law, we derive an index of -3.2. If the variable source is indeed the afterglow of GRB 040511, this would be among the fastest decay indices ever observed.