TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4215 SUBJECT: GRB 051109a: MDM Observations DATE: 05/11/09 03:52:12 GMT FROM: Jules Halpern at Columbia U. N. Mirabal (U. Michigan), J. P. Halpern, and S. Tonnesen (Columbia U.) report on behalf of the MDM Observatory GRB follow-up team: "We are observing the afterglow of Swift GRB 051109 detected by ROTSE-IIIb (Rykoff et al. GCN #4211, Schaefer et al. GCN #4214) using the MDM 2.4m telescope and a Gunn r filter under partly cloudy conditions. From 38 minutes to 148 minutes after the burst, the OT faded from R = 18.21 +/- 0.05 to R = 19.22 +/- 0.08. Magnitudes are referenced to a USNO B1.0 star at 22h01m14.60s +40d49'25.3" (J2000) having R=17.23. This corresponds to a power-law decay index of -0.68. Observations are continuing."