TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4214 SUBJECT: GRB 051109A: ROTSE-III Decline rate for Optical Counterpart DATE: 05/11/09 02:38:04 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State), E.S. Rykoff (U Mich), W. Rujopakarn (U Mich), F. Yuan (U Mich), R. Quimby (U Texas), K. Alatalo (Berkeley), T.A. McKay (U Mich), C. Akerlof (U Mich), report on behalf of the ROTSE collaboration: A fast fading optical transient has been discovered inside the error circle for GRB 051109A (Swift trigger 163136) by the ROTSE-IIIb telescope, located at McDonald Observatory (cf. Rykoff et al. GCN 4211). The source faded as a power law. At the central time of UT 01:16:05 (228 seconds after the Swift trigger), the magnitude was 16.21. (Magnitudes are unfiltered and calibrated against R-band magnitudes for nearby comparison stars.) The source is continuing to fade, but accurate magnitudes will have to await a more detailed analysis due to nearby faint stars. With these magnitudes, the power law index for the decline is -0.4. If this decline continues unbroken, then at a time around two hours after the burst (UT 03:12), the magnitude should be 17.7. Eight hours after the burst (UT 09:12), a magnitude of 18.3 might be expected.