TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2153 SUBJECT: ROTSE-III prompt optical detections of GRB 030418 DATE: 03/04/18 22:15:10 GMT FROM: Don Smith at U michigan Smith, D. A., Rykoff, E. S., McKay, T. A. report on behalf of the ROTSE collaboration: The ROTSE-IIIa robotic telescope at Siding Springs Observatory, NSW, responded automatically to the two HETE alerts for this burst, in each case beginning a ~45 minute sequence of unfiltered observations within six seconds of the alert time stamp, the first of which was 238 s after the burst. Each image sequence began with 10 5-s images and then continued with 90 20-s images. The 20-s images reached typical limiting magnitudes of 17.5 calibrated against the USNO A2.0 catalog in R band, while the 5-second images reached 16.9. Sensitivity was degraded by the full moon. Automated software searched these frames for variable sources not in the USNO catalog and failed to identify any counterpart candidates. In particular, no source was initially detected at the location of the optical counterpart reported by Price et al. (GCN Circ. 2148). Co-addition of multiple frames, however, yielded significant detections of the optical transient. We co-added our 200 observations into groups of ten, and found that from 10:05:23 to 10:50:05, and again from 11:43:07 to 12:20:42, we measure the transient to vary irregularly between 18.7 and 17.4, with errors of order 0.15 mag. This means that from 360 s to 2.3 h after the burst, the transient showed no evidence for decay, in marked contrast to other bursts that have been observed at early times, such as 990123 and 021211.