TITLE: GCN CIRCULAR NUMBER: 10613 SUBJECT: GRB 100418A: ROTSE-III Optical Limits DATE: 10/04/18 21:35:05 GMT FROM: Shashi Bhushan Pandey at ROTSE S. B. Pandey (U Mich), W. Zheng (U Mich), Limin Xiao (Louisiana State) and T. Guver (U Arizona), report on behalf of the ROTSE collaboration: ROTSE-IIId, located at the Turkish National Observatory at Bakirlitepe, Turkey, responded to GRB 100418A (Swift trigger 419797), producing images beginning 4.6 s after the GCN notice time. An automated response took the first image at 21:10:30.5 UT, 16.7 s after the burst, and during the gamma-ray emission, under fair conditions. We took 10 5-sec, 10 20-sec and 10 60-sec exposures. These unfiltered images are calibrated relative to USNO A2.0 (R). Imaging is on going. Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma Swift/BAT error circle or the XRT error circle, for both single images and coadding into sets of 10. Individual images have limiting magnitudes ranging from 15.1-16.2; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 21:10:30.6 21:10:35.6 5 15.4 16.7 N 21:10:30.6 21:12:37.9 127 16.8 16.7 Y Further observations are ongoing.