TITLE: GCN CIRCULAR NUMBER: 10629 SUBJECT: GRB 100420A: ROTSE-III Optical Limits DATE: 10/04/20 06:05:14 GMT FROM: Shashi Bhushan Pandey at ROTSE L. Xiao (Louisiana State), W. Rujopakarn (Steward), B. E. Schaefer (Louisiana State), T. Guver (U Arizona) and S. B. Pandey (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 100420A (Swift trigger 419932; W. H. Baumgartner et al., GCN 10628), producing images beginning 8.8 s after the GCN notice time. An automated response took the first image at 05:24:07.8 UT, 79.8 s after the burst, and during the gamma-ray emission. 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 15.3-16.8; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 05:24:07.8 05:25:14.9 67 16.7 79.8 Y 05:25:28.5 05:30:11.0 282 17.3 160.5 Y 05:30:20.3 05:41:42.0 681 17.6 452.3 Y