TITLE: GCN CIRCULAR NUMBER: 7633 SUBJECT: GRB 080420A: ROTSE-III Optical Limits on a possible short burst DATE: 08/04/20 04:50:49 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State), F. Yuan (U Mich), W. Rujopakarn (Steward), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 080420 (Swift trigger 309665; Vetere, L. et al., GCN 7632) a possible short burst, producing images beginning 8.1 s after the GCN notice time. An automated response took the first image at 04:13:10.2 UT, 23.0 s after the burst, under fair conditions. These observations were affected by the Full Moon 75 degrees away. We took 10 5-sec, and 60 20-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; the field is not crowded. Individual images have limiting magnitudes ranging from 16.1-17.1; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 04:13:10.2 04:13:15.2 5 16.1 23.0 N 04:13:10.2 04:14:17.3 67 18.1 23.0 Y 04:24:09.3 04:28:51.6 282 18.5 682.1 Y