F. V. Ferrante (SMU), G. Dhungana (SMU), T. Guver (Sabanci U), W. Zheng (UC Berkeley), H. Flewelling (IfA/Hawaii), F. Yuan (Australian National University), and R. Kehoe (SMU) report on behalf of the ROTSE collaboration:
ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 130420A (Swift trigger 553977; Guidorzi et al., GCN 14405; Page et al., GCN 14406; Guver et al., GCN 14407). The first image was at 07:29:51.5 UT, 82.0 s after the burst (7.5 s after the GCN notice time). We continued to monitor the source reported in GCN 14405, 14406, and 14407. The unfiltered images are calibrated relative to USNO A2.0. We took 220 20-sec exposures and observed a maximum magnitude of 15.5.
Further analysis of ROTSE-IIIb data using RPHOT indicates that the afterglow of GRB 130420A displays a single power-law decay with index alpha = - 0.70 up to ~1500 s after the burst. After the power law fall-off, the afterglow continued to be detected in subsequent images for ~6 ks. The minimum observed magnitude was 17.5.