GCN Circular 33431
Aaron Tohuvavohu (U Toronto), Jimmy DeLaunay (U Alabama), Samuele Ronchini (Penn State), Tyler Parsotan (GSFC/UMBC), Chris Matzner (U Toronto) report: The extremely bright GRB 230307A (GCN 33425) was detected in prompt emission by Swift/BAT outside the coded field of view, as reported previously. Soon after, a pre-planned observation serendipitously landed the GRB position near the center of the BAT field of view, with settled observations beginning at T0+910s. Using the BAT survey mode data, 300s duration 80-channel Detector Plane Histograms, we search for any detection of continued emission or bright afterglow from the burst between 910 to 1210 seconds after the burst. No emission is detected at the IPN position of the GRB, with a 5 sigma upper limit of 0.00656 mask-weighted counts/s (15-50 keV). Assuming an afterglow spectrum that can be described as a power law with photon index of 2.08 (as found for the BAT afterglow detection in GRB 221009A, Williams et al. 2023), this corresponds to an upper limit of 2.389e-9 erg/cm^2/s (15-50 keV). This constrains GRB 230307A's early afterglow (at T0+1 ks) to be at least ~1.5 orders of magnitude fainter than that of GRB 221009A at comparable times.