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GCN Circular 33431

GRB 230307A: Swift/BAT upper limits on the early afterglow
2023-03-09T01:49:22Z (9 months ago)
Aaron Tohuvavohu at U Toronto <>
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

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
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