TITLE: GCN CIRCULAR NUMBER: 41970 SUBJECT: GRB 250924A: KAIT optical observations DATE: 25/09/24 17:33:43 GMT FROM: Weikang Zheng at UC Berkeley weikang@berkeley.edu

WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on

behalf of the KAIT GRB team:

The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, automatically responded to the Swift GRB

250924A (Siegel et al., GCN 41959) starting at 08:21:50 UT, 201s

after the burst. Observations lasted for about 1 hour with degraded

sky condition (cloudy), and a set of clear (roughly R) filter images

were obtained. We clearly detected the reported fading afterglow

(Siegel et. al, GCN 41959; Becerra et. al, GCN 41960; Schneider

et. al, GCN 41963; Freeberg et. al, GCN 41968; Pankov et al.,

GCN 41969). We measure its brightness decayed from 16.41 +/- 0.03

mag (Vega) at 3.52 minutes to 19.7 +/- 0.3 mag at 59.33 minutes after

the burst. The data after 300s can be fit with a single power law

with a decay index of -0.96 +/- 0.02.

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