TITLE: GCN CIRCULAR NUMBER: 10294 SUBJECT: GRB 091221: Skynet/PROMPT Detections DATE: 09/12/23 22:11:00 GMT FROM: Josh Haislip at U.North Carolina J. Haislip, D. Reichart, K. Ivarsen, A. LaCluyze, R. Egger, A. Foster, J. Moore, A. Oza, M. Schubel, J. Styblova, A. Trotter, J. A. Crain, and M. Nysewander report: Skynet observed the Swift/BAT localization of GRB 091221 (Palmer et al., GCN 10283) with four of the 16" PROMPT telescopes at CTIO beginning 4.3 hours after the trigger in BVRI. We detect the afterglow (Palmer et al., GCN 10283; Zheng et al., GCN 10284). Stacking only images that increase the limiting magnitude yields: mean 1-sig. 1-sig. time sys. stat. since cal. cal. cal. trig. tel. exp. fil. magnitude stars unc. unc. (h) (# x s) (mag) (mag) 4.9 PROMPT-5 44 x 80 I > 21.1 (3 sigma) 143 USNO B1 0.338 0.000 + 1 x 40 4.9 PROMPT-4 42 x 80 R 21.49 +0.27 -0.22 139 USNO B1 0.362 0.000 4.9 PROMPT-3 41 x 80 B > 21.5 (3 sigma) 82 USNO B1 0.154 0.000 4.9 PROMPT-1 41 x 80 V 21.41 +0.42 -0.31 27 NOMAD 0.193 0.000 7.3 PROMPT-3 136 x 80 B > 22.1 (3 sigma) 79 USNO B1 0.153 0.000 7.3 PROMPT-5 134 x 80 I > 21.8 (3 sigma) 120 USNO B1 0.337 0.000 7.3 PROMPT-4 139 x 80 R 22.44 +0.38 -0.28 112 USNO B1 0.392 0.000 7.4 PROMPT-1 145 x 80 V > 22.0 (3 sigma) 28 NOMAD 0.112 0.000