TITLE: GCN CIRCULAR NUMBER: 39409 SUBJECT: GRB 250221A: Swift/UVOT Detection DATE: 25/02/21 15:27:39 GMT FROM: Sam Shilling at Lancaster University shilling.sam@gmail.com
S.P.R. Shilling (Lancaster U.), A.A. Breeveld (UCL-MSSL) and R. Caputo (NASA/GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250221A 107 s after the BAT trigger (Caputo et al., GCN Circ. 39396). A source consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 39404) is detected in the initial UVOT exposures and appears to be fading. An optical candidate in this field is also detected by Watson et al., (GCN Circ. 39397) and by Melandri et al., (GCN Circ. 39406).
Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 127 277 147 17.52 +/- 0.04 white 620 639 19 18.41 +/- 0.17 v 107 118 10 17.10 +/- 0.32 v 4484 4683 197 >18.8 b 595 615 19 >18.7 u 340 590 245 17.78 +/- 0.07 w1 4894 5041 144 >19.0 m2 4689 4889 197 >18.6 w2 4279 4479 197 >18.7
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.332 in the direction of the burst (Schlegel et al. 1998).
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