TITLE:   GCN CIRCULAR
NUMBER:  40185
SUBJECT: GRB 250419A: Swift/UVOT Follow up
DATE:    25/04/20 12:39:57 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

Paul Kuin (MSSL/UCL) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began observations of the field of SVOM detected GRB
250419A/(SVOM burst-id sb25041901) 3790s after the SVOM ECLAIR
trigger at 2025-04-19T02:29:32 (Wang et al., GCN Circ. 40168).
A source consistent with the position reported by Kin Ocelotl López
et al. (Colibri - GCN Circ. 40169) and Liping Xin at al. (SVOM-VT
GCN Circ 40170), also reported by Weikang Zheng et al. (KAIT - GCN
Circ. 40171) and Amit Kumar et al. (GCN Circ. 40172) has been
detected in the initial UVOT exposures.

The detections using the UVOT photometric system (Breeveld et al.
2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter         T_start(s)       Exp(s)         Mag (Vega)

u                 3790          1667         17.73 +/- 0.04
u                 9323           259         18.19 +/- 0.10
u                48658          1206         19.64 +/- 0.14
u                54296          1028         19.52 +/- 0.13

The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.085 in the direction
of the burst (Schlegel et al. 1998).