TITLE: GCN CIRCULAR NUMBER: 41344 SUBJECT: GRB 250812A: Swift/UVOT Detection DATE: 25/08/13 15:15:44 GMT FROM: Mike Siegel at PSU/Swift MOC mhs18@psu.edu

M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250812A 6.5 ks after the SVOM/ECLAIRs trigger (Xin et al., GCN Circ. 41322). An uncatalogued source consistent with the XRT position (Sbarrato et al., GCN Circ. 41325) and the optical transient (He et al., GCN Circ. 41324; Xin et al., GCN Circ. 41326; Rakotondrainibe et al., GCN Circ. 41328; Freeberg et al., GCN Circ. 41333) is detected in the UVOT exposures. However, we note that the source shows no evidence of fading in the 67 ks between the two Swift epochs.

The preliminary UVOT position is: RA (J2000) = 02:11:13.68 = 32.80701 (deg.) Dec (J2000) = -43:09:49.1 = -43.16363 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

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

u 6500 12380 1479 19.43+/-0.12 u 73801 90784 695 19.48+/-0.13 v 74444 91365 424 >19.24 white 74122 91182 695 19.96+/-0.10

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.012 in the direction of the burst (Schlegel et al. 1998).

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