TITLE: GCN CIRCULAR NUMBER: 39199 SUBJECT: GRB 250207A: Swift/UVOT Detection DATE: 25/02/07 10:42:24 GMT FROM: Paul Kuin at MSSL npkuin@gmail.com

Paul Kuin (MSSL/UCL) and M. Ferro (INAF-OAB) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250207A 113 s after the BAT trigger (Ferro et al., GCN Circ. 39182). The source was also reported by Angulo et al. GCN Circ. 39186.

The preliminary UVOT position is: RA (J2000) = 01:04:21.39 = 16.08912 (deg.) Dec (J2000) = -12:09:52.5 = -12.16459 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

Preliminary detections 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 113 263 147 15.00 +/- 0.02 v 655 674 19 16.89 +/- 0.20 b 581 600 19 16.56 +/- 0.09 u 326 575 246 15.65 +/- 0.03 uvm2 679 699 19 > 17.3 (3 sigma UL) uvw2 630 650 19 > 17.6 (3 sigma UL)

The lack of detection in the UVM2 band may indicate a redshift greater than 1.3.

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.027 in the direction of the burst (Schlegel et al. 1998).

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