TITLE: GCN CIRCULAR NUMBER: 22221 SUBJECT: GRB 171209A: Swift/UVOT Analysis of the burst DATE: 17/12/10 02:13:55 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and and S. J. LaPorte (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 171209A 107 s after the BAT trigger (LaPorte et al., GCN Circ. 22213). The proposed possible transient position was confirmed by independent observations of the LCO telescopes in Siding Springs in the SDSS r and i filters (Guidorzi et al, GCN Corc. 22214) and by the XRT (d'Avanzo et al. GCN Circ. 22218). Due to the nearby star (at 3.7"), the count rates were determined using a smaller than normal aperture of 2.5 arcsec, and for the reported magnitudes no aperture correction has been applied. The brightness is seen to rise to 16th magnitude around 400s past the trigger at which time there is a break in the UVOT observations until 4000s past the trigger at which time the source is seen to be decaying. After binning the white filter data in 10s bins, half a magnitude large variability is seen up to 175s after the trigger. There is no measurable flux in the uvw1,uvm2, and uvw1 filters which were observed after the gap, which suggests a redshifted source with a redshift in the range z = 1.8 - 2.8. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are: Filter T_start(s) T_stop(s) Exp(s) Mag white 175 185 541 17.4 v 4630 6265 393 17.3 b 4014 5649 393 18.2 u 320 350 30 16.5 w1 5039 6480 201 >19.6 m2 4834 6470 393 >20.2 w2 4425 6060 393 >20.1 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.15 in the direction of the burst (Schlegel et al. 1998).