TITLE: GCN CIRCULAR NUMBER: 29436 SUBJECT: GRB 210207B: Swift/UVOT Detection DATE: 21/02/09 07:45:09 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and A. Y. Lien (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210207B 82 s after the BAT trigger (Lien et al., GCN Circ. 29420). A source consistent with the XRT position (Lien et al. GCN Circ. 29420) is detected in the initial UVOT exposures, as reported in the initial circular and confirmed by ARIES (Kumar et al., GCN Circ. 29421), KAIT (Zheng & Filippenko, GCN Circ. 29430) and LCO (Strausbaugh & Cucchiara, GCN Circ. 29431). We note that the lack of detection in the two bluer NUV passbands is suggestive of a redshift in the 1.4-1.9 range. The preliminary UVOT position is: RA (J2000) = 18:02:32.40 = 270.63500 (deg.) Dec (J2000) = +53:40:55.8 = 53.68217 (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 (fc) 83 233 147 13.66+/-1.13 white 4081 5717 393 17.91+/-0.04 white 61938 62307 361 21.06+/-0.32 v 4492 4692 196 17.61+/-0.10 v 9433 10341 885 18.54+/-0.09 v 56912 74907 1297 >19.84 b 3876 5512 393 17.95+/-0.05 b 44686 45593 885 >20.99 b 61025 79012 1770 20.79+/-0.18 u (fc) 295 410 113 14.42+/-0.03 u 5106 5306 196 17.80+/-0.09 u 43773 44680 885 20.47+/-0.26 u 51795 69087 612 >19.80 uvw1 4901 5101 196 18.23+/-0.18 uvw1 11254 11428 171 >18.90 uvw1 39917 40544 616 >19.72 uvw1 50889 68646 1771 >20.53 uvm2 4697 4896 196 >19.12 uvm2 10346 11246 885 >20.23 uvw2 4287 5871 342 >19.39 uvw2 56005 74346 1771 >20.68 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.041 in the direction of the burst (Schlegel et al. 1998). [GCN OPS NOTE: Per author's request the rerferece to Circ 294311 was changed to the correct value Circ 29431.]