TITLE: GCN CIRCULAR NUMBER: 30278 SUBJECT: GRB 210619B: Swift/UVOT Detection DATE: 21/06/20 16:42:40 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and P. D'Avanzo (INAF-OAB) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210619B 136 s after the BAT trigger (D'Avanzo et al., GCN Circ. 30261). A source consistent with the XRT position (Beardmore et al., GCN Circ. 30267) was detected in the initial UVOT exposures consistent with the reported afterglow by Lipunov et al. (GCN Circ. 30259), Jelinek et al. (GCN Circ. 30263), Kong (GCN Circ. 30265), Kellegrin et al. (GCN Circ. 30268), and Zheng & Filippenko (GCN Circ. 30273). A redshift measurement of z=1.937 was reported by de Ugarte Postigo et al. (GCN Circ. 20272) which puts the Lyman break right at the center of the UVOT uvm2 filter. The preliminary UVOT position is: RA (J2000) = 21:18:52.38 = 319.71825 (deg.) Dec (J2000) = +33:51:01.6 = 33.85044 (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 white 136 285 147 14.67 +/- 0.02 white 794 814 19 16.66 +/- 0.06 v 679 699 19 15.59 +/- 0.10 b 604 624 20 16.39 +/- 0.08 u 348 598 246 15.60 +/- 0.03 w1 729 1107 39 17.22 +/- 0.19 m2 704 1776 117 >19.3 w2 655 1726 136 19.20 +/- 0.31 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.173 in the direction of the burst (Schlegel et al. 1998).