TITLE: GCN CIRCULAR NUMBER: 10878 SUBJECT: GRB 100621A: Swift/UVOT observations DATE: 10/06/21 14:30:51 GMT FROM: Samantha Oates at MSSL S. R. Oates (MSSL-UCL) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 100621A 85s after the BAT trigger (Ukwatta et al., GCN 10870). We detect a source at the enhanced Swift XRT position (Evans et al. GCN 10873) and at the location of the GROND source (Updike et al. GCN 10874). This source is also observed in the DSS and observed to be constant during the first 7000s and is thus likely to be the host galaxy, which is also suggested by Updike et al. (GCN 10874). The magnitudes and 3-sigma upper limits for the finding chart exposures (FC) and summed images for this possible host galaxy are: Filter T_start T_stop Exp(s) Magnitude/3-sigma upper limit ------------------------------------------------------------- white (FC) 85 234 147 20.87 +/- 0.35 white 576 7093 519 20.46 +/- 0.15 u (FC) 297 546 246 > 20.18 u 700 6683 360 > 20.14 v 626 6067 391 19.49 +/- 0.31 b 552 6888 372 20.27 +/- 0.28 uvw1 1107 6478 352 19.64 +/- 0.25 uvm2 6073 6273 197 > 19.66 uvw2 5663 5863 197 19.60 +/- 0.28 ------------------------------------------------------------- The quoted upper limits have not been corrected for the expected Galactic extinction along the line of sight of E_(B-V) = 0.03 mag. All photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627).