TITLE: GCN CIRCULAR NUMBER: 10436 SUBJECT: GRB 100219A: Swift/UVOT Detection of the Optical Afterglow DATE: 10/02/20 00:18:39 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/GSFC/USRA) and A. Rowlinson (U. Leicester) report on the behalf of the Swift UVOT team: Swift/UVOT began settled observations of the field of GRB 100219A starting 161 s after the BAT trigger (Rowlinson, et al. 2010, GCN Circ. 10430). Settled exposures started at T+183 s. The source reported by Holland, et al. (2010, GCN Circ. 104322) is seen in the white and u-band exposures. Preliminary 3-sigma magnitudes and upper limits are Filter T_start T_stop Exp(s) Mag Err ------------------------------------------------------ white 183 333 150 20.91 0.28 white 875 1718 225 >21.6 3-sigma UL v 673 1767 136 >19.7 3-sigma UL b 598 1693 117 >20.5 3-sigma UL u 341 1668 343 20.35 0.35 uvw1 722 1812 113 >19.9 3-sigma UL uvm2 697 1792 39 >18.3 3-sigma UL uvw2 648 1297 78 >19.5 3-sigma UL ------------------------------------------------------ The fading observed between T+183 s and T+875 s suggests that this is the afterglow of GRB 100219A superimposed on the host galaxy candidate of Bloom and Nugent (2010, GCN Circ. 10433). The weak detection in the u band suggests that this source has a redshift of less than approximately 2.8. The quoted magnitudes have not been corrected for the expected Galactic extinction along the line of sight corresponding to a reddening of E_{B-V} = 0.08 mag (Schlegel, et al., 1998, ApJS, 500, 525). All photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627).