TITLE: GCN CIRCULAR NUMBER: 9175 SUBJECT: GRB 090418A: Swift/UVOT Observations DATE: 09/04/20 19:47:16 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/GSFC/USRA) and V. Mangano (INAF-IASFPA) report on the behalf of the Swift UVOT team: The Swift/UVOT observed the field of GRB 090418A starting 84 s after the BAT trigger (Mangano, et al. 2009, GCN Circ. 9149). Settled exposures started at T+160 s. The refined UVOT position for the optical afterglow is RA (J2000) = 17:57:15.17 = 269.31321 (deg) Dec (J2000) = +33:24:21.1 = +33.40585 (deg) with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence, statistical + systematic). Preliminary 3-sigma magnitudes and upper limits are Filter T_start T_stop Exp(s) Mag Err ------------------------------------------------------------- u (fc) 160 410 246 17.31 0.09 v 466 485 19 17.31 0.24 b 415 435 19 18.22 0.25 u 539 559 19 18.55 0.39 uvw1 515 835 19 >18.1 3-sigma UL uvm2 490 510 19 >17.6 3-sigma UL uvw2 441 461 19 >18.1 3-sigma UL uvw1 515 11,831 1337 20.82 0.26 114,115 131,895 2064 >22.0 3-sigma UL uvm2 490 18,413 2109 >21.8 3-sigma UL uvw2 441 16,709 1357 >21.8 3-sigma UL ------------------------------------------------------------- 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} = 0.04 mag (Schlegel, et al., 1998, ApJS, 500, 525). All photometry is on the UVOT flight system described in Poole et al. (2008, MNRAS, 383, 627). The detection in the uvw1 filter, combined with the lack of a detection in the uvm2 filter, is consistent with this source having a redshift of z = 1.608 (Charnock, et al., 2009, GCN Circ. 9151).