TITLE: GCN CIRCULAR NUMBER: 10625 SUBJECT: GRB 100418A: Swift/UVOT Observations DATE: 10/04/19 21:54:40 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and F. Marshall (NASA/GSFC) report on behalf of the Swift/UVOT team. The Swift/UVOT began settled observations of the field of GRB 100418A 87s after the BAT trigger (Marshall et al., GCN Circ. 10612). Data summed from the first and second orbits reveals the source reported by Marshall et al. The source shows no fading between the first and second orbits within the photometric uncertainties. The photometry is consistent with a constant source with a white magnitude of 20.48+-0.13. This likely represents the GRB host galaxy reported by Malesani (GCN Circ. 10621) and not the afterglow reported by Filgas et al. (GCN Circ. 10617), Updike et al. (GCN Circ. 10619) and Antonelli et al. (GCN Circ. 10620). The UVOT position of the source is: RA(J2000.0) = 17:05:26.96 = 256.36278 (deg) Dec(J2000.0) = +11:27:41.9 = +11.46167 (deg) with a 90% confidence interval of 0."67. The 3-sigma upper limits for the finding chart (fc) and summed exposures from the first and second orbits are reported below: FILTER T_start(s) T_stop Exposure Mag/3UL ======================================================== white fc 87 237 147 20.62+-0.29 white 89 1548 390 20.58+-0.20 white 6143 6343 196 20.32+-0.20 v 628 6754 491 >19.90 b 554 6138 294 20.61+-.44 u 299 7348 697 20.47+-0.41 uvw1 678 7164 491 20.47+-0.42 uvm2 653 6959 491 20.09+-0.32 uvw2 604 6548 308 20.73+-0.54 ======================================================= The above magnitudes are not corrected for the Galactic extinction corresponding to a reddening of E_{B-V} = 0.07 (Schlegel et al., 1998, ApJS, 500, 525). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627).