TITLE: GCN CIRCULAR NUMBER: 10645 SUBJECT: GRB 100418A: Continuing Swift/UVOT Observations DATE: 10/04/21 19:26:57 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU), F. E. Marshall (NASA/GSFC), S. T. Holland (NASA/GSFC), M. M. Chester (PSU), M. de Pasquale (MSS-UCL) and S. R. Oates (MSSL-UCL) report on behalf of the Swift/UVOT team. Ongoing Swift/UVOT observations of the field of GRB 100418A (Marshall et al., GCN Circ. 10612) have continued to monitor the behavior of the optical transient. UVOT photometry taken about 50 ks after the burst shows an increase in brightness of about a magnitude compared to the initial detection, confirming the late increase in brightness reported by Bikamev et al. (GCN Circ. 10635). The OT has since decayed from this peak with a decay rate of -1.0+-0.1 in the UVOT white filter. This decay is slightly shallower than the decay observed by Malesani et al. (GCN Circ. 10631, 10643), Updike et al. (GCN Circ. 10637) and Filgas et al. (GCN Circ 10644). Such a late peak in brightness is very unusual and Swift will continue to monitor the GRB. Photometry from the summed exposures is reported below: FILTER T_start(s) T_stop Exposure Mag ========================================================= white fc 88 237 147 20.62+-0.29 white 88 1548 390 20.58+-0.20 white 6143 6343 196 20.32+-0.20 white 50996 52990 1959 19.38+-0.03 white 86204 92193 2308 19.77+-0.04 white 149282 156466 3812 20.54+-0.06 ========================================================= 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).