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GCN Circular 36184

GRB 240418A: Swift/UVOT Upper Limits
2024-04-19T16:06:19Z (3 months ago)
Sam Shilling at Lancaster University <>
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S. P. R. Shilling (Lancaster U.), S. R. Oates (Lancaster U.),  A. A. Breeveld (UCL-MSSL),  N. P. M. Kuin (UCL-MSSL) and M. H. Siegel (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 240418A
126 s after the BAT trigger (Siegel et al., GCN Circ. 36162).
No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 36166)
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           126          276          147         >20.6
u_FC               284          533          246         >19.4
white              126         5282          347         >21.1
v                 4072         4272          197         >19.3
b                  539         5093          216         >20.2
u                  284         4888          442         >19.9
w1                4482         4682          197         >19.7
m2                4276         4476          197         >19.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.057 in the direction of the burst
(Schlegel et al. 1998).
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