TITLE:   GCN CIRCULAR
NUMBER:  22510
SUBJECT: GRB 180316A: Swift/UVOT Detection
DATE:    18/03/16 15:48:27 GMT
FROM:    Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>

A. A. Breeveld (UCL-MSSL) and A. Melandri (INAF-OAB) report on behalf of 
the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180316A 
105 s after the BAT trigger (Melandri et al., GCN Circ. 22500).
A fading source consistent with the enhanced XRT position (Evans et al. 
GCN Circ. 22508) and the optical transient reported by Lipunov et al. 
(GCN Circ. 22503), Shady & Steinmassl (GCN Circ. 22504) and Martone et 
al. (GCN Circ. 22507), Butler et al. (GCN Circ. 22509) is detected in 
the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  17:41:42.87 = 265.42861 (deg.)
     Dec (J2000) = +00:44:53.7  =   0.74824 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT 
photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for 
the finding chart (white_fc) and early exposures are:

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

white_fc           105          254          147         16.27 ± 0.03
white              596          616           20         17.40 ± 0.11
v                  646          839           39         17.15 ± 0.18
b                  572          592           20         17.40 ± 0.15
u                  317          566          246         16.44 ± 0.04
w1                 695        22410         1175        >20.4
m2               12527        12578           50        >19.2
w2                 622         6753          255        >19.5

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.28 in the direction of the 
burst (Schlegel et al. 1998).