TITLE:   GCN CIRCULAR
NUMBER:  17739
SUBJECT: GRB 150423A: Swift/UVOT Upper Limits
DATE:    15/04/23 21:00:00 GMT
FROM:    Frank Marshall at GSFC  <femarsha@khamseen.gsfc.nasa.gov>

F.E. Marshall (NASA/GSFC) and C. Pagani (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150423A
74 s after the BAT trigger (Pagani et al., GCN Circ. 17728).
No optical afterglow consistent with the optical position
(Source 1 in Varela et al. GCN Circ. 17732; Perley GCN Circ. 17733) 
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) exposures and subsequent exposures are:

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

white_FC            74          223          147         >20.8
u_FC               286          535          246         >20.2
white               74         6067          733         >21.5
v                  615         6477          452         >20.9
b                  541         5862          255         >20.9
u                  286         6924          513         >20.8
w1                 665         6887          452         >20.9
m2                5048         6682          393         >20.8
w2                 591         6273          432         >21.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.03 in the direction of the burst
(Schlegel et al. 1998).