GCN Circular 17751

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

The Swift/UVOT began settled observations of the field of GRB 150424A
99 s after the BAT trigger (Beardmore et al., GCN Circ. 17743
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 ).
A source consistent with the position
of the suggested optical afterglow
(Perley and McConnell, GCN Circ. 17745
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 )
is detected in the initial UVOT exposures.
The source remains nearly constant in 6 exposures with the 
white filter starting 99 s after the trigger and ending
6786 s after the trigger. The magnitude is approximately that
reported by Perley and McConnell. This may be a detection
of the host galaxy.

The preliminary UVOT position is:
   RA  (J2000) =  10:09:13.38 = 152.30577 (deg.)
   Dec (J2000) = -26:37:51.1  = -26.63086 (deg.)
with an estimated uncertainty of 0.6 arcsec. (radius, 90% confidence).

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

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

white               99         6786          609         20.38 +/- 0.07
v                  641         5881          432        >19.8
b                  567         6701          432         20.71 +/- 0.21
u                  311         5061          462         20.13 +/- 0.20
w1                 690         6291          413         20.00 +/- 0.24
m2                4452         6086          393         19.91 +/- 0.29
w2                 616         5677          216         20.03 +/- 0.31

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.06 in the direction of the burst
(Schlegel et al. 1998).

[GCN OPS NOTE(24apr15): Per author's request the missing minus sign
on the hh:mm:ss format of the value was added; the decimal-deg value was correct.]