TITLE:   GCN CIRCULAR
NUMBER:  18856
SUBJECT: GRB 160104A: Swift/UVOT Detection
DATE:    16/01/12 22:20:40 GMT
FROM:    Frank Marshall at GSFC  <femarsha@khamseen.gsfc.nasa.gov>

F.E. Marshall (NASA/GSFC) and A. Melandri (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160104A
85 s after the BAT trigger (Melandri et al., GCN Circ. 18815).
A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 18821)
and the optical afterglow position (Guidorzi et al. GCN Circ. 18816)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  05:07:11.05 =  76.79603 (deg.)
    Dec (J2000) =  11:19:25.5  =  11.32375 (deg.)
with an estimated uncertainty of 0.64 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 early exposures are: 

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

white               85          235          141         20.48 +/- 0.22
white             1180         1893           94        >20.5
v                  627         1252           78         18.92 +/- 0.32
b                  553         1869          136        >19.8
u                  297         1843          363        >20.5
w1                 677         1819          136        >20.1
m2                 825         5587          203        >20.1
w2                 602         5249          333        >20.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.26 in the direction of the burst
(Schlegel et al. 1998).