GCN Circular 21207

M. H. Siegel (PSU) and K. L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170604A
128 s after the BAT trigger (Page et al., GCN Circ. 21191
Not found
 
 
).
We detect a fading afterglow consistent with the position given by XRT
(Page et al., GCN Circ 21196) and ground-based optical detections
(Izzy et al., GCN Circ 21197; Schady, GCN Circ. 21200
Not found
 
 
; Mazaeva et al., 
GCN Circ 21203).

The preliminary UVOT position is:
    RA  (J2000) =  22:50:37.52 = 342.65632 (deg.)
    Dec (J2000) = -15:24:43.9  = -15.41220 (deg.)
with an estimated uncertainty of 0.1 arc sec. (radius, 90% confidence).

Preliminary magnitudes and 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)         128          278          147          15.81+-0.03
white             3968         5603          393          18.72+-0.06
white            39053        39959          885          20.57+-0.18
v                 4378         6013          393          18.94+-0.31
v                34480        44756         1268         >19.89
b                 3763         5398          393          19.26+-0.15
b                38141        39048          885         >20.86
u (fc)             287          429          279          15.70+-0.03
u                 4993         5193          196          18.51+-0.16
u                28543        29132          576         >19.72
uvw1              4788         4988          196          18.47+-0.19
uvw1             23318        46354         1633          20.20+-0.27
uvm2              4583         6169          344          18.58+-0.18
uvm2             44762        45661          885         >20.37
uvw2              4174         5809          393          19.53+-0.26
uvw2             32392        40613         1700         >20.86

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).