GCN Circular 21399

GRB 170728B: Swift/UVOT Detection

S. J. LaPorte (PSU), S.W.K. Emery (MSSL), and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

After detailed analysis of initial data reported in LaPorte and Cenko, 
GCN Circ. 21388
Not found
 
 
, we do find a significant detection in initial optical 
filter exposures.

The Swift/UVOT began settled observations of the field of GRB 170728B
473 s after the BAT trigger (Cenko et al., GCN Circ. 21371
Not found
 
 
).
A fading source consistent with the optical position
(D'Avanzo et al. GCN Circ. 21373
Not found
 
 
)
is detected in the initial UVOT exposures.

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              876         1026          147         20.56 +/- 0.20
v                  630         8278          627         20.23 +/- 0.27
b                  728         2561          194         20.60 +/- 0.29
u                  703        13868         1276        >21.2
w1                 679        19251         2000        >21.6
m2                6847        18681         1279        >22.1
w2                6437         8073          393        >21.5

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).