TITLE:   GCN CIRCULAR
NUMBER:  20034
SUBJECT: GRB 161010A: Swift/UVOT Possible Detection
DATE:    16/10/13 12:53:23 GMT
FROM:    Frank Marshall at GSFC  <femarsha@khamseen.gsfc.nasa.gov>

F. E. Marshall (NASA/GSFC) and A. D'Ai (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 161010A
3.3 hours after the INTEGRAL trigger (Mereghetti et al., GCN Circ. 20021).
A source consistent with the XRT position
(McCauley et al., GCN Circ. 20025) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  18:20:51.27 = 275.21364 (deg.)
     Dec (J2000) = -28:47:06.7  = -28.78519 (deg.)
with an estimated uncertainty of about 1 arc sec. (radius, 90% confidence).

As shown in the table below, the source faded in the observations
using the U filter, suggesting that it could be the afterglow
of the GRB. However, the GRB is only 7.8 degrees from 
the direction of the Galactic Center, and this is a crowded
field for UVOT. Consequently, the association of the source
with the GRB is not certain.

Preliminary magnitudes using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag
u                12034        13628         1035         18.8 +/- 0.1
u                17802        19985          744         19.0 +/- 0.1
u                23563        25993          794         19.4 +/- 0.2
u                29338        30137          787         20.4 +/- 0.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.54 in the direction of the burst
(Schlegel et al. 1998).