TITLE:   GCN CIRCULAR
NUMBER:  18432
SUBJECT: GRB 151021A: Swift/UVOT Detection
DATE:    15/10/21 15:44:32 GMT
FROM:    Marissa McCaule at PSU  <marissamc@swift.psu.edu>

L. M. McCauley (PSU), and A. Melandri (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151021A
99 s after the BAT trigger (Melandri et al., GCN Circ. 18425).
A source consistent with the XRT position
(Osborne et al. GCN Circ. 18428)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  22:30:34.43 = 337.64335 (deg.)
    Dec (J2000) = -33:11:50.3  = -33.19715 (deg.)
with an estimated uncertainty of 0.42 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                  99         249      294   16.89
white                1668        5949      216  >20.86
v                      81          90        9   15.82
v                    1545        6359      235  >19.28
b                    1644        5744      216  >20.19
u                    1619        5538      216  >19.77
uvw1                 5133        5333      196  >19.53
uvm2                 4928        7484      316  >19.64
uvw2                 5955        6155      196  >19.97


The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).