TITLE:   GCN CIRCULAR
NUMBER:  20534
SUBJECT: GRB 170126A: Swift/UVOT Detection of a Fading Afterglow
DATE:    17/01/26 22:12:19 GMT
FROM:    Frank Marshall at GSFC  <femarsha@khamseen.gsfc.nasa.gov>

F. E. Marshall (NASA/GSFC) and D. Kocevski (NASA/MSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170126A
91 s after the BAT trigger (Kocevski et al., GCN Circ. 20530).
A fading source consistent with the enhanced XRT position
(Beardmore et al. GCN Circ. 20531)
is detected in the initial UVOT exposure.

The preliminary UVOT position is:
    RA  (J2000) =  17:34:31.97 = 263.63319 (deg.)
    Dec (J2000) = -64:45:37.3  = -64.76035 (deg.)
with an estimated uncertainty of 0.45 arc sec. (radius, 90% confidence).

Preliminary detection 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               91          222          129         18.10 +/- 0.05
white            11432        12036          590        >19.7
v                 1907         2273           58        >17.7
b                 1832         6164          132        >20.1
u                 1807         6084          255        >19.3
w1                1783         5879          255        >19.8
m2                1931         2297           58        >20.0
w2                1882         2248           58        >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.10 in the direction of the burst
(Schlegel et al. 1998).