[vsnet-grb-info 21261] GRB 181020A: Swift/UVOT Detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23360
SUBJECT: GRB 181020A: Swift/UVOT Detection
DATE:    18/10/21 18:39:11 GMT
FROM:    Jeffrey Gropp at PSU  <jdg44 at psu.edu>

J. D. Gropp (PSU), M. De Pasquale
(U. Instanbul) and Moss (George Washington University)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 181020A
122 s after the BAT trigger (Moss et al., GCN Circ. 23349).
A fading source consistent with the XRT enhanced position
(Goad et al. GCN Circ. 23351)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  00:55:55.77 =  13.98239 (deg.)
    Dec (J2000) = -47:22:51.9  = -47.38108 (deg.)
with an estimated uncertainty of 0.45 arc sec. (radius, 90% confidence).
This optical afterglow is also reported by VLT/X-shooter, Fynbo et al., GCN
Circ. 23356;
LCO Cerro Tololo, Cucchiara et al., GCN Circ. 23358;  and its position is
consistent
with the Fermi LAT Measurement reported in, Axelsson et al., GCN Circ.
23350;

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

b                     352             1027             97
17.18+-0.07
m2                  427               745             58      >18.06
u_fc                 96               6797           768
18.52+-0.09
u                      96                346            245
17.60+-0.08
v                      21                 720              67
 14.89+-0.04
w1                  452                 769              58       >18.20
w2                  379                1053             97       >18.56

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).

We note that the non-detection in the UV filters is expected for a source
at the reported
redshift z=2.94 (VLT/X-shooter, Fynbo et al., GCN Circ. 23356;), given
the Lyman forest absorption by intergalactic hydrogen.