TITLE:   GCN CIRCULAR
NUMBER:  13805
SUBJECT: GRB 120923A: Swift/UVOT Upper Limit
DATE:    12/09/23 12:50:40 GMT
FROM:    Alice Breeveld at MSSL-UCL  <aab@mssl.ucl.ac.uk>

A. A. Breeveld (MSSL-UCL), V. N. Yershov (MSSL-UCL) and N. P. M. Kuin 
(MSSL-UCL) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 120923A 
143 s after the BAT trigger (Yershov et al., GCN Circ. 13796).
No optical afterglow consistent with the XRT or Gemini position (Goad et 
al., GCN Circ. 13801, Levan et al., GCN Circ. 13802) is detected in the 
initial UVOT exposure.
A preliminary 3-sigma upper limit using the UVOT photometric system 
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding 
chart exposure is:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white              143          293          147         >21.3

The magnitude in the table is not corrected for the Galactic extinction 
due to the reddening of E(B-V) = 0.16 in the direction of the burst 
(Schlegel et al. 1998).