TITLE:   GCN CIRCULAR
NUMBER:  10704
SUBJECT: GRB 100420A: Terskol optical upper limit
DATE:    10/04/29 13:28:30 GMT
FROM:    Alina Volnova at SAI MSU  <alinusss@gmail.com>

M. Andreev,  A. Sergeev (Terskol Branch of  Institute of Astronomy),
A. Volnova (SAI MSU), A. Pozanenko (IKI) report on behalf of larger
GRB follow-up collaboration:


We observed the field of the Swift GRB 100420A (Baumgartner et al.,
GCN 10628) with the Z-600 telescope of Mt.Terskol observatory in
R-filter between (UT) Apr. 20 23:30 - Apr. 21 00:39 under good weather
conditions with mean seeing of about 1.2 arcsec. We do not detect any
source at the position of the enhanced XRT error circle (Beardmore et
al., GCN 10638).

The 3 sigma upper limit of the stacked image based on several
USNO-B1.0 field stars is following:


T0+      Filter,  Exposure,  mag.
(mid, d)           (s)

0.7789   R       20x180    >20.2