TITLE:   GCN CIRCULAR
NUMBER:  12040
SUBJECT: GRB 110521A: MASTER prompt optical  limit 
DATE:    11/05/23 09:31:10 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs  <gcncirc@observ.inetcomm.ru>

E. Gorbovskoy, V. Lipunov, V.Kornilov, D.Kuvshinov, A.Belinski, N.Tyurina,
N.Shatskiy, P.Balanutsa, V.V.Chazov, P.V.Kortunov, A.Kuznetsov, 
D.Zimnukhov, M.Kornilov, A.Sankovich
Sternberg Astronomical Institute, Moscow State University


O. Gres, K.Ivanov, V.A.Poleshchuk, S.Yazev, N.M.Budnev, O.Chuvalaev
Irkutsk State University

V.Krushinski, I.Zalozhnich, A. Popov
Ural State University, Kourovka

A. Tlatov, A.V. Parhomenko, D. Dormidontov,V. Senik
Kislovodsk Solar Station of the Pulkovo Observatory

V.Yurkov, Yu.Sergienko, I.Kudelina
Blagoveschensk Educational State University, Blagoveschensk


The Swift erros box has casually appeared inside FOV of the one of the 2 
Very Wide Field Cameras (Gres et al., GCN Circ 12033).
We have images before, during and after GRB .
The 3-sigma  upper limit has been about 12.0  mag (unfiltered, close 
to V)  each image (5 s exposition).
The upper limit of coadded images during 15 sec GRB is 12.7.
The corresponding optical fluence less than  10^-9 erg/cm^2
The best-fitting absorption column is  4.3 (+1.5, -2.6) x 10^21 cm^-2, in
excess of the Galactic value of 4.5 x 10^20 cm^-2  (Littlejohns et al., 
GCN Circ 12038).
The fluence in the 15-150 keV band is 4.4 +- 0.5 x 10^-7 erg/cm2 
(Barthelmy et al., GCN Circ 12035).
So including ~2 mag absorption:

                   Fluence_opt/Fluemce_gamma(15-150) < ~1/60

The video is available at 
http://observ.pereplet.ru/images/GRB110521A/grb110521A.gif .

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