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GCN Circular 21910

Subject
GRB 170921A: LCO 2m telescope observations
Date
2017-09-22T12:07:33Z (6 years ago)
From
Cristiano Guidorzi at Ferrara U,Italy <guidorzi@fe.infn.it>
C. Guidorzi (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell (U. Bath), 
A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU), A. Cucchiara, D. Morris, 
and B. Gendre (U. Virgin Islands) on behalf of a large collaboration report:

We began observing Swift GRB 170921A (D'Ai et al. GCN 21901) on 
September 21, from 14:52 to 15:05 UT (from 24 to 37 minutes since the 
GRB trigger time) with the LCO 2-m telescope in Hawaii in the SDSS R and 
I filters. Within the enhanced Swift-XRT circle (Beardmore et al. GCN 
21906) we do not detect any optical counterpart down to the following 
limits, in agreement with previous reports (Klotz et al. GCN 21903; 
Kuroda et al. GCN 21905; Feng et al. GCN 21907; Malesani et al. GCN 21909):

Mid Time����� Exposure������ Filter������ Magnitude (AB)
(min)���������� (s)
-------------------------------------------------------
26.8���������� 5x60��������� SDSS-R������� > 20.7
34.0���������� 3x60��������� SDSS-I������� > 19.8
-------------------------------------------------------

as calibrated against nearby SDSS objects.
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