TITLE:   GCN CIRCULAR
NUMBER:  21375
SUBJECT: GRB 170728B: LT afterglow confirmation
DATE:    17/07/29 04:50:11 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi@fe.infn.it>

C. Guidorzi (U. Ferrara), C.G. Mundell (U. Bath), A. Gomboc (U. Nova 
Gorica), S. Kobayashi, I.A. Steele (LJMU) on behalf of a large 
collaboration report:

We began observing Swift GRB 170728B (Cenko et al. GCN 21371) on July 
28, 23:06:47 UT (208 seconds since the GRB trigger) with 2-m Liverpool 
Telescope with RINGO3 polarimeter and IO:O camera in the SDSS R filter. 
We clearly detect the optical counterpart (D'Avanzo et al. GCN 21373; 
Heintz et al. GCN 21374) with r'=20.43 +- 0.12 mag in a 6x10s frame at a 
mid time of 36.5 minutes since GRB, as calibrated against nearby 
Pan-STARRS1 sources.