TITLE: GCN CIRCULAR NUMBER: 20080 SUBJECT: GRB 161017A: LT optical observations DATE: 16/10/19 08:29:40 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), S. Kobayashi, I.A. Steele (LJMU), C.G. Mundell (U. Bath), A. Gomboc (U. Nova Gorica), N.R. Tanvir (U. Leicester) on behalf of a large collaboration report: The 2-m Liverpool Telescope observed Swift GRB161017A (Troja et al., GCN 20064) in the SDSS r and i filters on October 19, from 03:45 to 05:37 UT (mid time~1.45 days since the GRB). We clearly detect the optical afterglow (Yurkov et al. GCN 20063; Troja et al. GCN 20064; de Ugarte-Postigo et al. GCN 20069; Fujiwara et al GCN 20070; Melandri et al. GCN 20071) with the following values, as calibrated against nearby SDSS stars: Mid Time Exposure Filter Magnitude (days) (s) ------------------------------------------------------- 1.42 5x120 SDSS-R 20.02 +- 0.03 1.42 5x120 SDSS-I 19.80 +- 0.03 ------------------------------------------------------- Compared with our previous report (Guidorzi et al. GCN 20072), we estimate an average power-law decay index alpha~1.3 between 0.68 and 1.45 days post burst.