TITLE: GCN CIRCULAR NUMBER: 21110 SUBJECT: GRB 170510A: LCO Siding Springs early observations DATE: 17/05/19 13:47:40 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), S. Kobayashi, I.A. Steele (LJMU), A. Gomboc (U. Nova Gorica), C.G. Mundell (U. Bath), A. Cucchiara, D. Morris, B. Gendre (U. of Virgin Islands) on behalf of a large collaboration report: We began observing Fermi GRB 170510A (Bissaldi et al. GCN 21081) on May 10, 12:14 UT (7.0 hours since the GRB) with a 1-m LCO unit with the SDSS i filter. Within the Swift-XRT error circle (Cholden-Brown et al. GCN 21103) we clearly detect the optical afterglow candidate (Cucchiara et al. GCN 21087; Lipunov et al. GCN 21091) at the following position: RA(J2000) = 10:40:36.89 DEC(J2000)= -39:12:45.2 with an error radius of 1.0", with the following value: Mid Time Exposure Filter Magnitude (AB) (hours) (s) ------------------------------------------------------- 7.1 5x120 SDSS-I 18.60 +- 0.07 ------------------------------------------------------- as calibrated against nearby UCAC4 stars.