TITLE: GCN CIRCULAR NUMBER: 10779 SUBJECT: GRB 100518A: optical afterglow decay DATE: 10/05/19 09:04:53 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy D. Kopac (U. Ljubljana), C. Guidorzi (U. Ferrara), Z. Cano (Liverpool JMU), A. Gomboc (U. Ljubljana), report on behalf of a larger collaboration: The 2-m Faulkes Telescope North observed the optical afterglow of GRB 100518A (Cano et al. GCN Circ. 10776) from 4.2 min to 3.7 hours from the INTEGRAL trigger time (Mereghetti et al. GCN Circ. 10772) in the BVRi filters. Within this time interval the afterglow is observed to decay in both R and i filters with an average power-law decay slope of 0.7 +/- 0.1, with the following initial magnitudes: Mid time from Total Exp Filter Magnitude trigger (min) (s) ------------------------------------------------- 4.9 3x10 R 19.4 +- 0.1 9.8 10 i 18.8 +- 0.2 ------------------------------------------------- Magnitudes have been calibrated from nearby USNOB-1 stars.