TITLE: GCN CIRCULAR NUMBER: 10027 SUBJECT: GRB 091010: optical afterglow candidate rejection DATE: 09/10/16 15:16:32 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), A. Melandri, C.G. Mundell, D. Bersier, Z. Cano, N.R. Clay, S. Kobayashi, C.J. Mottram, R.J. Smith, I.A. Steele (Liverpool JMU), A. Gomboc (U. Ljubljana), N.R. Tanvir (U. Leicester) on behalf of a large collaboration report: We re-observed the optical afterglow candidate (Rossi et al. GCN Circ. 10008) of the SuperAGILE GRB 091010 (Donnarumma et al. GCN Circ. 10004) with the 2-m Faulkes Telescope South (FTS) on October 16, 09:27:33 UT (6.3 days post burst) with the Ri' filters. The source is still clearly detected in both filters. Comparison with our previous values obtained at 1 day post burst (Guidorzi et al. GCN Circ. 10017) shows that the source did not fade appreciably. We derived an upper limit to the possible power-law decay index of alpha<0.5 (90% confidence) from 1 to 6 days post burst, flux~t^(-alpha). Given that a comparably shallow and long-lasting decay has never been observed, we can confidently rule out this source as the optical afterglow of GRB 091010. Telescope Mid Time Exposure Filter Mag (days) (s) ----------------------------------------------------- FTS 6.29 6x300 i' 22.2 +- 0.3 FTS 6.31 6x300 R 22.3 +- 0.3 ----------------------------------------------------- Calibration was the same as that used in our previous report (GCN Circ. 10017).