TITLE: GCN CIRCULAR NUMBER: 9677 SUBJECT: GRB 090715B: break in the optical afterglow decay DATE: 09/07/16 11:49:28 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), Z. Cano, A. Melandri, C.G. Mundell, R.J. Smith, I.A. Steele, D. Bersier, S. Kobayashi (Liverpool JMU), A. Gomboc (U. Ljubljana) on behalf of a large collaboration report: We began observing the Swift GRB 090715B (Vetere et al. GCN Circ. 9668) with the Liverpool Telescope (LT) on July 15, 21:14:19 UT (11.0 min post BAT trigger) with the SDSS riz filters, confirming the detection of the optical afterglow seen by Swift-UVOT (Smith et al. GCN Circ. 9670; Malesani et al GCN Circ. 9671; Antonelli et al. GCN Circ. 9675). Observations with the LT carried on until 5.6 hours post burst. Our measurements are in agreement with the value reported by Malesani et al. (GCN Circ. 9671) after correcting for the different calibration. We also observed the OT with the Faulkes Telescope North (FTN) from July 16, 09:45:46 UT (i.e., at 12.71 hours) with the R filter. The light curve in the r filter from 0.18 to 12.7 hours can be fitted with a broken power-law with alpha1=0.25 +/- 0.02, t_break =18 (-3, +12) ks, alpha2~0.7. Telescope Mid Time Exposure Filter Mag (hours) (s) ------------------------------------------------------ FTN 12.90 1200 R 20.98 +- 0.05 ------------------------------------------------------ Calibration was performed against the SDSS star mentioned by Malesani et al. (GCN 9671), assuming r'=17.74 and R=17.54.