TITLE: GCN CIRCULAR NUMBER: 17707 SUBJECT: GRB 150413A: additional P60 observations DATE: 15/04/15 11:29:50 GMT FROM: Daniel Perley at Caltech D. A. Perley (Caltech) and S. B. Cenko (GSFC) report: We have continued to follow the optical counterpart of GRB 150413A (Markwardt et al., GCN 17688; Ivanov et al., GCN 17689) with the Palomar 60-inch robotic telescope. A second griz epoch was acquired between 10:08:52 and 10:49:31 UT on the night of 2015-04-14, followed by two deeper epochs (one gri, one r only) on the night of UT 2015-04-15. Calibrating our r-band observations to a common set of USNO R-band reference stars, we derive the following photometry (statistical uncertainties only): dt_mid = 0.5602 day : R = 19.13 +/- 0.03 dt_mid = 0.8464 day : R = 20.23 +/- 0.06 dt_mid = 1.6034 day : R = 21.52 +/- 0.16 dt_mid = 1.8652 day : R = 21.74 +/- 0.18 This suggests steep decay over this period (decay index ~ 2.2).