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We report here the discovery by the Intermediate Palomar Transient Factory(iPTF) of iPTF14yb, a luminous ($M_{r}\approx-27.8$ mag), cosmological(redshift 1.9733), rapidly fading optical transient. We demonstrate, based onprobabilistic arguments and a comparison with the broader population, thatiPTF14yb is the optical afterglow of the long-duration gamma-ray burst GRB140226A. This marks the first unambiguous discovery of a GRB afterglow prior to(and thus entirely independent of) an associated high-energy trigger. Weestimate the rate of iPTF14yb-like sources (i.e., cosmologically distantrelativistic explosions) based on iPTF observations, inferring an all-sky valueof $\Re_{\mathrm{rel}}=610$ yr$^{-1}$ (68% confidence interval of 110-2000yr$^{-1}$). Our derived rate is consistent (within the large uncertainty) withthe all-sky rate of on-axis GRBs derived by the Swift satellite. Finally, webriefly discuss the implications of the nondetection to date of bona fide"orphan" afterglows (i.e., those lacking detectable high-energy emission) onGRB beaming and the degree of baryon loading in these relativistic jets.

iPTF14yb: The First Discovery of a GRB Afterglow Independent of a High-Energy Trigger