Accurate localizations have driven and enriched our understanding of gamma-ray
bursts. They could do the same for future gravitational-wave detections with LIGO
and Virgo. We report the discovery of the optical afterglow of the gamma-ray burst
(GRB) 130702A, identified upon searching 71 square degrees surrounding the Fermi
Gamma-ray Burst Monitor (GBM) localization. Discovered and characterized by the
intermediate Palomar Transient Factory (iPTF), iPTF13bxl is the first afterglow discovered
solely based on a GBM localization. Real-time image subtraction, machine
learning, human vetting, and rapid response multi-wavelength follow-up enabled us
to quickly narrow a list of 27,004 optical transient candidates to a single afterglow-like
source. The bright afterglow and emerging supernova offered an opportunity for
extensive panchromatic follow-up. Furthermore, our discovery of iPTF13bxl represents
an important step towards overcoming the challenges inherent in uncovering
faint optical counterparts to comparably localized gravitational wave events in the
Advanced LIGO and Virgo era
