Discovery and redshift of an optical afterglow in 71 square degrees:
  iPTF13bxl and GRB 130702A
  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. Detection of a new, fading X-ray source by Swift
and a radio counterpart by CARMA and the VLA confirmed the association between
iPTF13bxl and GRB 130702A. Spectroscopy with the Magellan and Palomar 200-inch
telescopes showed the afterglow to be at a redshift of z=0.145, placing GRB
130702A among the lowest redshift GRBs detected to date. The prompt gamma-ray
energy release and afterglow luminosity are intermediate between typical
cosmological GRBs and nearby sub-luminous events such as GRB 980425 and GRB
060218. The bright afterglow and emerging supernova offer an opportunity for
extensive panchromatic follow-up. Our discovery of iPTF13bxl demonstrates the
first observational proof-of-principle for ~10 Fermi-iPTF localizations
annually. Furthermore, it 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.
