The optical identification of events with poorly defined locations: the case of the Fermi GBM GRB 140801A

We report the early discovery of the optical afterglow of gamma-ray burst (GRB) 140801A in the 137 deg2 3-σ error-box of the <it>Fermi</it> Gamma-ray Burst Monitor (GBM). MASTER is the only observatory that automatically reacts to <it>all</it> <it>Fermi</it> alerts. GRB 140801A is one of the few GRBs whose optical counterpart was discovered solely from its GBM localization. The optical afterglow of GRB 140801A was found by MASTER Global Robotic Net 53 s after receiving the alert, making it the fastest optical detection of a GRB from a GBM error-box. Spectroscopy obtained with the 10.4-m Gran Telescopio Canarias and the 6-m Big Telescope Alt-azimuth of the Special Astrophysical Observatory of the Russian Academy of Sciences reveals a redshift of <it>z</it> = 1.32. We performed optical and near-infrared photometry of GRB 140801A using different telescopes with apertures ranging from 0.4 to 10.4 m. GRB 140801A is a typical burst in many ways. The rest-frame bolometric isotropic energy release and peak energy of the burst are <f>$E_\\mathrm{iso} = 5.54_{-0.24}^{+0.26} {\\times} 10^{52}$</f> erg and <it>E</it><inf>p, rest</inf> ≃ 280 keV, respectively, which is consistent with the Amati relation. The absence of a jet break in the optical light curve provides a lower limit on the half-opening angle of the jet θ = 6<f>$_{.}^{\\circ}$</f>1. The observed <it>E</it><inf>peak</inf> is consistent with the limit derived from the Ghirlanda relation. The joint <it>Fermi</it> GBM and Konus-<it>Wind</it> analysis show that GRB 140801A could belong to the class of intermediate duration. The rapid detection of the optical counterpart of GRB 140801A is especially important regarding the upcoming experiments with large coordinate error-box areas.