  We report on the observation of the bright, long gamma-ray burst, GRB
090926A, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT)
instruments on board the \Fermi\ Gamma-ray Space Telescope. GRB 090926A shares
several features with other bright LAT bursts. In particular, it clearly shows
a short spike in the light curve that is present in all detectors that see the
burst, and this in turn suggests that there is a common region of emission
across the entire \Fermi\ energy range. In addition, while a separate
high-energy power-law component has already been observed in other GRBs, here
we report for the first time the detection with good significance of a
high-energy spectral break (or cutoff) in this power-law component around 1.4
GeV in the time-integrated spectrum. If the spectral break is caused by opacity
to electron-positron pair production within the source, then this observation
allows us to compute the bulk Lorentz factor for the outflow, rather than a
lower limit.
