  Intrinsic variability was searched for in arrival times of six gamma-ray
bursts (GRBs) at high energies -- between 30 MeV and 2 GeV -- detected by the
Fermi satellite's Large Area Telescope (LAT). The GRBs were selected from the
Fermi LAT catalog with preference for events with numerous photons, a strong
initial pulse, and measured redshifts. Three long GRBs and three short GRBs
were selected and tested. Two different variability-detection algorithms were
deployed, one counting photons in pairs, and the other multiplying time gaps
between photons. In both tests, a real GRB was compared to 1000 Monte-Carlo
versions of itself smoothed over a wide range of different timescales. The
minimum detected variability timescales for long bursts (GRB 080916C, GRB
090926A, GRB 131108A) was found to be (0.005, 10.0, 10.0) seconds for the
photon pair test and (2.0, 20.0, 10.0) seconds for the time-gap multiplication
test. Additionally, the minimum detected variability timescales for the short
bursts (GRB 090510, GRB 140619B, GRB 160709A) was found to be (0.05, 0.01,
20.0) seconds for the photon pair test and (0.05, 0.01, 20.0) seconds for the
gap multiplication test. Statistical uncertainties in these times are about a
factor of 2. The durations of these variability timescales may be used to
constrain the geometry, dynamics, speed, cosmological dispersion,
Lorentz-invariance violations, weak equivalence principle violations, and GRB
models.
