  We present broadband (radio, optical, and X-ray) light curves and spectra of
the afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328,
090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large
Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral
bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large
isotropic energy releases (10e54 erg). Although rare, these events are
particularly important for testing GRB central-engine models. When combined
with spectroscopic redshifts, our afterglow data for these four events are able
to constrain jet collimation angles, the density structure of the circumburst
medium, and both the true radiated energy release and the kinetic energy of the
outflows. In agreement with our earlier work, we find that the relativistic
energy budget of at least one of these events (GRB 090926A) exceeds the
canonical value of 10e51 erg by an order of magnitude. Such energies pose a
severe challenge for models in which the GRB is powered by a magnetar or
neutrino-driven collapsar, but remain compatible with theoretical expectations
for magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles
(theta) are similar to those found for pre-Fermi GRBs, but the large initial
Lorentz factors (Gamma_0) inferred from the detection of GeV photons imply
theta Gamma_0 ~ 70-90, values which are above those predicted in MHD models of
jet acceleration. Finally, we find that these Fermi-LAT events preferentially
occur in a low-density circumburst environment, and we speculate that this
might result from the lower mass-loss rates of their lower-metallicity
progenitor stars. Future studies of Fermi-LAT afterglows in the radio with the
order-of-magnitude improvement in sensitivity offered by the EVLA should
definitively establish the relativistic energy budgets of these events.
