  We study the ground state and low energy excitations of vortices pinned to
columnar defects in superconductors, taking into account the long--range
interaction between the fluxons. We consider the ``underfilled'' situation in
the Bose glass phase, where each flux line is attached to one of the defects,
while some pins remain unoccupied. By exploiting an analogy with disordered
semiconductors, we calculate the spatial configurations in the ground state, as
well as the distribution of pinning energies, using a zero--temperature Monte
Carlo algorithm minimizing the total energy with respect to all possible
one--vortex transfers. Intervortex repulsion leads to strong correlations
whenever the London penetration depth exceeds the fluxon spacing. A pronounced
peak appears in the static structure factor $S(q)$ for low filling fractions $f
\leq 0.3$. Interactions lead to a broad Coulomb gap in the distribution of
pinning energies $g(\epsilon)$ near the chemical potential $\mu$, separating
the occupied and empty pins. The vanishing of $g(\epsilon)$ at $\mu$ leads to a
considerable reduction of variable--range hopping vortex transport by
correlated flux line pinning.
