  Particle acceleration to suprathermal energies in strong astrophysical shock
waves is a widespread phenomenon, generally explained by diffusive shock
acceleration. Such shocks can also amplify upstream magnetic field considerably
beyond simple compression. The complex plasma physics processes involved are
often parameterized by assuming that shocks put some fraction $\epsilon_e$ of
their energy into fast particles, and another fraction $\epsilon_B$ into
magnetic field. Modelers of shocks in supernovae, supernova remnants, and
gamma-ray bursters, among other locations, often assume typical values for
these fractions, presumed to remain constant in time. However, it is rare that
enough properties of a source are independently constrained that values of the
epsilons can be inferred directly. Supernova remnants (SNRs) can provide such
circumstances. Here we summarize results from global fits to spatially
integrated emission in six young SNRs, finding $10^{-4} \le \epsilon_e \le
0.05$ and $0.001 \le \epsilon_B \le 0.1$. These large variations might be put
down to the differing ages and environments of these SNRs, so we conduct a
detailed analysis of a single remnant, that of Kepler's supernova. Both
epsilons can be determined at seven different locations around the shock, and
we find even larger ranges for both epsilons, as well as for their ratio (thus
independent of the shock energy itself). We conclude that unknown factors have
a large influence on the efficiency of both processes. Shock obliquity,
upstream neutral fraction, or other possibilities need to be explored, while
calculations assuming fixed values of the epsilons should be regarded as
provisional.
