  We present Particle-in-Cell simulation results of relativistic shear boundary
layers between electron-ion and electron-positron plasmas and discuss their
potential applications to astrophysics. Specifically, we find in the case of a
fast electron-positron spine surrounded by a slow-moving or stationary
electron-ion sheath, lepton acceleration proceeds in a highly anisotropic
manner due to electromagnetic fields created at the shear interface. While the
highest-energy leptons still produce a beaming pattern (as seen in the
quasi-stationary frame of the sheath) of order 1/{\Gamma}, where {\Gamma} is
the bulk Lorentz factor of the spine, for lower-energy particles, the beaming
is much less pronounced. This is in stark contrast to the case of pure
electron-ion shear layers, in which anisotropic particle acceleration leads to
significantly narrower beaming patterns than 1/{\Gamma} for the highest-energy
particles. In either case, shear-layer acceleration is expected to produce
strongly angle-dependent lepton (and hence, emanating radiation) spectra, with
a significantly harder spectrum in the forward direction than viewed from
larger off-axis angles, much beyond the regular Doppler boosting effect from a
co-moving isotropic lepton distribution. This may solve the problem of the need
for high (and apparently arbitrarily chosen) minimum Lorentz factors of
radiating electrons, often plaguing current blazar and GRB jet modeling
efforts.
