  Cosmic rays are charged particles that are accelerated to relativistic speeds
by astrophysical shocks. Numerical models have been successful in confirming
the acceleration process for (quasi-)parallel shocks, which have the magnetic
field aligned with the direction of the shock motion. However, the process is
less clear when it comes to (quasi-)perpendicular shocks, where the field makes
a large angle with the shock-normal. For such shocks, the angle between the
magnetic field and flow ensures that only highly energetic particles can travel
upstream at all, reducing the upstream current. This process is further
inhibited for relativistic shocks, since the shock can become superluminal when
the required particle velocity exceeds the speed of light, effectively
inhibiting any upstream particle flow. In order to determine whether such
shocks can accelerate particles, we use the particle-in-cell (PIC) method to
determine what fraction of particles gets reflected initially at the shock. We
then use this as input for a new simulation that combines the PIC method with
grid-based magnetohydrodynamics to follow the acceleration (if any) of the
particles over a larger time-period in a two-dimensional grid. We find that
quasi-perpendicular, relativistic shocks are capable of accelerating particles
through the DSA process, provided that the shock has a sufficiently high
Alfvenic Mach number.
