  We present Horizon, a new graphics processing unit (GPU)-accelerated code to
solve the equations of general relativistic magnetohydrodynamics in a given
spacetime. We evaluate the code in several test cases, including magnetized
Riemann problems and rapidly rotating neutron stars, and measure the
performance benefits of the GPU acceleration in comparison to our CPU-based
code Thor. We find substantial performance gains in comparison to a quad-core
CPU both in single- and double-precision accuracy, and discuss these findings
in the context of future numerical modeling efforts.
