  We introduce SPHRAY, a Smoothed Particle Hydrodynamics (SPH) ray tracer
designed to solve the 3D, time dependent, radiative transfer (RT) equations for
arbitrary density fields. The SPH nature of SPHRAY makes the incorporation of
separate hydrodynamics and gravity solvers very natural. SPHRAY relies on a
Monte Carlo (MC) ray tracing scheme that does not interpolate the SPH particles
onto a grid but instead integrates directly through the SPH kernels. Given
initial conditions and a description of the sources of ionizing radiation, the
code will calculate the non-equilibrium ionization state (HI, HII, HeI, HeII,
HeIII, e) and temperature (internal energy/entropy) of each SPH particle. The
sources of radiation can include point like objects, diffuse recombination
radiation, and a background field from outside the computational volume. The MC
ray tracing implementation allows for the quick introduction of new physics and
is parallelization friendly. A quick Axis Aligned Bounding Box (AABB) test
taken from computer graphics applications allows for the acceleration of the
raytracing component. We present the algorithms used in SPHRAY and verify the
code by performing all the test problems detailed in the recent Radiative
Transfer Comparison Project of Iliev et. al. The Fortran 90 source code for
SPHRAY and example SPH density fields are made available on a companion website
(www.sphray.org).
