SPHRAY: a smoothed particle hydrodynamics ray tracer for radiative transfer

We introduce the publicly available code SPHRAY, a smoothed particle hydrodynamics (SPH) ray tracer designed to solve the 3D, time-dependent, radiative transfer equation for cosmological 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 on to a grid but instead integrates directly through the SPH kernels. Given an arbitrary (series of) SPH density field(s) and a description of the sources of ionizing radiation, the code will calculate the non-equilibrium ionization and temperature state of hydrogen (H I, H II) and helium (He I, He II, He III). 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 ray-tracing component. We present the algorithms used in SPHRAY and verify the code by performing the test problems detailed in the recent Radiative Transfer Comparison Project of Iliev et. al. The source code for SPHRAY and example SPH density fields are made available on a companion website (http://www.sphray.org).