$\text{GPUmonty}$: A GPU-accelerated relativistic Monte Carlo radiative transfer code

Abstract

We introduce $\text{GPUmonty}$, a CUDA/C-based Monte Carlo radiative transfer code accelerated using graphics processing units (GPUs). $\text{GPUmonty}$ derives from the CPU-based code $\text{grmonty}$ and offloads the most computationally expensive stages of the calculation -- superphoton generation, sampling, tracking, and scattering -- to the GPU. Whereas $\text{grmonty}$ handles photons sequentially, $\text{GPUmonty}$ processes large numbers of superphotons concurrently, leveraging the single-instruction, multiple-thread (SIMT) execution model of modern GPUs. Benchmarks demonstrate a speedup of about $12\times$ relative to the original CPU implementation on a single GPU, with runtime limited primarily by register pressure rather than compute or memory bandwidth saturation. We validate the implementation through analytic tests for a optically thin synchrotron sphere, as well as comparisons with $\text{igrmonty}$ for scattering synchrotron sphere and GRMHD simulation data. Relative errors remain below a percent level and convergence is consistent with the expected $N_{\mathrm{s}}^{-1/2}$ Monte Carlo scaling. By significantly reducing computational costs, GPUmonty enables the extensive parameter space surveys and faster spectra modeling required to interpret horizon-scale observations of supermassive black holes. $\text{GPUmonty}$ is publicly available under the GNU General Public License.

12 pages, 6 figures and 1 table. V2: Minor textual changes. Published in ApJ

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• Sensitivities of Black Hole Images from GRMHD Simulations