OSIRIS: A New Code for Ray Tracing Around Compact Objects

TL;DR

OSIRIS is a new FORTRAN code that efficiently computes null geodesics around compact objects, incorporating general relativistic effects, to aid in modeling and interpreting observations of black hole environments.

Contribution

The paper introduces OSIRIS, a stable and efficient FORTRAN-based ray tracing code for null geodesics around black holes, with detailed error analysis and validation through black hole shadow and accretion disk simulations.

Findings

Error in geodesic integration maintained below 10^{-11}

Successfully simulated black hole shadows and Einstein rings

Comparable computation times to parallelized codes

Abstract

The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic plasma orbiting close to the black hole event horizon, where strong gravitational effects are relevant. The observational data of such systems can be compared with theoretical models to infer the black hole and plasma properties. In the comparison process, ray tracing algorithms are essential to computing the trajectories followed by the photons from the source to our telescopes. In this paper, we present OSIRIS: a new stable FORTRAN code capable of efficiently computing null geodesics around compact objects, including general relativistic effects such as gravitational lensing, redshift, and relativistic boosting. The algorithm is based on the Hamiltonian formulation and uses different integration schemes to evolve null geodesics while tracking the error in the Hamiltonian constrain to ensure physical results. We found from an error analysis that the integration schemes are all stable, and the best one maintains an error below $10^{-11}$. Particularly, to test the robustness and ability of the code to evolve geodesics in curved spacetime, we compute the shadow and Einstein rings of a Kerr black hole with different rotation parameters and obtain the image of a thin Keplerian accretion disk around a Schwarzschild black hole. Although OSIRIS is parallelized neither with MPI nor with CUDA, the computation times are of the same order as those reported by other codes with these types of parallel computing platforms.