3D Photon Conserving Code for Time-dependent General Relativistic Radiative Transfer : CARTOON

TL;DR

CARTOON is a 3D general relativistic radiative transfer code that conserves photon number and energy, accurately modeling wavefronts and black hole shadows in curved spacetime.

Contribution

It extends the 2D ARTIST code to 3D, incorporating photon number and energy conservation in relativistic radiative transfer simulations around black holes.

Findings

Wavefront propagation matches analytical solutions.

Photon number remains conserved until the event horizon.

Black hole shadow can be reproduced in simulations.

Abstract

We develop the 3-dimensional general relativistic radiative transfer code: CARTOON (Calculation code of Authentic Radiative Transfer based On phOton Number conservation in curved space-time) which is improved from the 2-dimensional code: ARTIST developed by Takahashi & Umemura (2017). In CARTOON, the frequency-integrated general relativistic radiative transfer equation is solved in a photon number-conserving manner, and the isotropic and coherent scattering in the zero angular momentum observers (ZAMO) frame and the fluid rest frame is incorporated. By calculating the average energy of photons, energy conservation of the radiation is also guaranteed. With the test calculations in 2-dimensional and 3-dimensional space, we have demonstrated that the wavefront propagation in black hole space-time can be correctly solved in CARTOON conserving photon numbers. The position of the wavefront coincides with the analytical solution and the number of photons remains constant until the wavefront reaches the event horizon. We also solve the radiative transfer equation on the geodesic reaching the observer's screen. The time variation of the intensity map on the observer's screen can be simultaneously and consistently calculated with the time variation of the radiation field around the black hole. In addition, the black hole shadow can be reproduced in moderately optically thin situations.