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.

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