Validation of radiative transfer computation with Monte Carlo method for ultra-relativistic background flow

Ayako Ishii, Naofumi Ohnishi, Hiroki Nagakura, Hirotaka Ito, Shoichi Yamada

    Research output: Contribution to journalArticle

    Abstract

    We developed a three-dimensional radiative transfer code for an ultra-relativistic background flow-field by using the Monte Carlo (MC) method in the context of gamma-ray burst (GRB) emission. For obtaining reliable simulation results in the coupled computation of MC radiation transport with relativistic hydrodynamics which can reproduce GRB emission, we validated radiative transfer computation in the ultra-relativistic regime and assessed the appropriate simulation conditions. The radiative transfer code was validated through two test calculations: (1) computing in different inertial frames and (2) computing in flow-fields with discontinuous and smeared shock fronts. The simulation results of the angular distribution and spectrum were compared among three different inertial frames and in good agreement with each other. If the time duration for updating the flow-field was sufficiently small to resolve a mean free path of a photon into ten steps, the results were thoroughly converged. The spectrum computed in the flow-field with a discontinuous shock front obeyed a power-law in frequency whose index was positive in the range from 1 to 10 MeV. The number of photons in the high-energy side decreased with the smeared shock front because the photons were less scattered immediately behind the shock wave due to the small electron number density. The large optical depth near the shock front was needed for obtaining high-energy photons through bulk Compton scattering. Even one-dimensional structure of the shock wave could affect the results of radiation transport computation. Although we examined the effect of the shock structure on the emitted spectrum with a large number of cells, it is hard to employ so many computational cells per dimension in multi-dimensional simulations. Therefore, a further investigation with a smaller number of cells is required for obtaining realistic high-energy photons with multi-dimensional computations.

    Original languageEnglish
    Pages (from-to)612-633
    Number of pages22
    JournalJournal of Computational Physics
    Volume348
    DOIs
    Publication statusPublished - 2017 Nov 1

    Fingerprint

    Radiative transfer
    radiative transfer
    shock fronts
    Monte Carlo method
    Monte Carlo methods
    Photons
    Flow fields
    flow distribution
    photons
    radiation transport
    gamma ray bursts
    Shock waves
    Gamma rays
    shock waves
    simulation
    cells
    Compton scattering
    Radiation
    Angular distribution
    mean free path

    Keywords

    • Gamma-ray burst
    • Monte Carlo method
    • Radiative transfer
    • Relativistic jet

    ASJC Scopus subject areas

    • Physics and Astronomy (miscellaneous)
    • Computer Science Applications

    Cite this

    Validation of radiative transfer computation with Monte Carlo method for ultra-relativistic background flow. / Ishii, Ayako; Ohnishi, Naofumi; Nagakura, Hiroki; Ito, Hirotaka; Yamada, Shoichi.

    In: Journal of Computational Physics, Vol. 348, 01.11.2017, p. 612-633.

    Research output: Contribution to journalArticle

    Ishii, Ayako ; Ohnishi, Naofumi ; Nagakura, Hiroki ; Ito, Hirotaka ; Yamada, Shoichi. / Validation of radiative transfer computation with Monte Carlo method for ultra-relativistic background flow. In: Journal of Computational Physics. 2017 ; Vol. 348. pp. 612-633.
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    AB - We developed a three-dimensional radiative transfer code for an ultra-relativistic background flow-field by using the Monte Carlo (MC) method in the context of gamma-ray burst (GRB) emission. For obtaining reliable simulation results in the coupled computation of MC radiation transport with relativistic hydrodynamics which can reproduce GRB emission, we validated radiative transfer computation in the ultra-relativistic regime and assessed the appropriate simulation conditions. The radiative transfer code was validated through two test calculations: (1) computing in different inertial frames and (2) computing in flow-fields with discontinuous and smeared shock fronts. The simulation results of the angular distribution and spectrum were compared among three different inertial frames and in good agreement with each other. If the time duration for updating the flow-field was sufficiently small to resolve a mean free path of a photon into ten steps, the results were thoroughly converged. The spectrum computed in the flow-field with a discontinuous shock front obeyed a power-law in frequency whose index was positive in the range from 1 to 10 MeV. The number of photons in the high-energy side decreased with the smeared shock front because the photons were less scattered immediately behind the shock wave due to the small electron number density. The large optical depth near the shock front was needed for obtaining high-energy photons through bulk Compton scattering. Even one-dimensional structure of the shock wave could affect the results of radiation transport computation. Although we examined the effect of the shock structure on the emitted spectrum with a large number of cells, it is hard to employ so many computational cells per dimension in multi-dimensional simulations. Therefore, a further investigation with a smaller number of cells is required for obtaining realistic high-energy photons with multi-dimensional computations.

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