超相对论背景流中辐射转移蒙特卡罗计算的验证
Validation of Radiative Transfer Computation with Monte Carlo Method for Ultra-Relativistic Background Flow
摘要：
利用蒙特卡罗（MC）方法，建立了一个计算超相对论背景流场的三维辐射转移程序，模拟了伽马射线暴（GRB）辐射过程。为了在MC辐射输运与相对论流体力学耦合计算中获得可靠的模拟结果，重现GRB辐射，我们在超相对论区域验证了辐射转移计算，并评估了合适的模拟条件。通过两个试验计算验证了辐射转移程序的正确性：（1）在不同惯性系下的计算;（2）在激波阵面不连续和模糊的流场中的计算。对三种不同惯性系下的角分布和频谱进行了比较，结果表明，三种惯性系下的角分布和频谱的仿真结果基本一致。如果更新流场的持续时间足够小，可以将光子的平均自由程分解为十步，则结果完全收敛。在激波阵面不连续的流场中计算得到的谱在1 ~ 10 MeV范围内频率服从指数为正的幂律。高能侧的光子数随着模糊的冲击波阵面而减少，因为光子在冲击波后面由于小的电子数密度而散射较少。为了通过体康普顿散射获得高能光子，需要激波前沿附近的大光学深度。即使激波的一维结构也会影响辐射输运计算的结果。
英文摘要：
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.
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