相对论膨胀等离子体热辐射的时间演化
Temporal Evolution Of Thermal Emission From Relativistically Expanding Plasma
摘要：
光子在相对论膨胀等离子体外流中的传播，其特征在于稳定的洛伦兹因子伽玛被认为是。注入高光学深度区域的光子与气流平流，直到它们在光球层逃逸。在光球层下方，光子通过康普顿散射耦合到等离子体。我在这里表明，由于散射电子速度矢量的轻微不对准，（局部）同向运动光子能量随半径减小为ε '（r）~ r^{-2/3}。在电子绝热冷却较快的情况下，这种机制主导了光子冷却。然后，我表明，光球半径的相对论膨胀等离子体风强烈依赖于角度的视线，θ。对于θ <~ 1/Gamma，r_{ph}与θ无关，而对于θ>~ 1/Gamma，r_{ph} ~ θ ^2。我表明，θ的光球的依赖性意味着，对于流参数表征伽玛暴，热光子起源于下面的光球可以观察到长达几十秒的内部发动机活动衰减。我计算了热光子以半径r和角度θ逃离等离子体的概率密度函数P（r，θ）。使用这个函数，我表明，随着内部光子注入机制的终止，热通量随着F_{BB} ^{ob.} ~ t^{-2}，光子能量随半径的衰减导致观测温度T^{ob.}的幂律衰减。(t)~ t^{-2/3}，稍后变为t^{-1/2}。详细的数值结果与分析预测非常吻合。我讨论的后果，鉴于最近的证据，在伽玛暴的即时发射阶段观察到的热发射分量的时间行为。
英文摘要：
Propagation of photons in relativistically expanding plasma outflows, characterized by steady Lorentz factor Gamma is considered. Photons that are injected in regions of high optical depth are advected with the flow until they escape at the photosphere. Below the photosphere, the photons are coupled to the plasma via Compton scattering. I show here, that as a result of the slight misalignment of the scattering electrons velocity vectors, the (local) comoving photon energy decreases with radius as epsilon'(r) ~ r^{-2/3}. This mechanism dominates the photon cooling in scenarios of faster adiabatic cooling of the electrons. I then show that the photospheric radius of a relativistically expanding plasma wind strongly depends on the angle to the line of sight, theta. For theta <~ 1/Gamma, r_{ph} is theta independent, while for theta >~ 1/Gamma, r_{ph} ~ theta^2. I show that the theta dependence of the photosphere implies that for flow parameters characterizing GRBs, thermal photons originating from below the photosphere can be observed up to tens of seconds following the inner engine activity decay. I calculate the probability density function P(r,theta) of a thermal photon to escape the plasma at radius r and angle theta. Using this function, I show that following the termination of the internal photon injection mechanism, the thermal flux decreases as F_{BB}^{ob.} ~ t^{-2}, and that the decay of the photon energy with radius results in a power law decay of the observed temperature, T^{ob.}(t) ~ t^{-2/3} at early times, which changes to t^{-1/2} later. Detailed numerical results are in very good agreement with the analytical predictions. I discuss the consequences of this temporal behavior in view of the recent evidence for a thermal emission component observed during the prompt emission phase of GRBs.
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