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arXiv 1804.03053astro-ph.HE
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伽马射线暴中辐射介导的激波:次激波光子的产生

Radiation mediated shocks in gamma-ray bursts: Subshock photon production

Christoffer Lundman, Andrei Beloborodov

摘要:

内部冲击提供了一个合理的加热机制,在喷流的伽马射线暴(GRB)。在喷流光球层以下发生的激波是由辐射引起的。以前发现,伽玛射线暴喷流内的辐射介导冲击(RMS)是效率低下的光子生产者,介导RMS的光子必须来自爆炸的早期阶段。我们表明,这一结论只适用于非磁化射流。在中等磁化的等离子体中传播的RMS会产生一个无碰撞的次激波,将等离子体局部加热到相对论温度,热电子在RMS内发射出大量的同步加速器光子。我们发现,这种机制一般是有效的轻度相对论冲击,并可能是在伽玛暴中观察到的光子的主要来源。我们推导出一个简单的解析公式,每个质子产生的光子数,$Z$,它给出了$Z=10^5$ - $10^6$,与观察一致。光子最初以低能量注入,远低于观察到的GRB峰值。它们的数量受两个主要因素控制:(1)在冲击波中产生的电子-正电子对的丰度,这是自洽计算的,(2)由诱导下散射设定的软辐射亮度温度的上限。注入的软光子通过体康普顿化在RMS中获得能量,并形成其非热光谱。

英文摘要:

Internal shocks provide a plausible heating mechanism in the jets of gamma-ray bursts (GRBs). Shocks occurring below the jet photosphere are mediated by radiation. It was previously found that radiation mediated shocks (RMSs) inside GRB jets are inefficient photon producers, and the photons that mediate the RMS must originate from an earlier stage of the explosion. We show that this conclusion is valid only for non-magnetized jets. RMSs that propagate in moderately magnetized plasma develop a collisionless subshock which locally heats the plasma to a relativistic temperature, and the hot electrons emit copious synchrotron photons inside the RMS. We find that this mechanism is generally effective for mildly relativistic shocks and may be the main source of photons observed in GRBs. We derive a simple analytical formula for the generated photon number per proton, $Z$, which gives $Z=10^5$-$10^6$, consistent with observations. The photons are initially injected with low energies, well below the observed GRB peak. Their number is controlled by two main factors: (1) the abundance of electron-positron pairs created in the shock, which is self-consistently calculated, and (2) the upper limit on the brightness temperature of soft radiation set by induced downscattering. The injected soft photons that survive induced downscattering gain energy in the RMS via bulk Comptonization and shape its nonthermal spectrum.

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