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arXiv 1302.0235astro-ph.HE
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伽马射线暴中的瞬发热辐射

Prompt thermal emission in gamma-ray bursts

R. Hascoët, F. Daigne, R. Mochkovitch

摘要:

伽玛射线暴的光谱似乎是非热的,但最近对费米GBM的几次爆发的观测证实了BATSE先前的迹象,即存在潜在的热成分。当从中央引擎出现的相对论流出对自身辐射变得透明时,确实预期光球发射,在没有额外的亚光球耗散的情况下具有准黑体光谱。然而,它的强度强烈依赖于加速机制-热或磁-的流动。我们的目标是计算的热和非热的排放所产生的流出与可变的洛伦兹因子,其中在原点注入的功率是部分热(分数1 - 1- 1-th)和部分磁。热辐射产生于光球层,非热辐射产生于光学薄区。除了epsilon_th的值之外,我们还想测试其他模型参数如何影响观察到的热发射与非热发射的比率。如果非热辐射是由内激波引起的,我们自洽地得到了流中任意洛伦兹因子分布的热辐射和非热辐射的光变曲线和光谱。如果非热发射是由磁重联引起的,我们就不能产生光变曲线,只能比较相应的非热和热光谱。在不同的情况下,我们改变了模型参数,以查看光变曲线和/或光谱中的热分量何时可能出现,或者相反,隐藏。最后,我们将我们的结果与GRB谱中存在热分量的证据进行了比较。以GRB 090902 B和GRB 10072 B为例,我们展示了如何利用这些观测结果来约束GRB外流的性质和加速机制。

英文摘要:

GRB spectra appear non-thermal, but recent observations of a few bursts with Fermi GBM have confirmed previous indications from BATSE of the presence of an underlying thermal component. Photospheric emission is indeed expected when the relativistic outflow emerging from the central engine becomes transparent to its own radiation, with a quasi-blackbody spectrum in absence of additional sub-photospheric dissipation. However, its intensity strongly depends on the acceleration mechanism - thermal or magnetic - of the flow. We aim to compute the thermal and non-thermal emissions produced by an outflow with a variable Lorentz factor, where the power injected at the origin is partially thermal (fraction epsilon_th) and partially magnetic (fraction 1-epsilon_th). The thermal emission is produced at the photosphere, and the non-thermal emission in the optically thin regime. Apart from the value of epsilon_th, we want to test how the other model parameters affect the observed ratio of the thermal to non-thermal emission. If the non-thermal emission is made by internal shocks, we self-consistently obtained the light curves and spectra of the thermal and non-thermal components for any distribution of the Lorentz factor in the flow. If the non-thermal emission results from magnetic reconnection we were unable to produce a light curve and could only compare the respective non-thermal and thermal spectra. In the different considered cases, we varied the model parameters to see when the thermal component in the light curve and/or spectrum is likely to show up or, on the contrary, to be hidden. We finally compared our results to the proposed evidence for the presence of a thermal component in GRB spectra. Focussing on GRB 090902B and GRB 10072B, we showed how these observations can be used to constrain the nature and acceleration mechanism of GRB outflows.

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