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arXiv 1402.5364astro-ph.HE
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伽马射线暴准热光球辐射的低能谱指数和$E_{p}$演化

Low Energy Spectral Index and $E_{p}$ evolution of Quasi-thermal Photosphere Emission of Gamma-Ray Bursts

Wei Deng, Bing Zhang

摘要:

费米卫星最近的观测表明,光球发射分量对许多伽玛暴的观测光谱有贡献。一个重要的问题是光球成分是否能够解释具有典型低能光子谱指数$α\sim -1$的伽玛暴的典型“带”函数。我们通过逐步引入几个以前没有完全结合的物理成分来详细研究光球发射光谱,包括动态演变光球位置的概率分布,连续风中等到达时间“体积”的发射叠加,有限但不断演变的外边界风的光学深度的演变,以及不同顶帽风光度($L_w$)廓线的影响。通过假设光球中出现一个共同运动的黑体光谱,我们发现,对于一个恒定的或增加的$L_w$流出,低于峰值能量($E_{p}$)的低能光谱,可以修改为$F_ν\sim ν^{1.5}$($α\sim +0.5$)。在$L_w$减少阶段或高纬度发射主导阶段期间,可以获得较软($-1<α<+0.5$)或平坦($α=-1$)的光谱。在这个光球模型中,我们还研究了$E_{p}$随风和光球光度的演化。如果在无量纲熵$η$和$L_w$之间引入一定的正相关性,则可以再现$E_p-L$跟踪图案。然而,硬到软的演变模式不能复制,除非人为的条件被调用。为了解释能带谱,需要更复杂的光球模型或不同的能量耗散和辐射机制。

英文摘要:

Recent observations by the Fermi satellite suggest that a photosphere emission component is contributing to the observed spectrum of many GRBs. One important question is whether the photosphere component can interpret the typical ``Band'' function of GRBs with a typical low energy photon spectral index $α\sim -1$. We perform a detailed study of the photosphere emission spectrum by progressively introducing several physical ingredients previously not fully incorporated, including the probability distribution of the location of a dynamically evolving photosphere, superposition of emission from an equal-arrival-time ``volume'' in a continuous wind, the evolution of optical depth of a wind with finite but evolving outer boundary, as well as the effect of different top-hat wind luminosity ($L_w$) profiles. By assuming a co-moving blackbody spectrum emerging from the photosphere, we find that for an outflow with a constant or increasing $L_w$, the low-energy spectrum below the peak energy ($E_{p}$), can be modified to $F_ν\sim ν^{1.5}$ ($α\sim +0.5$). A softer ($-1<α<+0.5$) or flat ($α=-1$) spectrum can be obtained during the $L_w$ decreasing phase or high-latitude-emission-dominated phase. We also study the evolution of $E_{p}$ as a function of wind and photosphere luminosity in this photosphere model. An $E_p-L$ tracking pattern can be reproduced if a certain positive dependence between the dimensionless entropy $η$ and $L_w$ is introduced. However, the hard-to-soft evolution pattern cannot be reproduced unless a contrived condition is invoked. In order to interpret the Band spectrum, a more complicated photosphere model or a different energy dissipation and radiation mechanism are needed.

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