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arXiv 2409.17860astro-ph.HE
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伽马射线暴脉冲结构与辐射机制

Gamma-ray burst pulse structures and emission mechanisms

A Gowri, A. Pe'er, F. Ryde, H. Dereli-Bégué

AI总结:

本研究通过分析22个GRBs的61个脉冲,引入新函数量化脉冲不对称性,发现脉冲随时间变不对称且光谱变软,揭示了辐射机制从光球辐射向非热辐射的转变。

摘要:

伽马射线暴(GRBs)的瞬时阶段X射线和$γ$射线光变曲线表现出不规则且复杂的行为,通常具有多个脉冲。单个脉冲的时间形状通常被建模为“快上升指数衰减”(FRED)。在此,我们引入一种新的拟合函数来量化脉冲不对称性。我们对由 Fermi Gamma-ray Burst Monitor 探测到的来自 22 个 GRBs 的 61 个脉冲进行了光变曲线和时间分辨光谱分析。与先前的主张相反,我们发现样本中仅有约 50% 的脉冲光变曲线呈现 FRED 形状,约 25% 具有对称光变曲线,其余 25% 呈混合形状。此外,我们的分析揭示了一个明显趋势:在多脉冲暴中,初始脉冲倾向于表现出最对称的光变曲线,而后续脉冲变得越来越不对称,呈现更接近 FRED 的形状。另外,我们将脉冲的时间与光谱形状进行了关联。通过用经典的“Band”函数拟合光谱,我们发现脉冲不对称性与低能光谱指数 $α_{\max}$(覆盖单个脉冲的所有时间区间中的最大值)之间存在 0.23 的中等正 Spearman 相关系数。因此,在 GRBs 光变曲线期间,脉冲倾向于随时间变得更加不对称且光谱更软。我们将此解释为主导辐射机制从光球辐射(对称且硬)到光球上方非热辐射的转变,并表明该解释在许多情况下与数量级为几十的 GRB 喷流 Lorentz 因子相一致。

英文摘要:

The prompt phase X- and $γ$-ray light curves of gamma-ray bursts (GRBs) exhibit erratic and complex behaviour, often with multiple pulses. The temporal shape of individual pulses is often modelled as 'fast rise exponential decay' (FRED). Here, we introduce a novel fitting function to quantify pulse asymmetry. We conduct a light curve and a time-resolved spectral analysis on 61 pulses from 22 GRBs detected by the Fermi Gamma-ray Burst Monitor. Contrary to previous claims, we find that only $\sim 50\%$ of pulse lightcurves in our sample show a FRED shape, while about $25\%$ have a symmetric lightcurve, and the other $25\%$ have a mixed shape. Furthermore, our analysis reveals a clear trend: in multi-pulse bursts, the initial pulse tends to exhibit the most symmetric light curve, while subsequent pulses become increasingly asymmetric, adopting a more FRED-like shape. Additionally, we correlate the temporal and spectral shapes of the pulses. By fitting the spectra with the classical "Band" function, we find a moderate positive Spearman correlation index of 0.23 between pulse asymmetry and the low-energy spectral index $α_{\max}$ (the maximum value across all time bins covering an individual pulse). Thus, during GRB light curves, the pulses tend to get more asymmetric and spectrally softer with time. We interpret this as a transition in the dominant emission mechanism from photospheric (symmetric-like and hard) to non-thermal emission above the photosphere and show that this interpretation aligns with a GRB jet Lorentz factor of the order of a few 10s in many cases.

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