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arXiv 1706.01481astro-ph.HE
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亮暴和慢暴--高能截止点位于$\lesssim100\;$ MeV的伽玛暴100724 B\& 160509 A

The Bright and the Slow -- GRBs 100724B \& 160509A with high-energy cutoffs at $\lesssim100\;$MeV

G. Vianello, R. Gill, J. Granot, N. Omodei, J. Cohen-Tanugi, F. Longo

摘要:

我们分析了GRB 100724 B和GRB 160509 A的瞬发辐射,这两个GRB是费米在$\lesssim{\rm MeV}$能量下观测到的最亮的伽马射线暴(GRB),但在$\gtrsim100\;$ MeV能量下却令人惊讶地微弱。时间分辨光谱揭示了GRB~ 100724 B和GRB~ 160509 A在能量$E_c\sim20-60\;$ MeV和$E_c\sim80-150\;$ MeV处的尖锐高能截止。我们首先描述唯象的截止和它的时间演变。然后,我们将数据拟合到两个模型中,其中高能截止来自源($τ_{γγ}$)内的固有不透明度到对产生:(i)来自Granot等人(2008)的内部冲击激发模型的带谱$τ_{γγ}$,以及(ii)Gill & Thompson(2014)的光球模型。对于截止的其他解释,例如发射电子能量分布中的固有截止,似乎不太自然。这两个模型都提供了与数据的良好拟合,具有非常合理的物理参数,提供了对$Γ\sim 100-400$范围内的体洛伦兹因子的直接估计,该范围位于费米伽玛暴中通常观察到的低端。令人惊讶的是,与其他费米-LAT伽玛暴相比,它们较低的截止能量$E_c$并不主要来自较低的洛伦兹因子,但也来自可变时间,亮度和高能光子指数的差异。最后,特别低的$E_c$值可能会阻止费米-LAT的检测,从而在费米-LAT GRB样本中引入相对于具有低洛伦兹因子或可变时间的GRB的偏差。

英文摘要:

We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest Gamma-Ray Bursts (GRBs) observed by Fermi at $\lesssim{\rm MeV}$ energies but surprisingly faint at $\gtrsim100\;$MeV energies. Time-resolved spectroscopy reveals a sharp high-energy cutoff at energies $E_c\sim20-60\;$MeV for GRB~100724B and $E_c\sim80-150\;$MeV for GRB~160509A. We first characterize phenomenologically the cutoff and its time evolution. We then fit the data to two models where the high-energy cutoff arises from intrinsic opacity to pair production within the source ($τ_{γγ}$): (i) a Band spectrum with $τ_{γγ}$ from the internal-shocks motivated model of Granot et al. (2008), and (ii) the photospheric model of Gill & Thompson (2014). Alternative explanations for the cutoff, such as an intrinsic cutoff in the emitting electron energy distribution, appear to be less natural. Both models provide a good fit to the data with very reasonable physical parameters, providing a direct estimate of bulk Lorentz factors in the range $Γ\sim 100-400$, on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies $E_c$ compared to other Fermi-LAT GRBs arise not predominantly from the lower Lorentz factors, but also at a comparable level from differences in variability time, luminosity, and high-energy photon index. Finally, particularly low $E_c$ values may prevent detection by Fermi-LAT, thus introducing a bias in the Fermi-LAT GRB sample against GRBs with low Lorentz factors or variability times.

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