Using a set of general-relativistic magnetohydrodynamics simulations that include proper neutrino transfer, we assess for the first time the role played by the fallback accretion onto the remnant from a binary neutron star merger over a timescale of hundreds of seconds. In particular, we find that, independently of the equation of state, the properties of the binary, and the fate of the remnant, the fallback material reaches a total mass of ≳10⁻³ M _⊙, i.e., about 50% of the unbound matter, and that the fallback accretion rate follows a power law in time with slope ∼t ^−5/3. Interestingly, the timescale of the fallback and the corresponding accretion luminosity are in good agreement with the so-called “extended emission” observed in short gamma-ray bursts (GRBs). Using a simple electromagnetic emission model based on the self-consistent thermodynamical state of the fallback material heated by r-process nucleosynthesis, we show that this fallback material can shine in gamma and X-rays with luminosities ≳10⁴⁸ erg s⁻¹ for hundreds of seconds, thus making it a good and natural candidate to explain the extended emission in short GRBs. Additionally, our model for fallback emission reproduces well and rather naturally some of the phenomenological traits of extended emission, such as its softer spectra with respect to the prompt emission and the presence of exponential cutoffs in time. Our results clearly highlight that fallback flows onto merger remnants cannot be neglected, and the corresponding emission represents a very promising and largely unexplored avenue to explain the complex phenomenology of GRBs.

中文翻译：

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从后备吸积到合并残余物的“扩展排放”

使用一组包括适当中微子转移的广义相对论磁流体动力学模拟，我们首次评估了在数百秒的时间尺度内双中子星合并残余物的后退吸积所发挥的作用。特别是，我们发现，与状态方程、双星的性质以及残余物的命运无关，后备材料的总质量达到 ≳10 ⁻³ 中号 _⊙，即约 50% 的未束缚物质，并且后退吸积率遵循斜率 ∼ 的幂律t ^−5/3。有趣的是，回退的时间尺度和相应的吸积光度与短伽马射线暴（GRB）中观察到的所谓“扩展发射”非常一致。使用基于后备材料加热的自洽热力学状态的简单电磁发射模型r-过程核合成，我们证明这种后备材料可以在伽马射线和X射线中发光，光度≳10 ⁴⁸ erg s ^-1持续数百秒，从而使其成为解释短GRB中延长发射的良好且自然的候选者。此外，我们的后退发射模型很好且相当自然地再现了扩展发射的一些现象学特征，例如相对于即时发射的更柔和的光谱以及及时指数截止的存在。我们的结果清楚地表明，合并残余物的后退流不可忽视，相应的排放代表了解释伽玛暴复杂现象学的一个非常有前途且很大程度上尚未探索的途径。