自由中子衰变在千新星抛射物中的X射线/紫外闪光：非热效应

中文摘要 AI 辅助

含中子星的致密天体并合可产生千新星，这是一种由快速中子俘获（r-过程）合成的物质放射性衰变驱动的暂现事件。在并合过程中，部分抛射物以高速（≳0.4c）被抛出，这可能导致中子逃脱种子核的俘获，从而产生自由中子。随后，自由中子衰变并注入额外能量，为早期（≲1天）辐射提供动力。在此，我们利用Sedona，展示了首个非局部热动平衡且频率相关的不透明度辐射转移模拟网格，以预测自由中子衰变产生的早期X射线/紫外/光学辐射，该网格涵盖了自由中子质量、与r-过程物质的混合程度以及高速抛射物尾部的理论预期范围。辐射可通过一个SED来表征，该SED在最初约几分钟内从约几×10^41-10^42 erg s^-1的X射线迅速转变为在约几分钟到几小时时间尺度上的远紫外和近紫外峰值，随后对于足够大的自由中子质量，光学和红外辐射增强。原则上，自由中子抛射物的性质在极紫外和远紫外波段最为可区分，SED峰值通量和波长由抛射物的最大速度和混合程度决定。在远紫外和近紫外波段，自由中子抛射物质量小至10^-7 M_⊙时，与无中子模型相比表现出独特的凸起，这证明了即将开展的紫外任务如UVEX和ULTRASAT在约束中子星并合核合成环境方面的重要性。

英文摘要

Neutron star-bearing compact-object mergers can create a kilonova, a transient powered by the radioactive decay of material synthesized via rapid neutron capture ($r$-process). During the merger, a fraction of the ejecta is launched at high speeds ($\gtrsim$ 0.4c) that can lead to neutrons evading capture onto seed nuclei, resulting in free neutrons. The free neutrons then decay and inject additional energy that power early-time ($\lesssim$1 day) emission. Here, using \texttt{Sedona}, we present a grid of the first non-local thermodynamic equilibrium and frequency-dependent opacity radiative transfer simulations to predict early X-ray/UV/optical emission from free neutron decay that span the expected theoretical range of free neutron mass, mixing with $r$-process material, and extent of the high-velocity ejecta tail. The emission can be characterized by an SED that rapidly shifts from X-rays of $\sim\rm{few} \times10^{41}-10^{42}$ erg s$^{-1}$ in the first $\sim$ minutes to a far-UV and near-UV peak on the timescale of $\sim$ minutes to hours, followed by enhanced optical and IR emission for sufficiently large free neutron masses. The properties of the free neutron ejecta are most distinguishable, in principle, at extreme-UV and far-UV wavelengths, with SED peak flux and wavelength determined by the maximum velocity of the ejecta and the mixing. In the far-UV and near-UV, free neutron ejecta masses as small as 10$^{-7}$ M$_\odot$ exhibit a unique bump compared to neutron-free models, demonstrating the importance of upcoming UV missions like UVEX and ULTRASAT to constraining the nucleosynthetic environment of neutron star mergers.

发表机构

• University of California, Berkeley（加州大学伯克利分校）
• Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
• University of Oxford（牛津大学）

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