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arXiv 1411.2015astro-ph.HEastro-ph.SRgr-qc
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“时间反转”方案中的短伽马射线暴

Short gamma-ray bursts in the "time-reversal" scenario

Riccardo Ciolfi, Daniel M. Siegel

摘要:

短伽马射线暴(SGRB)是宇宙中最明亮的爆炸之一,其起源仍然不确定。观测证据支持与中子星或中子星-黑洞(NS-BH)双星合并的关联。领先的模型与SGRB的相对论性喷流推出的BH-环面系统合并产生的。然而,最近的观测显示,大部分SGRB事件伴随着X射线余辉的持续时间$\sim10^2\!-\!10^5~\mathrm{s}$,这表明连续的能量注入从一个长寿命的中央引擎,这是不兼容的短($\lesssim1~\mathrm{s}$)吸积时间尺度的BH-环面系统。超大质量NS的形成,在更长的自旋下降时间尺度上抵抗坍缩,可以解释这些余辉持续时间,但留下了严重的疑问,相对论性喷流是否可以在合并时发射。在这里,我们提出了一个新的情况下,容纳这两个方面,在SGRB的超大质量NS崩溃后产生的。早期的差异旋转和随后的自旋下降辐射在NS周围产生了一个光学厚的环境,由一个光子对星云和一个载有重子的喷出物外壳组成。虽然射流很容易穿过这种环境,但自旋下降辐射在更长的时间尺度上向外扩散,并积累了一个延迟,使SGRB能够在持久的X射线信号之前(部分)被观察到。通过分析扩散时间尺度的广泛的物理参数,我们发现延迟,一般可以达到$\sim10^5~\mathrm{s}$,与观测兼容。这一基本测试的成功使这种“时间反转”情景成为当前SGRB模型的一个有吸引力的替代方案。

英文摘要:

Short gamma-ray bursts (SGRBs) are among the most luminous explosions in the Universe and their origin still remains uncertain. Observational evidence favors the association with binary neutron star or neutron star-black hole (NS-BH) binary mergers. Leading models relate SGRBs to a relativistic jet launched by the BH-torus system resulting from the merger. However, recent observations have revealed a large fraction of SGRB events accompanied by X-ray afterglows with durations $\sim10^2\!-\!10^5~\mathrm{s}$, suggesting continuous energy injection from a long-lived central engine, which is incompatible with the short ($\lesssim1~\mathrm{s}$) accretion timescale of a BH-torus system. The formation of a supramassive NS, resisting the collapse on much longer spin-down timescales, can explain these afterglow durations, but leaves serious doubts on whether a relativistic jet can be launched at merger. Here we present a novel scenario accommodating both aspects, where the SGRB is produced after the collapse of a supramassive NS. Early differential rotation and subsequent spin-down emission generate an optically thick environment around the NS consisting of a photon-pair nebula and an outer shell of baryon-loaded ejecta. While the jet easily drills through this environment, spin-down radiation diffuses outwards on much longer timescales and accumulates a delay that allows the SGRB to be observed before (part of) the long-lasting X-ray signal. By analyzing diffusion timescales for a wide range of physical parameters, we find delays that can generally reach $\sim10^5~\mathrm{s}$, compatible with observations. The success of this fundamental test makes this "time-reversal" scenario an attractive alternative to current SGRB models.

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