Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars
Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars
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DOI：
10.1093/mnras/stu247
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发表时间：
2013-11
影响因子：
4.8
通讯作者：
B. Metzger;A. Piro
中科院分区：
文献类型：
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作者：
B. Metzger;A. Piro
中子双星（NS）的合并在某些情况下可能会产生一个稳定的大质量NS残留物，而不是黑洞。由于双星有相当大的角动量，这样的残余物诞生时会快速旋转，并可能获得强磁场（“毫秒磁星”）。磁星的旋转能量中有很大一部分来自磁星，而磁星的旋转能量是在合并过程中喷射出的少量物质的后面。如果磁星外流确实被困在喷出物后面（而不是将其大部分能量放入准直喷流），这有可能产生明亮的瞬变，这可能有助于确定合并中是否形成了NS或黑洞。我们研究了这种事件的预期特征，包括首次发现毫秒磁星注入周围星云的e^±对的重要影响。这些对通过同步加速器和逆康普顿发射冷却，产生对级联和硬X射线光谱。这些X射线中的一部分被喷出物壁吸收，并以热辐射的形式重新发射，导致光学/紫外瞬态峰值在几小时到几天的时间尺度上达到10^（43）-10 ^（44）erg s^（-1）的亮度。这比简单的分析模型预测的要暗，因为星云中的e^±对的光学深度很大，抑制了磁星自旋下降光度被热化的效率。然而，光学/紫外线发射比放射性供电的“千诺瓦”亮两个数量级以上。在某些情况下，星云X射线的亮度足以使喷出物重新发光，在这种情况下，非热X射线以与光辐射相似的峰值亮度和时间尺度无衰减地逸出喷出物。我们讨论了我们的结果的影响，时间延长的X射线发射，观察到以下一些短伽玛射线暴（GRB），包括kilonova候选人GRB 080503和GRB 130603 B。
The coalescence of binary neutron stars (NSs) may in some cases produce a stable massive NS remnant rather than a black hole. Due to the substantial angular momentum from the binary, such a remnant is born rapidly rotating and likely acquires a strong magnetic field (a ‘millisecond magnetar’). Magnetic spin-down deposits a large fraction of the rotational energy from the magnetar behind the small quantity of mass ejected during the merger. If the magnetar outflow is indeed trapped behind the ejecta (instead of placing most of its energy into a collimated jet), this has the potential for creating a bright transient that could be useful for determining whether an NS or black hole was formed in the merger. We investigate the expected signature of such an event, including for the first time the important impact of e^± pairs injected by the millisecond magnetar into the surrounding nebula. These pairs cool via synchrotron and inverse Compton emission, producing a pair cascade and hard X-ray spectrum. A fraction of these X-rays are absorbed by the ejecta walls and re-emitted as thermal radiation, leading to an optical/UV transient peaking at a luminosity of ∼10^(43)–10^(44) erg s^(−1) on a time-scale of several hours to days. This is dimmer than predicted by simpler analytic models because the large optical depth of e^± pairs across the nebula suppresses the efficiency with which the magnetar spin-down luminosity is thermalized. Nevertheless, the optical/UV emission is more than two orders of magnitude brighter than a radioactively powered ‘kilonova’. In some cases, nebular X-rays are sufficiently luminous to re-ionize the ejecta, in which case non-thermal X-rays escape the ejecta unattenuated with a similar peak luminosity and time-scale as the optical radiation. We discuss the implications of our results for the temporally extended X-ray emission that is observed to follow some short gamma-ray bursts (GRBs), including the kilonova candidates GRB 080503 and GRB 130603B.