Relativistic Envelopes and Gamma-Rays from Neutron Star Mergers
Relativistic Envelopes and Gamma-Rays from Neutron Star Mergers
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DOI：
10.3847/1538-4357/ab86a0
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发表时间：
2018-12
期刊：
影响因子：
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通讯作者：
A. Beloborodov;C. Lundman;Y. Levin
中科院分区：
文献类型：
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作者：
A. Beloborodov;C. Lundman;Y. Levin
我们认为，中子星合并抛出了质量为m∼10−7 M⊙的极端相对论包层，这有助于解释GW170817的伽马射线暴。一种抛射机制是中子星表面被中微子的爆发烧蚀，在前30μ的合并中S。另一种更有效的膨胀极端相对论包络的机制是合并产生的大规模喷出物中的内部冲击。如果合并产品是一颗磁星，它会发出离心力加速的风，预计会产生强烈的冲击。激波通过抛射物向外传播，并在半径为∼10910-1010厘米的外层加速，发射一个充满每个核子∼104个光子的极端相对论不透明包层。包层的洛伦兹因子分布向外上升，决定了包层的同源扩展，从而绝热地冷却了被捕获的光子。一旦磁星失去差动自转并塌缩成黑洞，就会形成一股强大的喷流。它驱使冲击波进入包层，追逐其外层，最终赶上r∼1012厘米处的包层光球层。延迟爆炸波的超相对论光球爆发在合并轴周围的宽广立体角度上发射出伽马射线暴。这个模型解释了GW170817与光度Lγ∼1047erg S−1、持续时间和特征光子能量∼100kev合并产生的伽马射线脉冲。包络光球处的冲击波洛伦兹因子与我们从观测到的爆发光曲线得到的Γ≳5是一致的。我们建议对该模型进行未来的测试。
We suggest that neutron star mergers eject an ultrarelativistic envelope of mass m ∼ 10−7 M⊙, which helps explain the gamma-ray burst from GW170817. One ejection mechanism is the ablation of the neutron star surface by the burst of neutrinos in the first 30 μs of the merger. Another, more efficient mechanism for inflating the ultrarelativistic envelope is an internal shock in the massive ejecta from the merger. A strong shock is expected if the merger product is a magnetar, which emits a centrifugally accelerated wind. The shock propagates outward through the ejecta and accelerates in its outer layers at radii r ∼ 109–1010 cm, launching an ultrarelativistic opaque envelope filled with ∼104 photons per nucleon. The Lorentz factor profile of the envelope rises outward and determines its homologous expansion, which adiabatically cools the trapped photons. Once the magnetar loses its differential rotation and collapses into a black hole, a powerful jet forms. It drives a blast wave into the envelope, chasing its outer layers, and eventually catching up with the envelope photosphere at r ∼ 1012 cm. The ultrarelativistic photospheric breakout of the delayed blast wave emits a gamma-ray burst in a broad solid angle around the merger axis. This model explains the gamma-ray pulse from merger GW170817 with luminosity Lγ ∼ 1047 erg s−1, duration , and characteristic photon energy ∼100 keV. The blast-wave Lorentz factor at the envelope photosphere is consistent with Γ ≳ 5, which we derive from the observed light curve of the burst. We suggest future tests of the model.