磁遍历星、喷流形成和伽马射线暴：遍历区与视界
Magnetic Ergostars, Jet Formation and Gamma-Ray Bursts: Ergoregions versus Horizons
摘要：
我们进行了第一次完全广义相对论，磁流体动力学模拟的动态稳定的超大质量中子星与没有ergoregions，以评估ergoregions对发射磁驱动的外流的影响。超大质量中子星是由一个可压缩的因果状态方程模拟的，并且最初被赋予了一个从恒星内部延伸到外部的偶极磁场。我们发现，经过几个阿尔文时间后，能星（包含能区的恒星）和正常恒星中的磁场线都被紧紧地缠绕成螺旋漏斗，物质在其中开始向外流动。流出物中的最大洛伦兹因子为$Γ_L \sim 2.5$，而无力参数保持在$B^2/8πρ_0\lesssim 10$。这些值与高度相对论性的、磁驱动的外流（喷流）和短$γ$射线爆发不相容。我们将这些结果与被磁化的无质量吸积盘包围的旋转黑洞的结果进行了比较，吸积盘发射出真正的喷流。我们的模拟表明，用于发射相对论性喷流的布兰德福德-兹纳杰克机制仅在存在黑洞时起作用，尽管在所有情况下的坡印廷光度都是可比的。因此，仅仅根据观测到的坡印廷光度值，我们无法区分一个磁化的吸积黑洞和一个所谓质量隙中的磁化超大质量中子星。这些结果补充了我们以前对超大质量遗迹的研究，并表明无论是正常的中子星还是超大质量状态的麦角星，都很难成为短$γ$射线爆发的祖先。
英文摘要：
We perform the first fully general relativistic, magnetohydrodynamic simulations of dynamically stable hypermassive neutron stars with and without ergoregions to assess the impact of ergoregions on launching magnetically--driven outflows. The hypermassive neutron stars are modeled by a compressible and causal equation of state and are initially endowed with a dipolar magnetic field extending from the stellar interior into its exterior. We find that, after a few Alfvén times, magnetic field lines in the ergostar (star that contains ergoregions) and the normal star have been tightly wound in both cases into a helical funnel within which matter begins to flow outward. The maximum Lorentz factor in the outflow is $Γ_L \sim 2.5$, while the force-free parameter holds at $B^2/8πρ_0\lesssim 10$. These values are incompatible with highly relativistic, magnetically-driven outflows (jets) and short $γ$-ray bursts. We compare these results with those of a spinning black hole surrounded by a magnetized, massless accretion disk that launches a bona fide jet. Our simulations suggest that the Blandford-Znajek mechanism for launching relativistic jets only operates when a black hole is present, though the Poynting luminosity in all cases is comparable. Therefore, one cannot distinguish a magnetized, accreting black hole from a magnetized hypermassive neutron star in the so-called mass-gap based solely on the value of the observed Poynting luminosity. These results complement our previous studies of supramassive remnants and suggest that it would be challenging for either normal neutron stars or ergostars in a hypermassive state to be the progenitors of short $γ$-ray bursts.
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