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arXiv 2207.08397gr-qchep-th
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静止黑洞赤道类时圆轨道的拓扑

Topology of equatorial timelike circular orbits around stationary black holes

Shao-Wen Wei, Yu-Xiao Liu

摘要:

本文用拓扑方法成功地研究了一般黑洞背景下的光环和零圆轨道的性质。然而,对于赤道类时圆轨道,与光环的情况完全不同,它的半径与测试粒子的能量和角动量密切相关。这一事实似乎限制了拓扑处理扩展到类时圆轨道。本文证明了角动量不影响所构造的矢量的渐近行为,其零点表示赤道类时圆轨道。结果,一个良好的拓扑结构来描述赤道类时圆轨道可以构建。我们的研究表明,对于一般的黑洞,类时圆轨道的总拓扑数为零,它依赖于粒子的能量。重要的是,它揭示了如果类时圆轨道存在,它们总是成对出现,角动量固定。同时,稳定和不稳定的类时圆轨道具有正或负的缠绕数。特别令人感兴趣的是,边缘稳定的圆形轨道对应于构造向量的零点的分叉点。此外,我们还研究了粒子能量作为控制参数的情况。结果表明,当粒子能量为1时,会发生拓扑相变。低于这个值,类时圆轨道总是成对出现,能量固定。否则,我们将多一个不稳定的类时圆形轨道。我们进一步将这种处理方法应用于Kerr黑洞。在一般的黑洞背景下给出的所有结果都得到了精确的再现。这有力地表明,我们的拓扑方法可以推广到赤道类时圆轨道。

英文摘要:

A topological approach has been successfully used to study the properties of the light ring and the null circular orbit, in a generic black hole background. However, for the equatorial timelike circular orbit, quite different from the light ring case, its radius is closely dependent of the energy and angular momentum of a test particle. This fact seems to restrict the extension of the topological treatment to the timelike circular orbit. In this paper, we confirm that the angular momentum does not affect the asymptotic behavior of the constructed vector with its zero points denoting the equatorial timelike circular orbits. As a result, a well-behaved topology to characterize the equatorial timelike circular orbits can be constructed. Our study shows that the total topological number of the timelike circular orbits vanishes for a generic black hole, which is dependent of the energy of the particle. Significantly, it reveals that if there the timelike circular orbits exist, they always come in pairs for fixed angular momentum. Meanwhile, the stable and unstable timelike circular orbits have positive or negative winding number. Of particular interest is that the marginally stable circular orbit corresponds to the bifurcation point of the zero point of the constructed vector. Moreover, we also examine the case when the particle energy acts as the control parameter. It is shown that there will be topological phase transition when the value of the particle energy is one. Below this value, the timelike circular orbits always come in pairs for fixed energy. Otherwise, we will have one more unstable timelike circular orbit. We further apply the treatment to the Kerr black hole. All the results given in a generic black hole background are exactly reproduced. These strongly indicate that our topological approach can be generalized to the equatorial timelike circular orbits.

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