On the Origin of Black Hole Spin in High-mass X-Ray Binaries
On the Origin of Black Hole Spin in High-mass X-Ray Binaries
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DOI：
10.3847/2041-8213/aaf97b
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发表时间：
2018-10
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影响因子：
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通讯作者：
Y. Qin;P. Marchant;T. Fragos;G. Meynet;V. Kalogera
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作者：
Y. Qin;P. Marchant;T. Fragos;G. Meynet;V. Kalogera
低质量 X 射线双星中的黑洞 (BH) 自旋涵盖了一系列值，这些值可以通过 BH 诞生后的吸积来解释。相比之下，高质量 X 射线双星 (HMXB) 中的三个 BH 自旋测量仅显示接近最大值的值，并且可能具有与 BH 恒星祖先相关的不同起源。我们在这里探讨了两种可能的情况来解释 HMXB 中 BH 的高自旋：在主序列期间经历传质（MT）的双星（MS；案例-A MT）中的形成，以及经历化学均质演化（CHE）的非常接近的双星。我们发现，如果祖星在 MS 演化后的内部角动量 (AM) 传输不太强（即恒星核心与其包层之间的弱耦合），这两种情况都能够产生高自旋 BH。相反，如果假设 AM 传输有效，我们会发现所得的 BH 自旋相对于观测结果总是太低。 Case-A MT 模型很好地拟合了 BH 自旋、两个组件的质量以及测量自旋的 HMXB 中三个 BH 中的两个的最终轨道周期。对于其中之一，预测的 BH 伴星质量明显低于观测到的质量，但这在很大程度上取决于假设的 MT 效率。 CHE 模型预测的轨道周期对于此处考虑的所有三种情况都太大。我们预计案例 A MT 在涉及的金属丰度上会更加频繁，因此我们得出结论，案例 A MT 场景是首选。最后，我们预测通过 Case-A MT 形成的 HMXB 的恒星伴星具有增强的氮表面丰度，这可以通过未来的观测进行测试。
Black hole (BH) spins in low-mass X-ray binaries cover a range of values that can be explained by accretion after BH birth. In contrast, the three BH spin measurements in high-mass X-ray binaries (HMXBs) show only values near the maximum and likely have a different origin connected to the BH stellar progenitor. We explore here two possible scenarios to explain the high spins of BHs in HMXBs: formation in binaries that undergo mass transfer (MT) during the main sequence (MS; Case-A MT), and very close binaries undergoing chemically homogeneous evolution (CHE). We find that both scenarios are able to produce high-spin BHs if internal angular momentum (AM) transport in the progenitor star after its MS evolution is not too strong (i.e., weak coupling between the stellar core and its envelope). If instead efficient AM transport is assumed, we find that the resulting BH spins are always too low with respect to observations. The Case-A MT model provides a good fit for the BH spins, the masses of the two components, and the final orbital periods for two of the three BHs in HMXBs with measured spins. For one of them, the mass predicted for the BH companion is significantly lower than observed, but this depends strongly on the assumed efficiency of MT. The CHE models predict orbital periods that are too large for all three cases considered here. We expect the Case-A MT to be much more frequent at the metallicities involved, so we conclude that the Case-A MT scenario is preferred. Finally, we predict that the stellar companions of HMXBs formed through the Case-A MT have enhanced nitrogen surface abundances, which can be tested by future observations.