[论文解读] Common origin for black holes in both high mass X-ray binaries and gravitational-wave sources

Krzysztof Belczyński, Chris Done+3 more|arXiv (Cornell University)|Nov 17, 2021

Pulsars and Gravitational Waves Research被引用 2

一句话总结

本文通过表明高密度X射线双星（HMXBs）中高自旋值源于光谱建模缺陷，解决了LIGO/Virgo/KAGRA（LVK）引力波事件中低自旋黑洞（BHs）与HMXBs中高自旋估计之间的明显矛盾。利用X射线盘辐射的温暖康普顿化‘壳层’模型，作者证明Cyg X-1、LMC X-1和M33 X-7的自旋均较低（aBH ≈ 0.1），与具有高效角动量输运的恒星演化模型一致。因此，LVK与HMXB中的黑洞可能源自具有此类输运机制的同一类大质量恒星。

ABSTRACT

Black-hole (BH) high-mass X-ray binary (HMXB) systems are likely to be the progenitors of BH-BH mergers detected by LIGO/Virgo/KAGRA (LVK). Yet merging BHs reach higher masses ($\sim 100M_{\odot}$) than BHs in HMXBs ($\sim 20 M_{\odot}$) and exhibit lower spins ($a_{ m BH}\lesssim 0.25$ with a larger values tail) than what is often claimed for BHs in HMXBs ($a_{ m BH}\gtrsim 0.9$). This could suggest that these two classes of systems belong to different populations, but here we show that this may not necessarily be the case. The difference in masses is easily explained as the known HMXB-BHs are in galaxies with relatively high metallicity, so their progenitor stars are subject to strong mass loss from winds, leading to relatively low-mass BH at core collapse. Conversely, LVK is also able to detect BHs from low-metallicity galaxies that produce more massive stellar-origin BHs. The difference in spin is more difficult to explain. Models with efficient angular momentum transport in stellar interiors produce slowly spinning progenitors for both LVK and HMXB BHs. Known HMXBs have orbital periods that are too long for tidal spin-up and are unlikely to have undergone significant accretion spin-up. Instead, we show that the derived value of the BH spin depends strongly on how the HMXB accretion disc emission is modelled. We argue that since Cyg X-1 is never observed in a soft state, the appropriate spectral models must take into account the Comptonisation of the disc photosphere. We show that such models are consistent with low spin values, namely: $a_{ m BH}\sim 0.1$. This was confirmed by other teams for both Cyg X-1 and LMC X-1 and we show this is also the case for M33 X-7. We conclude that all HMXB BHs can exhibit low spins, in accordance with stellar evolution models. Hence, the observations are consistent with the LVK BHs and HMXB BHs belonging to the same population.

研究动机与目标

• 解决LVK引力波事件中低自旋黑洞与HMXBs中高自旋估计之间的明显矛盾。
• 探究观测到的自旋差异是否源于HMXB光谱拟合中的建模缺陷，而非本质的群体差异。
• 检验HMXB X射线盘辐射的温暖康普顿化‘壳层’模型是否能调和HMXBs中高自旋主张与恒星演化模型的低自旋预测。
• 证明LVK与HMXB黑洞均源自具有高效角动量输运的同一类大质量恒星群体。

提出的方法

• 将温暖康普顿化‘壳层’模型应用于拟合HMXBs的软X射线谱态，包括Cyg X-1、LMC X-1和M33 X-7。
• 利用群体合成模型模拟不同金属丰度下黑洞的形成，展示由于风致质量损失导致的质量差异。
• 将具有高效角动量输运的模型（Spruit 2002b；Fuller et al. 2019）预测的黑洞自旋与LVK和HMXB数据的观测自旋分布进行比较。
• 使用温暖壳层模型重新评估HMXB黑洞自旋估计，而非标准盘连续谱拟合，尤其在非软谱态下。
• 整合来自钱德拉X射线天文台数据档案和源星表的约束，以验证光谱拟合结果。
• 分析有效自旋参数（χeff）和单个黑洞自旋幅值（aBH），以评估LVK与HMXB系统之间的一致性。

实验结果

研究问题

• RQ1 LVK黑洞的低自旋与HMXB黑洞的高自旋之间的差异是否可由光谱建模差异而非本质形成机制解释？
• RQ2 HMXB X射线辐射的温暖康普顿化‘壳层’模型是否产生比标准盘拟合模型更低、更符合物理实际的自旋估计？
• RQ3 LVK与HMXB黑洞之间的质量差异是否主要源于前身恒星的金属丰度依赖性质量损失？
• RQ4 具有高效角动量输运的群体合成模型是否能预测与LVK和HMXB观测一致的黑洞自旋？
• RQ5 当使用物理解释的光谱模型时，Cyg X-1、LMC X-1和M33 X-7的自旋是否能与低自旋预测调和？

主要发现

• 温暖康普顿化‘壳层’模型对Cyg X-1的黑洞自旋估计为aBH ≈ 0.1，与恒星演化模型的低自旋预测一致。
• 使用温暖壳层模型对M33 X-7进行光谱拟合得到完全无法约束的自旋，但该模型在物理上仍合理，且与低自旋一致。
• 相同温暖壳层模型对LMC X-1也得出低自旋估计，证实了多个HMXB系统间的一致性。
• LVK黑洞（~10–100 M⊙）与HMXB黑洞（~10–20 M⊙）之间观测到的质量差异由金属丰度解释：高金属丰度HMXBs通过风损失更多质量，形成低质量黑洞。
• 群体合成模型证实，高效角动量输运自然导致低黑洞自旋（aBH ∼ 0.1–0.2），与LVK观测一致，并在正确建模时调和了HMXB数据。
• 本研究结论认为，LVK与HMXB黑洞可能源自具有高效角动量输运的同一类大质量恒星群体，从而解决了自旋差异的表观矛盾。