[论文解读] A magnetic accretion disk-outflow model for state transition in X-ray binaries

Xinwu Cao, Bei You+1 more|arXiv (Cornell University)|Aug 20, 2021

Astrophysical Phenomena and Observations参考文献 72被引用 10

一句话总结

本文提出了一种磁性吸积盘-喷流模型，用以解释X射线双星中硬X射线状态转换光度与软X射线峰值光度之间的紧密相关性。该模型将此相关性归因于外盘中通过逆级联产生的大尺度磁场所驱动的磁性喷流，这些喷流增强了角动量损失，并提高了ADAF抑制的临界吸积率，其中中子星X射线双星（XRBs）所需的黏滞参数（α ∼0.15–0.4）高于黑洞X射线双星（α ∼0.05–0.15）。

ABSTRACT

The hard to soft state transition of the outbursts in X-ray binaries (XRBs) is triggered by the rising of the mass accretion rate due to the disk instability. In order to explain the observed correlation between the hard X-ray transition luminosity and the soft X-ray peak luminosity in the soft state, we construct a magnetic disk-outflow model for the state transition in XRBs. We assume that the large-scale magnetic field in the outer thin disk is formed through inverse cascade of small-scale dynamo generated field, and it is then advected by the inner advection dominated accretion flow (ADAF), which accelerates a fraction of the gas into the outflows. During the outbursts, the heating front moves inwards, and the field strength at the heating front of the outer disk is proportional to the accretion rate of the disk. Much angular momentum of the inner ADAF is carried away by the outflows for a stronger magnetic field, which leads to a high radial velocity of the ADAF. This makes the critical mass accretion rate of the ADAF increases with the field strength, and it therefore leads to a correlation between transition luminosity and the peak luminosity in the thermal state. We found that the values of the viscosity parameter $\alpha$ of the neutron star XRBs are systematically higher for those of the black hole (BH) XRBs ($\alpha\sim 0.05-0.15$ for BHs, and $\alpha\sim 0.15-0.4$ for neutron stars). Our model predicts the transition luminosity may be higher than the peak luminosity provided $\alpha$ is sufficiently high, which is able to explain a substantial fraction of outbursts in BHXRBs not reaching the thermally dominant accretion state.

研究动机与目标

• 解释X射线双星爆发中观测到的硬X射线状态转换光度与软X射线峰值光度之间的紧密相关性。
• 解决理论预测与观测之间的差异，即状态转换的临界吸积率变化幅度超出预期。
• 解释中子星与黑洞X射线双星在黏滞参数（α）上的系统性差异。
• 解释为何部分黑洞X射线双星爆发未能达到热主导状态。

提出的方法

• 假设大尺度磁场通过外薄盘中小尺度发电机磁场的逆级联形成。
• 建立磁性喷流模型，从内层ADAF中携带角动量，从而增加其径向速度。
• 计算ADAF抑制的临界质量吸积率作为磁场强度和黏滞参数α的函数。
• 采用具有磁驱动喷流的稳态ADAF模型，转换半径为rtr ∼20–30。
• 基于观测约束和ISCO附近能量释放，采用rtr ∼20–30。
• 利用中子星XRBs和黑洞XRBs的数据，将模型预测与观测到的λtr与λpeak相关性进行比较。

实验结果

研究问题

• RQ1 为何X射线双星爆发中硬X射线状态转换光度与软X射线峰值光度紧密相关？
• RQ2 何种物理机制可在不假设普遍临界吸积率的前提下解释观测到的相关性？
• RQ3 为何在状态转换背景下，中子星X射线双星所需的黏滞参数（α）高于黑洞X射线双星？
• RQ4 该模型如何解释未能达到热主导状态的爆发？
• RQ5 磁场输运在改变ADAF抑制临界吸积率方面起何种作用？

主要发现

• 该模型通过将相关性与磁场强度联系起来解释了观测到的λtr–λpeak相关性，而磁场强度随外盘吸积率增加而增强。
• 磁性喷流提高了ADAF抑制的临界质量吸积率，从而提升了转换光度。
• 中子星X射线双星所需的α值（α ∼0.15–0.4）高于黑洞X射线双星（α ∼0.05–0.15），与观测一致。
• 该模型预测当α足够高时，转换光度可超过峰值光度，从而解释黑洞X射线双星中非热爆发现象。
• ADAF中的磁场输运使磁场沿径向向内增强，从而在整个流体中提高临界吸积率。
• 该模型与观测到的爆发 timescales 一致，因为α−1决定了吸积 timescale，与黑洞X射线双星中较长的上升/衰减时间相符。