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arXiv 2209.07264astro-ph.HE
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M87喷流速度剖面的含义:缓慢旋转的黑洞磁层的可能性

Implications from the velocity profile of the M87 jet: a possibility of a slowly rotating black hole magnetosphere

Motoki Kino, Masaaki Takahashi, Tomohisa Kawashima, Jongho Park, Kazuhiro Hada, Hyunwook Ro, Yuzhu Cui

摘要:

Park等人使用KVN和VERA阵列(KaVA)测量的M87喷流的速度剖面表明,喷流加速的起始位置比广义相对论磁流体动力学模拟中预测的更远离喷流的中央发动机,我们探索如何减轻模拟和KaVA观测之间的明显差异。我们使用Tomimatsu和Takahashi提出的半解析射流模型。一致地求解跨磁场结构,但忽略任何耗散效应。通过与实测的M87喷流速度剖面的比较,发现该模型能再现速度剖面的对数特征,并且在选取$c/(100r_{g}) \lesssim Ω_{F} \lesssim c/(70r_{g})$($r_{g}$为引力半径)时能很好地拟合观测数据。虽然喷流的总比能(${\cal E}$)改变喷流的终端体洛仑兹因子,但黑洞磁层(漏斗区域)($Ω_{F}$)的较慢角速度使光柱半径($r_{\rm lc}$)变大,并且其因此推出喷流加速的起始点的位置。利用估算的$Ω_{F}$,假设Blandford-Znajek(BZ)过程起作用,进一步估算了M87事件视界尺度上的磁场强度。穿过M87事件视界的相应磁通量与磁制动盘(MAD)机制吻合得很好。

英文摘要:

Motivated by the measured velocity profile of the M87 jet using the KVN and VERA Array (KaVA) by Park et al. indicating that the starting position of the jet acceleration is farther from the central engine of the jet than predicted in general relativistic magnetohydrodynamic simulations, we explore how to mitigate the apparent discrepancy between the simulations and the KaVA observation. We use a semi-analytic jet model proposed by Tomimatsu and Takahashi. consistently solving the trans-magnetic field structure but neglecting any dissipation effects. By comparing the jet model with the observed M87 jet velocity profile, we find that the model can reproduce the logarithmic feature of the velocity profile, and can fit the observed data when choosing $c/(100r_{g}) \lesssim Ω_{F} \lesssim c/(70r_{g})$ where $r_{g}$ is the gravitational radius. While a total specific energy (${\cal E}$) of the jet changes the terminal bulk Lorentz factor of the jet, a slower angular velocity of the black hole magnetosphere (funnel region) ($Ω_{F}$) makes a light-cylinder radius ($r_{\rm lc}$) larger and it consequently pushes out a location of a starting point of the jet acceleration. Using the estimated $Ω_{F}$ we further estimate the magnetic field strength on the event horizon scale in M87 by assuming Blandford-Znajek (BZ) process is in action. The corresponding magnetic flux threading the event horizon of M87 is in good agreement with a magnetically arrested disc (MAD) regime.

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