A radiation-hydrodynamic model of accretion columns for ultra-luminous X-ray pulsars
A radiation-hydrodynamic model of accretion columns for ultra-luminous X-ray pulsars
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DOI：
10.1093/pasj/psw075
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发表时间：
2016-08
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影响因子：
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通讯作者：
T. Kawashima;S. Mineshige;K. Ohsuga;Takumi Ogawa
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作者：
T. Kawashima;S. Mineshige;K. Ohsuga;Takumi Ogawa
基于最近发现的超亮X射线源M82 X-2（“ULX脉冲星”）的脉冲辐射，我们对超临界吸积流通过狭窄的吸积柱到达中子星星进行了二维辐射流体动力学模拟。在中子星星上方设置一个填充密度为10 ^{-4} {\rmg}~ {\rmcm}^{-3}$的稀薄气体的圆锥形吸积柱，求解吸积柱内气体的二维运动和辐射输运。侧边界被设置为使得辐射可以自由逸出，而气体不能。由于初始气体层不处于流体静力平衡，柱状气体福尔斯落在中子星表面，从而产生激波。因此，正如Basko \& Sunyaev（1976年）所指出的那样，吸积柱由两个区域组成：一个接近自由下落的上部区域和一个较低的沉降区域。平均吸积率非常高，为10^{2-3} L_{\rm E}/c^2$（其中L_{\rm E}$为爱丁顿光度），因此辐射能在整个柱内占主导地位。尽管有很高的吸积率，但在沉降区距离r$处，实验室框架中的辐射通量几乎保持在L_{\rm E}/（4\pi r^2）$以下，因此物质可以缓慢地吸积。这种调整是可能的，因为大量的光子通过物质的动能耗散产生的可以通过侧边界逃逸。总的光度可以大大超过$L_{\rm E}$几个数量级，而从柱的顶部观察到的视光度要小得多。由于这种高度各向异性的辐射场，观测到的通量应该表现出随旋转周期的周期性变化，前提是旋转轴和磁轴不重合。
Prompted by the recent discovery of pulsed emission from an ultra-luminous X-ray source, M82 X-2 ("ULX-pulsar"), we perform a two-dimensional radiation-hydrodynamic simulation of a super-critical accretion flow onto a neutron star through a narrow accretion column. We set an accretion column with a cone shape filled with tenuous gas with density of $10^{-4} {\rm g}~ {\rm cm}^{-3}$ above a neutron star and solve the two dimensional gas motion and radiative transfer within the column. The side boundaries are set such that radiation can freely escape, while gas cannot. Since the initial gas layer is not in a hydrostatic balance, the column gas falls onto the neutron-star surface, thereby a shock being generated. As a result, the accretion column is composed of two regions: an upper, nearly free-fall region and a lower settling region, as was noted by Basko \& Sunyaev (1976). The average accretion rate is very high; ${\dot M}\sim 10^{2-3} L_{\rm E}/c^2$ (with $L_{\rm E}$ being the Eddington luminosity), and so radiation energy dominates over gas internal energy entirely within the column. Despite the high accretion rate, the radiation flux in the laboratory frame is kept barely below $L_{\rm E}/(4\pi r^2)$ at a distance $r$ in the settling region so that matter can slowly accrete. This adjustment is made possible, since large amount of photons produced via dissipation of kinetic energy of matter can escape through the side boundaries. The total luminosity can greatly exceed $L_{\rm E}$ by several orders of magnitude, whereas the apparent luminosity observed from the top of the column is much less. Due to such highly anisotropic radiation fields, observed flux should exhibit periodic variations with the rotation period, provided that the rotation and magnetic axes are misaligned.