Variability associated with α accretion disc theory for standard and advection-dominated discs
Variability associated with α accretion disc theory for standard and advection-dominated discs
复制标题
与标准盘和平流主导盘的 α 吸积盘理论相关的变异性
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发表时间：
1998
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影响因子：
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通讯作者：
E. Blackman
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作者：
E. Blackman
吸积盘的α湍流粘性形式必须被解释为平均场理论，只在比湍流更大的空间尺度或时间尺度上模拟稳态。尺度分离的程度决定了光度预测的相对精度误差（RPE）。湍流和α的使用意味着：（1）磁力线拉伸给出的磁压一般为总压的<$α^2/6，对于薄圆盘，α与压力比之间存在一对一的关系;（2）对于给定的观测持续时间和中心质量，平流主导吸积流（ADAF）中的大湍流尺度预测的L_v精度低于薄圆盘。频率ν处的允许变异性（或RPE）随着贡献区域的大小而增加。对于X射线双星ADAF，当R ≤ 1000 Schwarzchild半径（R_s）时，平均值超过1000 s时，RPE约为5%。然而，对于像NGC 4258和M87这样的星系，即使在R ≤ 100 R_S的情况下，目前的数据也给出了50- 100%的L_v RPE。需要更多的数据，但ADAF预测的系统偏差比随机偏差更显着，可能会限制湍流，吸积模式，稳定状态或吸积率的假设的属性。
The α turbulent viscosity formalism for accretion discs must be interpreted as a mean field theory, modelling a steady state only on spatial or time-scales greater than those of the turbulence. The extent of the scale separation determines the relative precision error (RPE) of the predicted luminosity L_ν. Turbulence and the use of α implies that (1) field line stretching gives a magnetic pressure ≳ α^2/6 of the total pressure generally, and a one-to-one relation between α and the pressure ratio for thin discs, and (2) large turbulent scales in advection-dominated accretion flows (ADAFs) predict a lower L_ν precision than thin discs for a given observation duration and central mass. The allowed variability (or RPE) at frequency ν increases with the size of the contributing region. For X-ray binary ADAFs, the RPE ∼5 per cent at R ≤ 1000 Schwarzchild radii (R_s) for averages over ≳1000 s. However, current data for galaxies like NGC 4258 and M87 give RPEs in L_ν of 50–100 per cent even at R ≤ 100 R_S. More data are required, but systematic deviations from ADAF predictions are more significant than random deviations, and may constrain properties of the turbulence, the accretion mode, the assumption of a steady state or the accretion rate.