黑洞吸积的双模式特征

黑洞吸积必定是跨声速的。对于静态、绝热吸积流,比能量E、比角动量L和质量吸积率M都是空间的常量。跨声速解的非奇异条件,F(E,L,M)=0,使独立参数减为只有两个。对于一对给定的E和L的符合条件E_c>E>E_(Barr)的值(这里E_c是一临界值,E_(Barr)是引力和离心力的联合势垒),上述非奇异条件给出两个不同的吸积率值,对应着两个不同的吸积流声速点位置。然而,物理上合理的整体解却是唯一的,它总是使两个吸积率值中之较小者得到实现。 对于一个不转动的黑洞,吸积以两种模式之一进行。一是类球吸积或称Bondi吸积,角动量的影响和相对论效应均微不足道;另一是盘吸积,这两个因素起决定性作用。两种模式之间的转换是基于声速点位置的间断性跳跃,而这种跳跃是由吸积流参数(例如角动量)的连续变化所引发。Bondi吸积可称为高态而盘吸积为低态,因为前者总对应着较高的吸积率。 随时间变化的吸积流很可能在这两种模式之间来回振荡,呈现出周期性或准周期性或无规则行为。这可以用来解释天鹅座X-1和若干活动星系核的光变现象,从而为黑洞的存在提供有力的观测依据。

Bimodal Character of Black Hole Accretion

Accretion onto a black hole must be transonic. For a stationary, adiabatic flow the specific energy E, specific angular momentum L and mass accretion rate M are constant in space. The condition for regularity of the transonic solution, F(E, L,M)=0, reduces the number of independent parameters to two. For a fixed pair of E(>0) and L satisfying Ec>E>EBarr(L), where E0 is a critical value and EBarr is the potential barrier connected with the centrifugal force, the regularity condition equation gives two different formal accretion rates corresponding to different locations of the sonic point in the flow. However, the physically acceptable global solution is unique: it is always realized for the smaller of the two accretion rates.For a non-rotating black hole the accretion occurs in one of the two modes: quasi-spherical or the Bondi type in which both the rotational and relativistic effects are negligible in the transonic part, and disk type in which they are dominant. Transition between the two, which is based on a discontinuous jump in the location of the sonic point, is caused by a continuous change in the flow parameter (angular momentum, say). The Bondi type accretion defines a high state and the disk type a low state, in the sense that the former always requires a higher accretion rate.A time-dependent accretion flow would probably oscillate berween the two modes, exhibiting a periodic or quasi-periodic or chaotic behaviour. This could be used to explain the luminosity variability of active galactic nuclei and Cyg X-1, thus providing a strong observational support for the existence of black holes.