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Date: 14-12-2015
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Gravitational capture
Let us consider now the motion of a test particle in which its trajectory terminates in the black hole. Two types of such a motion are possible. First, the trajectory of the particle starts at infinity and ends in the black hole. Second, the trajectory starts and ends in the black hole. Of course, a particle cannot be ejected from the black hole. Hence, the motion on the second-type trajectory becomes possible either if the particle was placed on this trajectory via a non-geodesic curve or if the particle was created close to the black hole.
The gravitational capture of a particle coming from infinity is of special interest. Let us have a better look at this case. It is clear from the analysis of motion given in the preceding section that a particle coming from infinity can be captured if its specific energy Ẽ is greater, for a given ˜L, than the maximum (Ẽmax) of the curve V (r ). Let us consider the gravitational capture in two limiting cases, one for a particle whose velocity at infinity is much lower than the speed of light (v∞/c << 1) and another for a particle which is ultrarelativistic at infinity.
In the former case, Ẽ ≈ 1. The curve V (r ), which has Ẽmax = 1, corresponds to ˜Lcr = 2 (line c in figure 5.2). The maximum of this curve lies at r = 2rS. This radius is minimal for the periastra of the orbits of the particles with v∞ = 0 which approach the black hole and again recede to infinity. If ˜L ≤ 2, gravitational capture takes place. The angular momentum of a particle moving with the velocity v∞ at infinity is L = mv∞b, where b is the impact parameter. The condition ˜L ≡ L/mcrS = 2 defines the critical value bcr,nonrel = 2rS (c/v∞) of the impact parameter for which the capture takes place. The capture cross section for a non-relativistic particle is
(1.1)
For an ultrarelativistic particle, bcr = 3√3rS/2, and the capture cross section is
(1.2)
Owing to a possible gravitational capture, not every particle whose velocity exceeds the escape limit flies away to infinity. In addition, it is necessary that the angle ψ between the direction to the black hole center and the trajectory be greater than a certain critical value ψcr. For the velocity equal to the escape threshold this critical angle is given by the expression
(1.3)
The plus sign is chosen for r > 2rS (ψcr < 90o), and the minus sign for r < 2rS (ψcr > 90o).
For an ultrarelativistic particle, the critical angle is given by the formula
(1.4)
The plus sign is taken for r > 1.5rS and the minus for r < 1.5rS.
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