Centrifugal acceleration (astrophysics)

Centrifugal acceleration of astroparticles to relativistic energies might take place in rotating astrophysical objects (see also Fermi acceleration). It is strongly believed that active galactic nuclei and pulsars have rotating magnetospheres, therefore, they potentially can drive charged particles to high and ultra-high energies. It is a proposed explanation for ultra-high-energy cosmic rays (UHECRs) and extreme-energy cosmic rays (EECRs) exceeding the Greisen–Zatsepin–Kuzmin limit.

Acceleration to high energies

It is well known that the magnetospheres of AGNs and pulsars are characterized by strong magnetic fields that force charged particles to follow the field lines. If the magnetic field is rotating (which is the case for such astrophysical objects), the particles will inevitably undergo centrifugal acceleration. The pioneering work by Machabeli & Rogava was a thought experiment in which a bead moves inside a straight rotating pipe. Dynamics of the particle were analyzed both analytically and numerically and it was shown that if the rigid rotation is maintained for a sufficiently long time energy of the bead will asymptotically increase. In particular, Rieger & Mannheim, building on the theory of Machabeli & Rogava, showed that the Lorentz factor of the bead behaves as

where

γ

0

{\displaystyle \gamma _{0}}

is the initial Lorentz factor, Ω is the angular velocity of rotation,

r

{\displaystyle r}

is the radial coordinate of the particle, and

c

{\displaystyle c}

is the speed of light. From this behavior it is evident that radial motion will exhibit a nontrivial character. In due course of motion the particle will reach the light cylinder surface (a hypothetical area where the linear velocity of rotation exactly equals the speed of light), leading to the increase of the poloidal component of velocity.