A Gravitational Mechanism for the Acceleration of Ultrarelativistic   Particles
  Imagine a swarm of free particles near a point P outside a gravitating mass M and a free reference particle at P that is on a radial escape trajectory away from M. Relative to this reference particle and in a Fermi normal coordinate system constructed along its worldline, the particles in the swarm that move along the radial direction and are ultrarelativistic (that is, they have speeds above c/sqrt(2)) decelerate toward this terminal speed. On the other hand, the swarm particles that are ultrarelativistic and move in directions normal to the radial (jet) directions accelerate to almost the speed of light by the gravitational tidal force of the mass M. The implications of these effects as well as the influence of the higher-order terms on the tidal acceleration mechanism are investigated. The observational evidence in support of these general relativistic effects is briefly discussed. 
C. Chicone and B. Mashhoon