Where do ulta-high-energy (UHE) cosmic rays come from?

Clues:

1. UHEs are isotropic, meaning they come equally from every direction. This indicates that they originate extragalactically (outside our galaxy,) since their distribution does not reflect the distribution of the galaxy around us. (You can see that the galaxy is distributed unevenly around us: the Milky Way (which is our galaxy) shows that our galaxy is thick enough to see is some directions, but not in others. However, more UHEs do not come from the direction of the Milky Way, or any other direction!)
   
2. UHEs make it all the way to the earth, meaning that they originate within 30 million light years distance. If they came from further away, they would lose their energy through interaction with the cosmic background radiation. (The cosmic background radiation is the energy left over from the Big Bang--a very low-temperature radiation that fills the whole universe.)
3. UHEs have lots of energy, so the source must be something that can accelerate particles to extremely high speeds.

   Therefor the source of UHEs must be outside our galaxy, but closer than 30 million light years away, and very violently energetic.

   Possible sources:

   1. Relativistic jets spewing from black holes.
      
      

      Do the highest energy Cosmic Rays come from objects such as this? On the left is a radio telescope image of the active galaxy NGC-4261. The width of the image covers a region of space 88,000 light years across.Notice the bright orange jets that spray out above and below the centre. To the right is a close up image from the Hubble Space Telescope covering just 400 light years. It shows a doughnut shaped ring at the centre. This ring is thought to orbit a giant black hole with a mass more than a hundred million times that of our sun. From http://ast.leeds.ac.uk/haverah/origin.shtml

      
      
      
   2. Explosions of colliding neutron stars. Neutron stars are super-dense stars--so dense that the electrons have been stripped of of the atoms that make up the star, and only the nucleus of the atoms remain. When two of these super-dense stars collide, they may cause an explosion energetic enough to create UHEs.
   3. Collapsing topological defects in the fabric of space-time. This is tougher to explain, because it involves space-time, which is a way to explain gravity using a theory called general relativity. The basic idea is that the universe is made up of space-time, which can be thought of as a fabric. When the universe formed, there were knots, kinks and other defects in space-time. As the universe evloves and space-time smoothes out, these knots and kinks flatten out and release huge amounts of energy, which could accelerate particles as UHEs.