A mysterious "hot spot" in the sky is emitting unusual numbers of powerful cosmic rays.

It is centred two hand-widths below the "handle" of the Big Dipper, or Plough, an arrangement of seven stars within the Great Bear constellation.

The discovery may shed new light on the origin of ultra-high energy cosmic rays, which still cannot be fully explained.

"All we see is a blob in the sky, and inside this blob there is all sorts of stuff - various types of objects that could be the source," said US astronomer Professor Gordon Thomson, from the University of Utah. "Now we know where to look."

While lower energy cosmic rays come from the sun, other stars, and exploding stars, the origin of the highest energy rays has been a decades-long mystery.

Discovered in 1912, cosmic rays are actually fast-moving particles - either bare protons from stripped hydrogen atoms or the nuclei of heavier elements such as carbon, oxygen, nitrogen and iron.

Possible sources of high energy cosmic rays include super-dense neutron stars, noisy radio galaxies and shock waves from colliding galaxies.

Previous research has suggested that very high energy cosmic rays are not only rare but can appear from any point in the sky.

But the scientists at the £14 million Telescope Array cosmic ray observatory in Utah found 19 high energy cosmic rays coming from the direction of the hot spot, compared with the 4.5 that would have been expected if their origin was random.

The odds that the hot spot is a statistical fluke are said to be only 1.4 in 10,000.

Professor Charlie Jui, from the University of Utah, another member of the team whose findings are to appear in Astrophysical Journal Letters, said the distribution of ultra-high energy cosmic rays in the northern sky was consistent with the large scale structure of the universe.