A mysterious "hot spot" in the sky is emitting unusual numbers of powerful cosmic rays, say scientists.
The discovery may shed new light on the origin of ultra-high energy cosmic rays, which still cannot be fully explained.
The hot spot is centred two hand-widths below the "handle" of the Big Dipper, or Plough, an arrangement of seven stars within the Great Bear constellation.
"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.
More exotic explanations include decaying "cosmic strings" - hypothetical thread-like objects thinner than an atom - and massive particles left over from the Big Bang that gave birth to the universe 13.8 billion years ago.
The most powerful cosmic rays pack quite a punch in terms of energy - more than a billion, billion electrovolts.
Being hit on the head by just one of the particles would feel like a blow from a fast-pitched baseball, according to Dr Thomson's team, which operates the £14 million Telescope Array cosmic ray observatory in Utah.
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 Telescope Array scientists 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.
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.
The rays appeared to come from places where matter is concentrated in clusters and superclusters of galaxies.
"It tells us there is at least a good chance these are coming from matter we can see as opposed to a different class of mechanisms where you are producing these particles with exotic processes," said Prof Jui.Suggest a correction