Cosmic ray is a catch-all term for the high-energy particles that constantly bombard the Earth's upper atmosphere. The majority of them are deflected by the Earth's magnetic field or absorbed by the atmosphere, but a very few are energetic enough to reach the surface of the planet. About 90% of cosmic rays are protons; a good chunk of the remaining ten percent are alpha particles (helium nuclei, consisting of two protons and two neutrons bound together). The rest are varying mixes of particles from the subatomic zoo, sometimes even including positrons and antiprotons -- particles of antimatter. They were discovered in 1912 by Austrian-American physicist Victor Hess in 1912, for which he won the 1936 Nobel Prize in Physics.
The lion's share of cosmic rays that strike the Earth originate from the Sun, but some come from much farther away. As we've seen here several times at Skeptophilia, the universe is an energetic and often violent place, not lacking in mechanisms for sending bits of matter careening across the universe at a significant fraction of the speed of light. As you might expect, supernovae produce cosmic rays; so do gamma ray bursters, Wolf-Rayet stars, and quasars. The last-mentioned are thought to be supermassive black holes surrounded by an inward-spiraling accretion disk of gas and dust, which accelerates as it tumbles toward the event horizon and gives of one final death scream of radiation. This makes quasars one of the brightest objects in the known universe, with luminosities tens of thousands of times that of the Milky Way.
Trying to pinpoint the origin of particular cosmic rays is tricky. Being mostly made of charged particles, they're deflected by magnetic fields; so even if you find one and know the direction it was traveling when it hit your detector, you can't just trace the line backwards and assume that's the point in the sky where it originated. So scientists who are interested in figuring out where the highest-energy cosmic rays come from -- ones that almost certainly weren't created by our placid, stable home star -- have a difficult task.
A team led by Laura Olivera-Nieto of the Max Planck Institute for Nuclear Physics has tackled this problem, and in a paper published last week in Science, came up with an answer for at least some of these mysterious particles. Working at the High-Energy Stereoscopic System (HESS -- a nice nod to the discoverer of cosmic rays) in Namibia, Olivera-Nieto and her team are studying a curious source of cosmic rays -- black holes that are in a binary system with another star.
The current study is of an object called SS 433, a source of x-rays so powerful it's been nicknamed a "microquasar." It lies in the middle of the Manatee Nebula in the constellation Aquila, a shell of gas and dust blown outward when a star went supernova between ten and a hundred thousand years ago. The supernova resulted in a black hole as the doomed star's core collapsed, but its companion star lived on.
Well, after a fashion. The enormous gravitational pull of the black hole is siphoning off matter from the companion star, and as that plume of gas spirals inward, it accelerates and gives off radiation -- just as the accretion disk of a quasar does. The result is a jet of cosmic rays, including not only the typical charged particles but x-rays and gamma rays, which (unlike charged particles) are unaffected by magnetic fields. This allows astronomers to pinpoint their sources.
So in the midst of this seemingly placid bit of space is a whirling hurricane of gas and dust that is accelerated so strongly it creates jets of particles moving at nearly the speed of light. (Exactly the speed of light, in the case of the x-rays and gamma rays.) Some of those particles eventually reach the Earth -- a few of which are picked up by Olivera-Nieto's team at HESS.
And those cosmic rays allows us to discern the fingerprints of an incredibly violent process taking place eighteen thousand light years away.