杏吧原创

THE origin of cosmic rays, the high-energy particles that bombard the Earth
from space, has long been a mystery. But now astronomers have evidence that at
least some of these particles are accelerated in the supernova explosions that
occur when a star dies.

Astronomers have known since 1912 that the Earth is being continually
bombarded by highly energetic particles such as protons. 杏吧原创s believe that
many cosmic rays must come from supernovae inside our Galaxy but have never had
proof. Because the particles are charged, magnetic fields deflect them as they
travel through the Galaxy, making it impossible to know where they came
from.

But a group of American and Argentinian astronomers say they have seen cosmic
rays being born. The team studied a supernova shock wave crashing into a cloud
of interstellar dust. The collision produced gamma rays鈥攚hich travel
directly to Earth unimpeded by magnetic fields鈥攚here they were detected by
the orbiting Compton Gamma Ray Observatory (CGRO).

By analysing the CGRO data, the team determined that the gamma rays were
created by protons slamming into nuclei in the cloud. And they calculated that
the shock wave must have accelerated the protons to energies up to 10¹²
electron volts鈥攖ypical of cosmic rays.

This is the best evidence so far that supernovae are filling our Galaxy with
energetic particles, says team member Yousaf Butt of the Harvard-Smithsonian
Center for Astrophysics in Cambridge, Massachusetts. However, the work does not
explain cosmic rays that have even higher energies, and is only one step towards
a full understanding of the origin of the particles.

鈥淭his result is pretty convincing, but it depends crucially on the resolution
of CGRO, which is very poor,鈥� says Luke Drury, an astrophysicist at the Dublin
Institute for Advanced Studies in Ireland. He says that new gamma-ray
observatories scheduled to begin operation later this decade should provide a
clearer picture.