Cosmic rays reveal their roots
Apr 25, 2002
Two groups of scientists have shed light on the origin of the streams of high-energy particles known as cosmic rays that continually bombard the Earth. Ryoji Enomoto of the University of Tokyo and co-workers have found the first strong evidence that cosmic rays with energies up to 1015 eV are produced by remnants of supernovas (R Enomoto et al 2002 Nature 416 823). Meanwhile, a team of researchers from NASA and Princeton University has proposed that cosmic rays with energies of over 1020 eV are made by black holes in ancient quasar galaxies.
Cosmic rays were first detected in 1912 but there is still no consensus on where they are produced or how they are accelerated to such high energies. Scientists have speculated that supernovas – the huge explosions produced by collapsing stars – could be responsible. This is because the combined energy of cosmic rays in our galaxy is a significant fraction of the total energy released by galactic supernovae. In addition, the mechanism by which this energy could be transferred – through the shock waves generated by supernovae – can account for the observed energy distribution of the cosmic rays that reach the Earth.
The observations by Enomoto and colleagues support this theory. Using the CANGAROO telescope in Australia, they detected showers of optical photons resulting from gamma-rays hitting the Earth’s upper atmosphere with energies of about 1012 eV (1 TeV), from the direction of the supernova remnant RX J1713.7-3946. Such gamma rays could result from the decay of short-lived particles called pions, which are produced by the interaction of protons – the main constituent of cosmic rays – with the interstellar gas surrounding a supernova remnant.
Gamma rays with energies of the order of 1 TeV have previously been detected from two other supernova remnants. But in these cases the gamma rays could have been produced by high-energy electrons that scattered and energized photons from the microwave radiation left over from the big bang, the so-called cosmic microwave background. In contrast, the energy spectrum of the gamma rays detected by Enomoto and colleagues closely matches that expected from the radiation produced by protons rather than electrons.
In a related discovery, Diego Torres of Princeton University and Elihu Boldt and colleagues at NASA’s Goddard Space Flight Center have found that four elliptical galaxies relatively close to Earth may be responsible for cosmic rays with energies of at least 1020 eV. These ultra-high-energy cosmic rays must originate from within 200 million light years of Earth, otherwise their energy would be diminished by interactions with the cosmic microwave background. At a press conference earlier this week the scientists announced that these cosmic rays appear to arrive on Earth from the direction of these galaxies.
But in order to generate cosmic rays, the supermassive black holes known to exist at the cores of these galaxies must spin. Torres and colleagues admit that they do not know if this is the case, but point out that at least one supermassive black hole in the universe is known to spin.
About the author
Edwin Cartlidge is News Editor of Physics World