Astrophysicists presume that the principal drivers that accelerate cosmic rays to energies as high as 1015 eV are shock waves in supernova remnants. But the evidence has been inconclusive. Obvious pointers back to CR sources would seem to be GeV photons from the decay of pions produced in collisions of CR protons with interstellar gas. The diffuse gamma-ray emission from the Milky Way’s disk is attributed largely to such photons from a myriad of SN remnants. But for observers viewing the disk edge-on from inside the galaxy, it’s been impossible to resolve the diffuse emission into discrete sources. And detections of TeV gammas from local SN remnants by ground-based Cherenkov telescopes have been ambiguous. But now, two Cherenkov telescopes (HESS and VERITAS) and the new Fermi gamma-ray satellite have yielded the first reports of faint gamma emission from ordinary galaxies—unlike the so-called active galactic nuclei whose brilliance in gammas is powered by actively accreting supermassive black holes. In two papers, the Fermi team reports gamma emissions from three nearby galaxies with known “starburst” regions of prolific ongoing star formation, which are bound to have high supernova rates. In the closest of these, the Large Magellanic Cloud, the Fermi data are able to localize the strongest gamma emission to the starburst region 30 Doradus (see the figure). In each galaxy, the observed gamma intensity and spectrum (up to 20 GeV) agree impressively with theoretical predictions that assume the gammas come from interactions of CRs accelerated in supernova remnants. That assumption is strengthened by the TeV spectra from those same starburst galaxies recorded by HESS and VERITAS. ( F. Acero et al., HESS collaboration, http://arxiv.org/abs/0909.4651; V. A. Acciari et al., VERITAS collaboration, http://arxiv.org/abs/0911.0873; A. Abdo et al., Fermi collaboration, Astron. Astrophys., in press; A. A. Abdo et al., http://arxiv.org/abs/0911.5327.)—Bertram Schwarzschild
The correct URL for the Fermi detection of the starburst galaxies is http://arxiv.org/abs/0911.5327 [corrected in article 12/2/2009].
This really does not say too much about the source of cosmic rays....e.g. one would expect more pulsars in high star formation Galaxies like NGC253 and M82. So pulsars could be implicated as the sources, just as well.
Why the overhyped reporting?
"Says geochemist Douglas Hammond of the University of Southern California (USC) in Los Angeles: 'Everybody always assumes radioactive decay to be totally independent of temperature, pressure, and chemical form. It seems there are some exceptions.' ...When Fritz Bosch and his colleagues at the Gesellschaft für Schwerionenforschung in Darmstadt, Germany, stripped away all the electrons from rhenium nuclei... its half-life plummeted from 42 billion years to 33 years [that's from 42,000,000,000 to a mere 33]. But, until now, researchers have detected only tiny variations (or none at all) in the decay rate of beryllium and other atoms under Earth-like conditions..." Tweaking the Clock of Radioactive Decay, Richard A. Kerr, Science 286, 882-883 (1999)
In fact, in principle - there are NO exceptions in physics.
The connection of cosmic rays here to supernova remnants is quite correct. The halo is constantly replenished with such material (also from novae and other nuclear processes) that are radioactive or turn radioactive in the new environment.
The diffuse gamma-ray emission from the Milky Way would thus come not only from its disk but also from its highly rarefied and enormously larger (invisible) halo as well. Even "stable" nuclei would disintegrate with a drastically shortened half-life, especially in the outermost regions of the halo.
The full length story in the January issue of the magazine will address this issue.