Seeing cosmic-ray sources

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