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Session 44 - New Light on Supernova Remnants.
Display session, Wednesday, June 11
South Main Hall,
Young supernova remnants are often assumed to be the source of cosmic rays up to energies approaching the slight steepening at around 1000 TeV known as the ``knee'' in the cosmic-ray energy spectrum. We show that the observed X-ray emission of seven shell remnants, including all five historical shells, can be used to put limits on E_m, the energy at which the electron energy distribution must steepen from its slope at radio-emitting energies. Six of the remnants show thermal spectra, so any nonthermal component must fall below the observed X-ray fluxes. Assuming homogeneous emitting volumes with a mean magnetic-field strength of 10 \ \muG, no object could reach 1000 TeV, and only one, Cas A, has an upper limit on E_m near 100 TeV. All the other historical shells have limits at or below 50 TeV. These limits scale as B^-1/2, so absurdly low magnetic fields would be required to get E_m much higher. E_m is probably set by the finite remnant lifetime rather than by synchrotron losses for remnants younger than a few thousand years, so that an observed electron steepening implies that the proton spectrum should steepen at that energy as well. More complicated, inhomogeneous models could allow higher values of E_m in parts of the remnant, but the emission-weighted average value, that characteristic of typical electrons, should obey these limits. The historical remnants should be among the best in the Galaxy at producing the highest-energy Galactic cosmic rays; if they cannot do it, this picture of cosmic-ray origin may need major alteration.
Program listing for Wednesday