Constraints on Cosmic-ray Origin Theories from Tev Gamma-ray Observations

If supernova remnants (SNRs) are the sites of cosmic-ray acceleration, the associated nuclear interactions should result in observable fluxes of TeV gamma-rays from the nearest SNRs. Measurements of the gamma-ray flux from six nearby, radio-bright, SNRs have been made with the Whipple Observatory gamma-ray telescope. No significant emission has been detected and upper limits on the >300 GeV flux are reported. Three of these SNRs (IC443, gamma-Cygni and W44) are spatially coincident with low latitude unidentified sources detected with EGRET. These upper limits weaken the case for the simplest models of shock acceleration and energy dependent propagation. INTRODUCTION It is generally believed that cosmic rays with energies less than ∼ 100 TeV originate in the galaxy and are accelerated in shock waves in shell-type SNRs. This hypothesis is supported by several strong arguments. First, supernova blast shocks are one of the few galactic sites capable of sustaining the galactic cosmic ray population against loss by escape, nuclear interactions and ionization energy loss assuming a SN rate of about 1 per 30 years and a 10% efficiency for converting the mechanical energy into relativistic particles. Second, models of diffuse shock acceleration provide a plausible mechanism for efficiently converting this explosion energy into accelerated particles with energies ∼ 1014 − 1015 eV and naturally give a power-law spectrum similar to that inferred from the cosmic ray data after correcting for energy dependent propagation effects. Finally, observations of nonthermal X-ray emission in SN1006 (Koyama, et al., 1995) and IC443 (Keohane, et al., 1997) suggest the presence of electrons accelerated to ∼ 100 TeV and ∼ 10 TeV respectively. If SNRs are sites for cosmic ray production, there will be interactions between the accelerated particles and the local swept-up interstellar matter. Drury, Aharonian and Volk (1994) (DAV) and Naito and Takahara (1994) have calculated the expected gamma-ray flux from secondary pion production using the model of diffusive shock acceleration. The expected intensity (DAV) is