Physics
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aps..apr..d801b&link_type=abstract
American Physical Society, APS/AAPT Joint April Meeting, April 18-21, 1998 Columbus, Ohio, abstract #D8.01
Physics
Scientific paper
If supernova remnants (SNRs) are the site of cosmic-ray acceleration, the associated nuclear interactions should result in an observable flux of π^0-decay γ-rays for the nearest SNRs. Measurements of the TeV γ-ray flux from nearby, radio-bright SNRs have been made with the Whipple imaging air Cherenkov telescope but reveal no significant emission (Buckley et al. 1998). Three of these SNRs (IC443, γ-Cygni and W44) are spatially coincident with low-latitude unidentified sources detected with the Energetic Gamma-Ray Experiment Telescope (EGRET). If the EGRET γ-ray fluxes result from cosmic-ray interactions, then the EGRET and Whipple data are found to be collectively inconsistent with a cosmic-ray source spectrum flatter than ~ E-2.4. The Whipple upper limits for IC443 and γ-Cygni are also inconsistent with a priori predictions if these remnants are indeed expanding into regions where the average density of the interstellar medium is enhanced by the presence of molecular clouds. Recent observations of nonthermal X-rays in the limbs of a number of shell-type SNRs (including IC443, SN 1006 and Cassiopeia A) signify the presence of very high energy electrons (E>10 TeV) in the vicinity of SNR shells. More Recently, the CANGAROO atmospheric Cherenkov telescope has detected significant TeV emission from the remnant SN 1006 (Tanimori et al. 1998), but in this case the TeV emission most probably arises from inverse-Compton scattering by energetic electrons. Despite the growing body of evidence for shock acceleration of electrons in SNRs, there is still no direct evidence pointing to the source of cosmic ray nuclei and the data are beginning to require a modification of the simplest models of shock acceleration and energy dependent propagation of cosmic rays.
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