Mechanism for spectral break in cosmic ray proton spectrum from a supernova remnant sur-rounded by dense gas

摘要

Observations of the W44, IC443 and other SNRs by the Fermi spacecraft observatory strongly support the ideathat the bulk of galactic cosmic rays is accelerated in such remnant shocks by the diffusive mechanism. However many SNRshocks expand into weakly ionized dense gases. In this paper we provide the necessary modifications to the diffusive shockacceleration mechnism and demonstrate that strong ion-neutral collisions in remnant surroundings lead to the steepening ofthe energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescenceleading to the fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons withthe ambient gas is also calculated and successfully fitted to the Fermi Observatory data. The parent proton spectrum of theW44, for example, is best represented by a classical test particle power law E 2 , steepening to E 3 at Ebr 7GeV due todeteriorated particle confinement.