Cosmic-ray acceleration at the forward shock in Tycho's supernova remnant:: Evidence from Chandra X-ray observations

We present evidence for cosmic-ray acceleration at the forward shock in Tycho's supernova remnant (SNR) from three X-ray observables: ( 1) the proximity of the contact discontinuity to the forward shock, or blast wave, ( 2) the morphology of the emission from the rim of Tycho, and (3) the spectral nature of the rim emission. We determine the locations of the blast wave (BW), contact discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova remnant using a principal component analysis and other methods applied to new Chandra data. The azimuthal-angle-averaged radius of the BW is 251. For the CD and RS we find average radii of 241 and 183, respectively. Taking account of projection effects, we find ratios of 1:0.93:0.70 (BW: CD: RS). We show these values to be inconsistent with adiabatic hydrodynamic models of SNR evolution. The CD: BW ratio can be explained if cosmic-ray acceleration of ions is occurring at the forward shock. The RS : BW ratio, as well as the strong Fe K alpha emission from the Tycho ejecta, imply that the RS is not accelerating cosmic rays. We also extract radial profiles from similar to 34% of the rim of Tycho and compare them to models of surface brightness profiles behind the BW for a purely thermal plasma with an adiabatic shock. The observed morphology of the rim is much more strongly peaked than predicted by the model, indicating that such thermal emission is implausible here. Spectral analysis also implies that the rim emission is nonthermal in nature, lending further support to the idea that Tycho's forward shock is accelerating cosmic rays.