Cosmic‐Ray Acceleration at the Forward Shock in Tycho’s Supernova Remnant: Evidence fromChandraX‐Ray Observations

We present evidence for cosmic ray acceleration at the forward shock inTycho's supernova remnant (SNR) from three X-ray observables: (1) the proximityof the contact discontinuity to the forward shock, or blast wave, (2) themorphology of the emission from the rim of Tycho, and (3) the spectral natureof the rim emission. We determine the locations of the blast wave (BW), contactdiscontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernovaremnant using a principal component analysis and other methods applied to newChandra data. The azimuthal-angle-averaged radius of the BW is 251". For the CDand RS we find average radii of 241" and 183", respectively. Taking account ofprojection effects, we find ratios of 1:0.93:0.70 (BW:CD:RS). We show thesevalues to be inconsistent with adiabatic hydrodynamical models of SNRevolution. The CD:BW ratio can be explained if cosmic ray acceleration of ionsis occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Kaemission from the Tycho ejecta, imply that the RS is not accelerating cosmicrays. We also extract radial profiles from ~34% of the rim of Tycho and comparethem to models of surface brightness profiles behind the BW for a purelythermal plasma with an adiabatic shock. The observed morphology of the rim ismuch more strongly peaked than predicted by the model, indicating that suchthermal emission is implausible here. Spectral analysis also implies that therim emission is non-thermal in nature, lending further support to the idea thatTycho's forward shock is accelerating cosmic rays.Comment: 39 pages, 10 figures, accepted by Ap