Thermal and Nonthermal X‐Ray Emission from the Forward Shock in Tycho’s Supernova Remnant

We present Chandra X-ray images of Tycho's supernova remnant that delineateits outer shock as a thin, smooth rim along the straight northeastern edge andmost of the circular western half. The images also show that the Si and Sejecta are highly clumpy, and have reached near the forward shock at numerouslocations. Most of the X-ray spectra that we examine along the rim showevidence of line emission from Si and S ejecta, while the continuum iswell-represented by either a thermal or nonthermal model. If the continuum isassumed to be thermal, the electron temperatures at the rim are all similar atabout 2 keV, while the ionization ages are very low, because of the overallweakness of the line emission. These electron temperatures are substantiallybelow those expected for equilibration of the electron and ion temperatures,assuming shock velocities inferred from radio and X-ray expansion measurements;the electron to mean temperature ratios are <~0.1-0.2, indicating thatcollisionless heating of the electrons at the shock is modest. The nonthermalcontribution to these spectra may be important, but cannot be stronglyconstrained by these data. It could account for as much as half of the flux inthe 4-6 keV energy range, based on an extrapolation of the hard X-ray spectrumabove 10 keV.Comment: ApJ, in press; 32 pages LaTeX, 9 postscript figures; replaced version to better match ApJ versio