Multifrequency studies of bright radio supernova remnants. III. X-ray and radio

Radio-bright, presumably young supernova remnants offer the opportunity of studying strong-shock physics and the nature of the interaction of ejected material with the surrounding medium. The relation between radio and X-ray morphology varies considerably among supernova remnants, with important imp lications for the physics of the emission processes at different wavelength s. We use Very Large Array (VLA) and Roentgen Satellite (ROSAT) images of t he radio-bright supernova remnant 3C 397 (G41.1-0.3) to examine the shock s tructure in both thermal X-ray emission and nonthermal radio emission. The unusual rectangular morphology can be seen in VLA maps at 20 and 6 cm wavel ength at a resolution of 6" and in ROSAT HRI images. The:X-ray images resem ble the radio strongly, except for a small, possibly unresolved X-ray hot s pot near the center. There is no variation in the X-ray hardness ratio from ROSAT Position Sensitive Proportional Counter data across the remnant, sug gesting that at least between 0.4 and 2 keV the interior emission is not di fferent in character from that in the bright shell regions. The remnant is unpolarized at 20 cm and has a mean fractional polarization of 1.5% +/- 0.1 % at 6 cm. The polarized flux, and polarized fraction, peak inside the remn ant at a location not coincident with either an internal maximum in total-i ntensity radio emission or with the X-ray hot spot. Spectral index maps bet ween 6 and 20 cm do not show any systematic differences associated with int erior emission; there appears to be no "plerionic" dr pulsar-driven compone nt in 3C 397, at least as normally characterized by high polarization and a flat radio spectrum. Spectral index values spread about the mean by about Delta alpha similar to 0.2, a result consistent with previous work. The ste ep total-intensity profile off the southwest edge of 3C 397 allows an infer ence of the upstream electron diffusion coefficient and implies a mean free path for electron scattering shorter than in the general interstellar medi um but longer than that similarly inferred for Tycho and SN 1006. A simple analysis based on the observed X-ray flux gives an estimate of the mean den sity in 3C 397 of about 4 cm(-3), which would also be enough to depolarize the 20 cm emission completely, as observed. The remnant age is then of orde r 10(3) yr, and the current shock velocity is about 1600 km s(-1). Finally, we speculate on possible mechanisms producing the X-ray hot spot.