Multifrequency Studies of Bright Radio Supernova Remnants. III. X-Ray and Radio Observations of 3C 397

Abstract

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. We use VLA and ROSAT images of the radio-bright supernova remnant 3C 397 (G41.1--0.3) to examine the shock structure 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 wavelength at a resolution of 6", and in ROSAT HRI images. The X-ray images resemble the radio strongly, except for a small, possibly un resolved X-ray hot spot near the center. There is no variation in the X-ray hardness ratio from ROSAT Position Sensitive Proportional Counter data across the remnant, suggesting that at least between 0.4 and 2 keV, the interior emission is not different in character from that in the bright shell regions. Thus 3C 397 is not a member of the ``thermal composite'' or ``mixed-morphology'' class (Rho 1998). 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 remnant at a location not coincident with either an internal maximum in total-intensity radio emission, or with the X-ray hot spot. Spectral-index maps between 6 and 20 cm do not show any systematic differences associated with interior emission; there appears to be no ``plerionic'' or pulsar-driven component 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 0.2, a result consistent with previous work. We make calculations of the upstream electron diffusion coefficient and the mean density. Finally, we speculate on possible mechanisms producing the X-ray hot spot.