Distance and Evolutionary State of the Supernova Remnant 3C 397 (G41.1-0.3)

In this paper, we analyze 1420 MHz continuum and H I observations of the supernova remnant (SNR) 3C 397 (G41.1-0.3). The H I absorption spectra show clear absorption up to the tangent point velocity and also the absence of absorption at 50-60 km s^-1. This yields lower and upper limits to the distances of 6.3 ± 0.1 and 9.7 ± 0.3 kpc, which are better and more robust than previous estimates. We apply generalized SNR models to 3C 397, including the ejecta-dominated phase and the transition-to-Sedov phase. Using emission measures from the X-ray and mean gas density from the infrared, we show that the hard X-ray component has the dominant filling factor and the soft X-ray component has a very small filling factor. The models are required to be consistent with 3C 397's measured properties, including the observed shock temperatures and shock radii. Consistent models are found if 3C 397 has a distance in the range of ≃8-9.7 kpc. For an 8 kpc distance, the estimated age is ≃1350 years and the explosion energy is 1.0 × 10⁵¹ erg, while for 9.7 kpc, the the most probable age is ≃1750 years and the energy 1.5 × 10⁵¹ erg.