ASCA & RXTE observations of a unique SNR: 3C397

3C397 (G41.1-0.3) is one of the brightest radio SNR's, whose classification remains ambiguous. While in the radio, it is classified as a `Shell'; in X-rays, it has a shell-like plus a centrally bright component, giving the remnant a `Composite' morphology. We present the ASCA & RXTE observations in order to better understand the nature of this unique SNR. The ASCA spectrum is dominated by thermal emission from Mg, Si, S, Ar, and Fe. One-temperature component models fail to account for the hard X-ray emission above ~ 4 keV; and at least two components are required: a soft component dominated by line emission from heavily enriched material, and a hot component, characterized by a prominent Fe-K emission line. The total observed flux is ~ 2.6 x 10(-11) erg cm(-2) s(-1) , which corresponds to an unabsorbed luminosity L_x (1-9 keV) = 4 x 10(36) erg s(-1) , assuming a distance of 10 kpc. The combined ASCA & RXTE PCA data favor the thermal interpretation of the hard component, and suggest the presence of a weak non-thermal third component, whose parameters are poorly determined because of the contamination of the RXTE signal with the emission from the Galactic ridge. We discuss the X-ray spectrum in the light of thermal emission from an ejecta-dominated and a blast wave component, and speculate on the nature of a compact object possibly hiding in this unusual remnant. Our broadband imaging and spectral study suggests that 3C397 is unlike the thermal or plerionic Composites, and that it is dynamically young ( ~ 1,500 yr-old), perhaps a transition object from the young Shells into the more evolved Composites.