A broadband X-ray study of supernova remnant 3C 397

We present a broadband imaging and spectral study of the radio-bright super nova remnant (SNR) 3C 397 with ROSAT, ASCA, and RXTE. A bright X-ray spot s een in the HRI image hints at the presence of a pulsar-powered component an d gives this SNR a composite X-ray morphology. Combined ROSAT and ASCA imag ing shows that the remnant is highly asymmetric, with its X-ray emission pe aking at the western lobe. The hard-band images obtained with the ASCA Gas Imaging Spectrometer show that much of the hard X-ray emission arises from the western lobe, associated with the SNR shell, with little hard X-ray emi ssion associated with the central hot spot. The spectrum from 3C 397 is hea vily absorbed and dominated by thermal emission with emission lines evident from Mg, Si, S, Ar and Fe. Single-component models fail to describe the X- ray spectrum, and at least two components are required : a soft component c haracterized by a low temperature and a large ionization timescale, and a h ard component required to account for the Fe-K emission line and characteri zed by a much lower ionization timescale. We use a set of nonequilibrium io nization (NEI) models (Borkowski et al., in preparation), and find that the fitted parameters are robust. The temperatures from the soft and hard comp onents are similar to0.2 keV and similar to1.6 keV respectively. The corres ponding ionization timescales n(0) t (n(0) being the preshock hydrogen dens ity) are similar to6 x 10(12) cm(-3) s and similar to6 x 10(10) cm(-3) s, r espectively. The large n(0) t of the soft component suggests it is approach ing ionization equilibrium; thus it can be Dt equally well with a collision al equilibrium ionization model. The spectrum obtained with the Proportiona l Counter Array (PCA) of RXTE is contaminated by emission from the Galactic ridge, with only similar to 15% of the count rate originating from 3C 397 in the 5-15 keV range. The PCA spectrum allowed us to confirm the thermal n ature of the hard X-ray emission. A third component originating from a puls ar-driven component is possible, but the contamination of the source signal by the Galactic ridge did not allow us to determine its parameters or find pulsations from any hidden pulsar. We discuss the X-ray spectrum in the li ght of two scenarios : a young ejecta-dominated remnant of a core-collapse SN, and a middle-aged SNR expanding in a dense ISM. In the first scenario, the hot component arises from the SNR shell, and the soft component from an ejecta-dominated component. 3C 397 would be a young SNR (a few thousand ye ars old), but intermediate in dynamical age between the young historical sh ells (like Tycho or Kepler), and those that are well into the Sedov phase o f evolution (like Vela). In the second scenario, the soft component represe nts the blast wave propagating in a dense medium, and the hard component is associated with hot gas encountering a fast shock, or arising from thermal conduction. In this latter scenario, the SNR would be similar to twice as old, and transitioning into the radiative phase. The current picture we pre sent in this paper is marginally consistent with this scenario, but it cann ot be excluded. A spatially resolved spectroscopic study is needed to resol ve the soft and hard components and differentiate between the two scenarios . Future Chandra and XMM data will also address the nature of the mysteriou s central (radio-quiet) X-ray spot.