X-Ray Spectrum of a Peculiar Supernova Remnant, G 359.1-0.5

We present Suzaku results of a supernova remnant (SNR), G 359.1-0.5, in the direction of the Galactic center region. From the SNR, we have found prominent K-shell lines of highly ionized Si and S ions, together with unusual structures at 2.5-3.0 and 3.1-3.6 keV. No canonical SNR plasma model, in either ionization equilibrium or under-ionization, can explain the structures. The energies and shapes of the structures are similar to those of the radiative transitions of free electrons to the K-shell of He-like Si and S ions (radiative recombination continuum: RRC). The presence of strong RRC structures indicates that the plasma is in over-ionization. In fact, the observed spectrum has been well-fitted with an over-ionized plasma model. The best-fit electron temperature of 0.29 keV is far smaller than the ionization temperature of 0.77 keV, which means that G 359.1-0.5 is in an extreme condition of over-ionization. We report some cautions on the physical parameters, and comment about possible origins for the over-ionized plasma.