Localized recombining plasma in G166.0+4.3: A supernova remnant with an unusual morphology

Abstract We observed the Galactic mixed-morphology supernova remnant G166.0+4.3 with Suzaku. The X-ray spectrum in the western part of the remnant is well represented by a one-component ionizing plasma model. The spectrum in the northeastern region can be explained by two components. One is an Fe-rich component with electron temperature $kT_{\rm e} = 0.87_{-0.03}^{+0.02}\:$ keV. The other is a recombining plasma (RP) component of lighter elements with kT e = 0.46 ± 0.03 keV, initial temperature kT init = 3 keV (fixed), and ionization parameter $n_{\rm e}t = (6.1_{-0.4}^{+0.5}) \times 10^{11}\:$ cm−3 s. As the formation process of the recombining plasma, both rarefaction and thermal conduction scenarios are considered. The former would not be favored since we found the recombining plasma only in the northeastern region, whereas the latter would explain the origin of the RP. In the latter scenario, an RP is anticipated where blast waves in part of the remnant are in contact with cool dense gas. The emission measure suggests higher ambient gas density in the northeastern region. The morphology of the radio shell and a GeV gamma ray emission also suggest a molecular cloud in the region.