Recombining plasma and hard x-ray filament in the mixed-morphology supernova remnant w44

Abstract

We report on new features of the typical mixed-morphology supernova remnantW44. In X-ray spectra obtained with Suzaku, radiative recombination continua of highly ionized atoms were detected for the first time. The spectra are well reproduced by a thermal plasma in a recombining phase. The best-fit parameters suggest that the electron temperature of the shock-heated matter rapidly cooled down from -1 keV to 0.5 keV, possibly due to adiabatic expansion (rarefaction), occurred 20000 yr ago. We also discovered hard X-ray emission, which shows an arclike structure spatially correlating with a radio continuum filament. The surface-brightness distribution has a clear anticorrelation with 12CO (J = 2-1) emission from a molecular cloud observed with NANTEN2. While the hard X-ray is most likely due to a synchrotron enhancement in the vicinity of the cloud, no current model can quantitatively predict the observed flux. © 2012. Astronomical Society of Japan.