Recombining Plasma and Hard X-ray Filament in the Mixed-Morphology Supernova Remnant W44

TL;DR

This study reveals recombining plasma and arc-like hard X-ray emission in the supernova remnant W44, indicating rapid cooling and possible synchrotron enhancement near molecular clouds, with implications for SNR evolution.

Contribution

First detection of radiative recombination continua in W44's X-ray spectra, showing a recombining plasma phase and new insights into shock cooling and particle acceleration.

Findings

Detection of RRCs indicating recombining plasma phase.

Hard X-ray arc correlates with radio filament and anti-correlates with molecular cloud emission.

Electron temperature cooled rapidly approximately 20,000 years ago.

Abstract

We report new features of the typical mixed-morphology (MM) supernova remnant (SNR) W44. In the X-ray spectra obtained with Suzaku, radiative recombination continua (RRCs) of highly ionized atoms are 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 matters cooled down rapidly from $\sim1$,keV to $\sim 0.5$,keV, possibly due to adiabatic expansion (rarefaction) occurred $\sim20,000$ years ago. We also discover hard X-ray emission which shows an arc-like structure spatially-correlated with a radio continuum filament. The surface brightness distribution shows a clear anti-correlation with $^{12}$CO (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.