Recombining Plasma and Hard X-Ray Filament in the Mixed-Morphology Supernova Remnant W 44

Abstract

We report on new features of the typical mixed-morphology supernova remnant W 44. 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 $\sim$ 1 keV to $\sim$ 0.5 keV, possibly due to adiabatic expansion (rarefaction), occurred $\sim$ 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 $^{12}$ 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.

Supernova Remnants Interacting with Molecular Clouds: X-Ray and Gamma-Ray Signatures

“…IC 443 shows an enhanced intensity ratio for H-like to He-like Si, indicating an overionized plasma (Kawasaki et al 2002), and also distinct RRC features ), for example. RRC features are also observed for W44, which also shows enhanced emission from H-like Si (Uchida et al 2012), and from W49B ). Interestingly, the nature of such overionized plasmas may actually be only indirectly associated with the presence of MCs.…”

“…Interestingly, the nature of such overionized plasmas may actually be only indirectly associated with the presence of MCs. Although thermal conduction has been suggested as a mechanism by which heat flow from the hot SNR interior to regions with cold clouds could bring lower the electron temperature to values below the current ionization temperature of the ions, most calculations indicate that this process is too slow to operate efficiently in the typical lifetime of an SNR (e.g., Uchida et al 2012). Instead, early evolution through a dense medium such as that from a stellar wind may have created a high ionization state, with subsequent rapid cooling as the remnant expands adiabatically into a low density cavity leading to a temperature that is lower than the ionization temperature (e.g., Uchida et al 2012;Moriya 2012).…”

“…Although thermal conduction has been suggested as a mechanism by which heat flow from the hot SNR interior to regions with cold clouds could bring lower the electron temperature to values below the current ionization temperature of the ions, most calculations indicate that this process is too slow to operate efficiently in the typical lifetime of an SNR (e.g., Uchida et al 2012). Instead, early evolution through a dense medium such as that from a stellar wind may have created a high ionization state, with subsequent rapid cooling as the remnant expands adiabatically into a low density cavity leading to a temperature that is lower than the ionization temperature (e.g., Uchida et al 2012;Moriya 2012). Maps of the ionization state in W49B indicate higher states of overionization in the central and western regions Lopez et al 2013), supporting the notion that rapid expansion in the direction opposite that of a dense MC in the east is responsible for the cooling.…”

The Morphologies and Kinematics of Supernova Remnants

“…In these cases, the electron temperature kT e derived from the bremsstrahlung continuum is systematically lower than the effective ionization temperature kT z given by the line ratios. Subsequent Suzaku observations of MM SNRs detected radiative recombination continuum (RRC) features in their spectra (e.g., Yamaguchi et al 2009, Uchida et al 2012, conclusive evidence of overionization.…”

Suzaku X-ray observations of the mixed-morphology supernova remnant CTB 1

“…Recently, X-ray observations found recombining plasmas (RPs) in many MM SNRs (e.g., Kawasaki et al 2002;Yamaguchi et al 2009;Ozawa et al 2009;Ohnishi et al 2011;Sawada & Koyama 2012;Uchida et al 2012;Matsumura et al 2017a). An RP indicates the unusual evolution of thermal plasmas, because the ionization timescale is longer ( > ∼ 10 5 yr) than the typical age of SNRs in the typical density of electrons (0.1-1 cm −3 ) and an ionizing plasma is expected in SNRs.…”