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Register a new userUnraveling the Origin of Overionized Plasma in the Galactic Supernova Remnant W49B
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Laura A. Lopez
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Recent observations have shown several supernova remnants (SNRs) have overionized plasmas, those where ions are stripped of more electrons than they would be if in equilibrium with the electron temperature. Rapid electron cooling is necessary to produce this situation, yet the physical origin of that cooling remains uncertain. To assess the cooling scenario responsible for overionization, in this paper, we identify and map the overionized plasma in the Galactic SNR W49B based on a 220 ks Chandra Advanced CCD Imaging Spectrometer (ACIS) observation. We performed a spatially-resolved spectroscopic analysis, measuring the electron temperature by modeling the continuum and comparing it to the temperature given by the flux ratio of the He-like and H-like lines of sulfur and of argon. Using these results, we find that W49B is overionized in the west, with a gradient of increasing overionization from east to west. As the ejecta expansion is impeded by molecular material in the east but not in the west, our overionization maps suggest the dominant cooling mechanism is adiabatic expansion of the hot plasma.
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The physical origin of the overionized recombining plasmas (RPs) in supernova remnants (SNRs) has been attracting attention because its understanding provides new insight into SNR evolution. However, the process of the overionization, although it has been discussed in some RP-SNRs, is not yet fully understood. Here we report on spatially resolved spectroscopy of X-ray emission from IC~443 with {it XMM-Newton}. We find that RPs in regions interacting with dense molecular clouds tend to have lower electron temperature and lower recombination timescale. These tendencies indicate that RPs in these regions are cooler and more strongly overionized, which is naturally interpreted as a result of rapid cooling by the molecular clouds via thermal conduction. Our result on IC~443 is similar to that on W44 showing evidence for thermal conduction as the origin of RPs at least in older remnants. We suggest that evaporation of clumpy gas embedded in a hot plasma rapidly cools the plasma as was also found in the W44 case. We also discuss if ionization by protons accelerated in IC~443 is responsible for RPs. Based on the energetics of particle acceleration, we conclude that the proton bombardment is unlikely to explain the observed properties of RPs.
We report on the results of our detailed analyses on the peculiar recombining plasma of the supernova remnant (SNR) G359.1$-$0.5, and the interacting CO clouds. Combining {it Chandra} and {it Suzaku} data, we estimated the ionization state of the plasma with a careful treatment of the background spectrum. The average spectrum showed a remarkably large deviation of the electron temperature ($sim$0.17 keV) from the initial temperature ($>$ 16 keV), indicating that the plasma is in a highly recombination-dominant state. On the other hand, the recombination timescale $({it n_{rm e} t})$ is comparable to those of the other recombining SNRs ($sim4.2 times 10^{11}$ cm$^{-3}$ s). We also searched for spatial variation of the plasma parameters, but found no significant differences. Using $^{12}$CO($J$=2--1) data obtained with NANTEN2, we found a new, plausible candidate for the interacting CO cloud, which has a line-of-sight velocity of $sim -$20 km s$^{-1}$. This indicates that the SNR is located at a distance of $sim$4 kpc, which is the foreground of the Galactic center, as previously reported. The associated CO cloud does not show clear spatial coincidence with the nearby GeV/TeV emission, indicating that the origins of the GeV/TeV emission are likely unrelated to G359.1$-$0.5.
X-ray observations of supernova remnants (SNRs) in the last decade have shown that the presence of recombining plasmas is somewhat common in a certain type of object. The SNR W49B is the youngest, hottest, and most highly ionized among such objects and hence provides crucial information about how the recombination phase is reached during the early evolutionary phase of SNRs. In particular, spectral properties of radiative recombination continuum (RRC) from Fe are the key for constraining the detailed plasma conditions. Here we present imaging and spectral studies of W49B with Nuclear Spectroscopic Telescope Array (NuSTAR), utilizing the highest-ever sensitivity to the Fe RRC at > 8.8keV. We confirm that the Fe RRC is the most prominent at the western part of the SNR because of the lowest electron temperature (~ 1.2 keV) achieved there. Our spatially-resolved spectral analysis reveals a positive correlation between the electron temperature and the recombination timescale with a uniform initial temperature of ~ 4 keV, which is consistent with the rapid adiabatic cooling scenario as an origin of the overionization. This work demonstrates NuSTARs suitability for studies of thermal emission, in addition to hard nonthermal X-rays, from young and middle-aged SNRs.
We report on NuSTAR observations of the mixed morphology supernova remnant (SNR) W49B, focusing on its nonthermal emission. Whereas radio observations as well as recent gamma-ray observations evidenced particle acceleration in this SNR, nonthermal X-ray emission has not been reported so far. With the unprecedented sensitivity of NuSTAR in the hard X-ray band, we detect a significant power-law-like component extending up to $sim 20~{rm keV}$, most probably of nonthermal origin. The newly discovered component has a photon index of $Gamma =1.4^{+1.0}_{-1.1}$ with an energy flux between 10 and 20 keV of $(3.3 pm 0.7) times 10^{-13}~{rm erg}~{rm cm}^{-2}~{rm s}^{-1}$. The emission mechanism is discussed based on the NuSTAR data combined with those in other wavelengths in the literature. The NuSTAR data, in terms both of the spectral slope and of the flux, are best interpreted as nonthermal electron bremsstrahlung. If this scenario is the case, then the NuSTAR emission provides a new probe to sub-relativistic particles accelerated in the SNR.
The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). $gamma$-ray observations of SNR/MC associations are a powerful tool to constrain the origin of Galactic cosmic-rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. The detection of a $gamma$-ray source coincident with W49B at very high energies (VHE; E > 100 GeV) with the H.E.S.S. Cherenkov telescopes is reported together with a study of the source with 5 years of Fermi-LAT high energy $gamma$-ray (0.06 - 300 GeV) data. The smoothly-connected combined source spectrum, measured from 60 MeV to multi-TeV energies, shows two significant spectral breaks at $304pm20$ MeV and $8.4_{-2.5}^{+2.2}$ GeV, the latter being constrained by the joint fit from the two instruments. The detected spectral features are similar to those observed in several other SNR/MC associations and are found to be indicative of $gamma$-ray emission produced through neutral-pion decay.
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