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Register a new userEvidence for Rapid Adiabatic Cooling as an Origin of the Recombining Plasma in the Supernova Remnant W49B Revealed by NuSTAR Observations
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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.
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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.
Recent X-ray studies revealed over-ionized recombining plasmas (RPs) in a dozen mixed-morphology (MM) supernova remnants (SNRs). However, the physical process of the over-ionization has not been fully understood yet. Here we report on spatially resolved spectroscopy of X-ray emission from W44, one of the over-ionized MM-SNRs, using XMM-Newton data from deep observations, aiming to clarify the physical origin of the over-ionization. We find that combination of low electron temperature and low recombination timescale is achieved in the region interacting with dense molecular clouds. Moreover, a clear anti-correlation between the electron temperature and the recombining timescale is obtained from each of the regions with and without the molecular clouds. The results are well explained if the plasma was over-ionized by rapid cooling through thermal conduction with the dense clouds hit by the blast wave of W44. Given that a few other over-ionized SNRs show evidence for adiabatic expansion as the major driver of the rapid cooling, our new result indicates that both processes can contribute to over-ionization in SNRs, with the dominant channel depending on the evolutionary stage.
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.
Recent X-ray study of middle-aged supernova remnants (SNRs) reveals strong radiative recombination continua (RRCs) associated with overionized plasmas, of which the origin still remains uncertain. We report our discovery of an RRC in the middle-aged SNR 3C 391. If the X-ray spectrum is fitted with a two-temperature plasma model in collisional ionization equilibrium (CIE), residuals of Si XIV Ly alpha line at 2.006 keV, S XVI Ly alpha line at 2.623 keV and the edge of RRC of Si XIII at 2.666 keV are found. The X-ray spectrum is better described by a composite model consisting of a CIE plasma and a recombining plasma (RP). The abundance pattern suggests that the RP is associated to the ejecta from a core-collapse supernova with a progenitor star of 15 solar mass. There is no significant difference of the recombining plasma parameters between the southeast region and the northwest region surrounded by dense molecular clouds. We also find a hint of Fe I K alpha line at 6.4 keV (~2.4 sigma detection) from the southeast region of the SNR.
We carried out new CO($J$ = 2-1) observations toward the mixed-morphology supernova remnant (SNR) W49B with the Atacama Large Millimeter/submillimeter Array (ALMA). We found that CO clouds at $sim$10 km s$^{-1}$ show a good spatial correspondence with synchrotron radio continuum as well as an X-ray deformed shell. The bulk mass of molecular clouds accounts for the western part of the shell, not for the eastern shell where near-infrared H$_2$ emission is detected. The molecular clouds at $sim$10 km s$^{-1}$ show higher kinetic temperature of $sim$20-60 K, suggesting that modest shock-heating occurred. The expanding motion of the clouds with $Delta V sim$6 km s$^{-1}$ was formed by strong winds from the progenitor system. We argue that the barrel-like structure of Fe rich ejecta was possibly formed not only by an asymmetric explosion, but also by interactions with dense molecular clouds. We also found a negative correlation between the CO intensity and the electron temperature of recombining plasma, implying that the origin of the high-temperature recombining plasma in W49B can be understood as the thermal conduction model. The total energy of accelerated cosmic-ray protons $W_mathrm{p}$ is estimated to be $sim$$2times 10^{49}$ erg by adopting an averaged gas density of $sim$$650pm200$ cm$^{-3}$. The SNR age-$W_mathrm{p}$ diagram indicates that W49B shows one of the highest in-situ values of $W_mathrm{p}$ in the gamma-ray bright SNRs.
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