Recombinant Plasma Research Articles

Origin and Composition of the Galactic Diffuse X-Ray Emission Spectra by Unresolved X-Ray Sources

Galactic diffuse X-ray emission (GDXE) can be spatially segmented into Galactic center X-ray emission (GCXE), Galactic ridge X-ray emission (GRXE), and Galactic bulge X-ray emission (GBXE). The X-ray spectra of GDXE are expressed by the assembly of compact X-ray sources, which are either white dwarfs (WDs) or X-ray active stars consisting of binaries with late-type stars. WDs have either a strong magnetic field or a weak magnetic field. WDs and X-ray active stars are collectively called compact X-ray stars. However, spectral fittings by the assembly of all compact X-ray stars for GCXE, GRXE, and GBXE are rejected, leaving significant excess near the energies of the Kα, Heα, and Lyα lines. These excesses are found in the collisional ionization equilibrium (CIE) plasma. Thus, the spectra of GRXE and GBXE are improved by adding CIE supernova remnants (SNRs). However, the GCXE spectrum is still unacceptable, with significant data excess due to radiative recombination emission (recombining plasma (RP)). The GCXE fit is then significantly improved by adding aged RP-SNRs. Aged RP-SNRs are made by a past big flare of Sgr A* emitting either hard X-rays or low-energy cosmic rays. The big flares may excite Fe and Ni atoms in cold diffuse gas (cold matter (CM)) and emit fluorescent X-ray lines. The CIE-SNRs, RP-SNRs, and CM are called diffuse X-ray sources. This paper presents the spectral fits by the assembly of all the compact and diffuse X-ray sources together with high-quality spectra and a combined fit among all the GDXE of GCXE, GRXE, and GBXE. This provides the first scenario to quantitatively and comprehensively predict the origin of the GDXE spectra.

Initial state of the recombining plasma in supernova remnant W 28

Abstract We have investigated supernova remnant (SNR) W 28 using Suzaku archival data, and we report here the results of our spatially resolved analyses. We carried out spectral analyses using a recombining-plasma (RP) model with element-dependent initial ionization temperatures, obtaining ∼0.5 keV for Ne, ∼0.7 keV for Mg, ∼1.0 keV for Si, ∼1.2 keV for S, ∼1.4 keV for Ar, ∼1.7 keV for Ca, and ∼0.7 keV for Fe in the initial RP phase. In addition to the north-east regions where RPs have been reported previously, we find that the ionization temperatures in the south-east and south-west regions exhibit trends that are similar to those in the central region in the initial RP phase. We also find that the time elapsed from the initial phase of the RP to the present is shorter in the central region, ∼300 yr, and longer in the outside regions, ∼103–104 yr. Our results cannot be explained by simple scenarios involving thermal conduction into molecular clouds or by adiabatic cooling (due to rarefaction), indicating that more complex mechanisms or other scenarios are required. We also estimate the ejecta mass to be ≳14 M⊙, which indicates that this SNR was produced by a massive star.

Investigation of the Physical Origin of Overionized Recombining Plasma in the Supernova Remnant IC 443 with XMM-Newton

Abstract 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 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.

A systematic comparison of ionization temperatures between ionizing and recombining plasmas in supernova remnants

Abstract The temperatures of the plasma in supernova remnants (SNRs) are initially very low just after the shock heating. The electron temperature (kTe) increases quickly by Coulomb interaction, and then the energetic electrons gradually ionize atoms to increase the ionization temperature (kTi). The observational fact is that most young and middle-to-old SNRs have lower kTi than kTe after the shock heating. The temperature evolution in shell-like SNRs has been explained by this ionizing plasma (IP) scenario. On the other hand, in the last decade, a significant fraction of mixed-morphology SNRs was found to exhibit a recombining plasma (RP) with higher kTi than kTe. The origin and the evolution mechanism of RP SNRs have been puzzling. To address this puzzle, this paper presents kTe and kTi profiles using follow-up Suzaku observations, and then proposes a new scenario for the temperature and morphology evolution in IP and RP SNRs.

Discovery of recombining plasma associated with the candidate supernova remnant G189.6+3.3 with Suzaku

Abstract We present the results of an X-ray spectral analysis of the northeast region of the candidate supernova remnant G189.6+3.3 with Suzaku. K-shell lines from highly ionized Ne, Mg, Si, and S were detected in the spectrum for the first time. In addition, a radiative recombining continuum (RRC) from He-like Si was clearly seen near 2.5 keV. This detection of an RRC reveals for the first time that G189.6+3.3 possesses an X-ray-emitting recombining plasma (RP). The extracted X-ray spectrum in the 0.6–10.0 keV energy band is well fitted with a model consisting of a collisional ionization equilibrium plasma component (associated with the interstellar medium) and an RP component (associated with the ejecta). The spectral feature shows that G189.6+3.3 is most likely to be a middle-aged SNR with an RP.

An XMM-Newton X-Ray View of Supernova Remnant W49B: Revisiting Its Recombining Plasmas and Progenitor Type

Abstract We present a comprehensive X-ray spectroscopy and imaging study of supernova remnant W49B using archival XMM-Newton observations. The overionization state of the shocked ejecta in W49B is clearly indicated by the radiative recombination continua of Si xiv, S xv, and Fe xxv, combined with the Lyα lines of Ca and Fe. The line flux images of W49B indicate high emission measures of the central bar-like region for almost all of the emission lines, while the equivalent width maps reveal a stratified structure for the metal abundance distributions. The global spectrum of W49B is well reproduced by a model containing one collisional ionization equilibrium (CIE) plasma component and two recombining plasma (RP) components. The CIE plasma represents the shocked interstellar medium, which dominates the X-ray emitting volume in W49B with a mass ∼450 . The two RP components with a total mass ∼4.6 are both dominated by the ejecta material, but characterized by different electron temperatures (∼1.60 and ∼0.64 keV). The recombination ages of the RP components are estimated as ∼6000 yr and ∼3400 yr, respectively. We then reveal the possibility of a thermal conduction origin for the high-temperature RP in W49B by calculating the conduction timescale. The metal abundance ratios of the ejecta in W49B are roughly consistent with a core-collapse explosion model with a ≲15 progenitor, except for a rather high Mn/Fe. A Type Ia origin can explain the Mn abundance, while it predicts much higher ejecta masses than observed values for all of the metal species considered in our analysis.

Discovery of recombining plasma inside the extended gamma-ray supernova remnant HB9

ABSTRACT We present the results from the Suzaku X-ray Imaging Spectrometer observation of the mixed-morphology supernova remnant (SNR) HB9 (G160.9+2.6). We discovered recombining plasma (RP) in the western Suzaku observation region and the spectra here are well described by a model having collisional ionization equilibrium (CIE) and RP components. On the other hand, the X-ray spectra from the eastern Suzaku observation region are best reproduced by the CIE and non-equilibrium ionization model. We discuss possible scenarios to explain the origin of the RP emission based on the observational properties and concluded that the rarefaction scenario is a possible explanation for the existence of RP. In addition, the gamma-ray emission morphology and spectrum within the energy range of 0.2–300 GeV are investigated using 10 yr of data from the Fermi Large Area Telescope (LAT). The gamma-ray morphology of HB9 is best described by the spatial template of radio continuum emission. The spectrum is well fit to a log-parabola function and its detection significance was found to be 25σ. Moreover, a new gamma-ray point source located just outside the south-east region of the SNR’s shell was detected with a significance of 6σ. We also investigated the archival H i and CO data and detected an expanding shell structure in the velocity range of $-10.5$ and $+1.8$ km s−1 that is coinciding with a region of gamma-ray enhancement at the southern rim of the HB9 shell.

X-ray spectra of Sagittarius A East and diffuse X-ray background near the Galactic center

AbstractThis paper reports the analysis procedure and results of simultaneous spectral fits of the Suzaku archive data for Sagittarius (Sgr) A East and the nearby Galactic center X-ray emission (GCXE). The results are that the mixed-morphology supernova remnant Sgr A East has a recombining plasma (RP) with Cr and Mn Heα lines, and a power-law component (PL) with an Fe i Kα line. The nearby GCXE has a ∼1.5-times larger surface brightness than the mean GCXE far from Sgr A East, although the spectral shape is almost identical. Based on these results, we interpret that the origins of the RP and the PL with the Fe i Kα line are past big flares of Sgr A⋆.

New scenario of plasma evolution in IC 443

Abstract Most young and middle-aged supernova remnants (SNRs) exhibit an ionizing plasma (IP), an ionizing process following a shock-heated SNR gas. On the other hand, significant fractions of SNRs exhibit a recombining plasma (RP). The origin and the mechanisms of the RP, however, are not yet well understood. This paper proposes a new model that the RP follows after the IP process taken at the first epoch of the SNR evolution. Using the high-quality and wide-band (0.6–10 keV) spectrum of IC 443, we fitted it with a model of two RPs (two-RP model) plus a power law (PL) with an Fe i Kα line component. The ionization temperature in one RP monotonically increases from Ne–Ca, while that in the other RP shows a drastic increase from Cr–Ni. The origin and mechanism of the two-RP and PL with an Fe i Kα line components are possibly due to different evolution of two plasmas and ionization by low-energy cosmic rays.

Discovery of recombining plasma from the faintest GeV supernova remnant HB 21 and a possible scenario for cosmic rays escaping from supernova remnant shocks

Abstract We present an X-ray study of the GeV gamma-ray supernova remnant (SNR) HB 21 with Suzaku. HB 21 is interacting with molecular clouds, and is the faintest in the GeV band among known GeV SNRs. We discovered strong radiative recombination continua of Si and S from the center of the remnant, which provide direct evidence of a recombining plasma (RP). The total emission can be explained with the RP and ionizing plasma components. The electron temperature and recombination timescale of the RP component were estimated as 0.17 (0.15–0.18) keV and 3.2 (2.0–4.8) × 1011 s cm−3, respectively. The estimated age of the RP (∼170 kyr) is the longest among known recombining GeV SNRs, because of a very low density of electrons (∼0.05 cm−3). We have examined the dependencies of GeV spectral indices on each of RP ages and SNR diameters for nine recombining GeV SNRs. Both showed possible positive correlations, indicating that both the parameters can be good indicators of properties of accelerated protons, for instance the degree of escape from SNR shocks. A possible scenario for a process of proton escape is introduced: interaction with molecular clouds makes weaker magnetic turbulence and cosmic-ray protons escape, simultaneously cooling down the thermal electrons and generating an RP.

The origin of recombining plasma and the detection of the Fe-K line in the supernova remnant W 28

Abstract Overionized recombining plasmas (RPs) have been discovered from a dozen mixed-morphology (MM) supernova remnants (SNRs). However, their formation process is still under debate. As pointed out by many previous studies, spatial variations of plasma temperature and ionization state provide clues to understanding the physical origin of RPs. We report on spatially resolved X-ray spectroscopy of W 28, which is one of the largest MM SNRs found in our Galaxy. Two observations with Suzaku XIS cover the center of W 28 to the northeastern rim where the shock is interacting with molecular clouds. The X-ray spectra in the inner regions are reproduced well by a combination of two RP models with different temperatures and ionization states, whereas that in the northeastern rim is explained with a single RP model. Our discovery of the RP in the northeastern rim suggests an effect of thermal conduction between the cloud and hot plasma, which may be the production process of the RP. The X-ray spectrum of the northeastern rim also shows an excess emission of the Fe i K α line. The most probable process to explain the line would be inner shell ionization of Fe in the molecular cloud by cosmic ray particles accelerated in W 28.

Toward the Understanding of the Physical Origin of Recombining Plasma in the Supernova Remnant IC 443

Abstract We perform a spatially resolved spectroscopic analysis of X-ray emission from the supernova remnant (SNR) IC 443 with Suzaku. All of the spectra are well reproduced by a model consisting of a collisional ionization equilibrium (CIE) and two recombining plasma (RP) components. Although previous X-ray studies found an RP in the northeastern region, this is the first report on RPs in the other parts of the remnant. The electron temperature, kT e , of the CIE component is almost uniform at ∼0.2 keV across the remnant. The CIE plasma has metal abundances consistent with solar and is concentrated toward the rim of the remnant, suggesting that it is of shocked interstellar medium origin. The two RP components have different kT e : one in the range of 0.16–0.28 keV and the other in the range of 0.48–0.67 keV. The electron temperatures of both RP components decrease toward the southeast, where the SNR shock is known to be interacting with a molecular cloud. We also find the normalization ratio of the lower-kT e RP to higher-kT e RP components increases toward the southeast. Both results suggest the X-ray emitting plasma in the southeastern region is significantly cooled by some mechanism. One of the plausible cooling mechanisms is a thermal conduction between the hot plasma and the molecular cloud. If the cooling proceeds faster than the recombination timescale of the plasma, the same mechanism can account for the recombining plasma as well.

Recombining Plasma and Gamma-Ray Emission in the Mixed-morphology Supernova Remnant 3C 400.2

Abstract 3C 400.2 belongs to the mixed-morphology supernova remnant class, showing center-filled X-ray and shell-like radio morphology. We present a study of 3C 400.2 with archival Suzaku and Fermi-LAT observations. We find recombining plasma (RP) in the Suzaku spectra of north–east and south–east regions. The spectra of these regions are well described by two-component thermal plasma models: the hard component is in RP, while the soft component is in collisional ionization equilibrium (CIE) conditions. The RP has enhanced abundances, indicating that the X-ray emission has an ejecta origin, while the CIE has solar abundances associated with the interstellar material. The X-ray spectra of north–west and south–west regions are best fitted by a two-component thermal plasma model: an ionizing and a CIE plasma. We have detected GeV gamma-ray emission from 3C 400.2 at the level of ∼5σ, assuming a point-like source model with a power-law (PL) type spectrum. We have also detected a new GeV source at the level of ∼13σ, assuming a Gaussian extension model with a PL-type spectrum in the neighborhood of the supernova remnant. We report the analysis results of 3C 400.2 and the new extended gamma-ray source, and discuss the nature of gamma-ray emission of 3C 400.2 in the context of existing NANTEN CO data, Dominion Radio Astrophysical Observatory H i data, and the Suzaku X-ray analysis results.

Localized recombining plasma in G166.0+4.3: A supernova remnant with an unusual morphology

Abstract We observed the Galactic mixed-morphology supernova remnant G166.0+4.3 with Suzaku. The X-ray spectrum in the western part of the remnant is well represented by a one-component ionizing plasma model. The spectrum in the northeastern region can be explained by two components. One is an Fe-rich component with electron temperature $kT_{\rm e} = 0.87_{-0.03}^{+0.02}\:$keV. The other is a recombining plasma (RP) component of lighter elements with kTe = 0.46 ± 0.03 keV, initial temperature kTinit = 3 keV (fixed), and ionization parameter $n_{\rm e}t = (6.1_{-0.4}^{+0.5}) \times 10^{11}\:$cm−3 s. As the formation process of the recombining plasma, both rarefaction and thermal conduction scenarios are considered. The former would not be favored since we found the recombining plasma only in the northeastern region, whereas the latter would explain the origin of the RP. In the latter scenario, an RP is anticipated where blast waves in part of the remnant are in contact with cool dense gas. The emission measure suggests higher ambient gas density in the northeastern region. The morphology of the radio shell and a GeV gamma ray emission also suggest a molecular cloud in the region.

N49: THE FIRST ROBUST DISCOVERY OF RECOMBINING PLASMA IN AN EXTRA GALACTIC SUPERNOVA REMNANT

Recent discoveries of recombining plasmas (RPs) in supernova remnants (SNRs) have dramatically changed our understanding of SNR evolution. To date, a dozen of RP SNRs have been identified in the Galaxy. Here we present Suzaku deep observations of four SNRs in the Large Magellanic Cloud (LMC), N49, N49B, N23, and DEM L71, for accurate determination of their plasma state. Our uniform analysis reveals that only N49 is in the recombining state among them, which is the first robust discovery of a RP from an extra-galactic SNR. Given that RPs have been identified only in core-collapse SNRs, our result strongly suggests a massive star origin of this SNR. On the other hand, no clear evidence for a RP is confirmed in N23, from which detection of recombination lines and continua was previously claimed. Comparing the physical properties of the RP SNRs identified so far, we find that all of them are categorized into the "mixed-morphology" class and interacting with surrounding molecular clouds. This might be a key to solve formation mechanisms of the RPs.

Discovery of recombining plasma in the supernova remnant 3C 391

Abstract Recent X-ray study of middle-aged supernova remnants (SNRs) reveals strong radiative recombination continua (RRCs) associated with overionized plasmas, which origin still remains uncertain. We report a 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 a Si xiv Lyα line at 2.006 keV, a S xvi Lyα line at 2.623 keV and the edge of a RRC of Si xiii at 2.666 keV are found. In fact, the X-ray spectrum is well described by the composite model consisting of the CIE plasma and a recombining plasma (RP). The abundance pattern suggests that the RP is due to ejecta from core-collapse supernova of the progenitor mass of about 15 M⊙. There is no significant difference of the recombining parameters between the south-east region and the north-west region surrounded by dense molecular clouds. We also find a hint of a Fe i Kα line at 6.4 keV (2.4 σ detection) from the south-east region of the SNR.