Kuniaki Masai

SUZAKU DISCOVERY OF THE STRONG RADIATIVE RECOMBINATION CONTINUUM OF IRON FROM THE SUPERNOVA REMNANT W49B

We present a hard X-ray spectrum of unprecedented quality of the Galactic supernova remnant (SNR) W49B obtained with the Suzaku satellite. The spectrum exhibits an unusual structure consisting of a saw-edged bump above 8 keV. This bump cannot be explained by any combination of high-temperature plasmas in ionization equilibrium. We firmly conclude that this bump is caused by the strong radiative recombination continuum (RRC) of iron, detected for the first time in a SNR. The electron temperature derived from the bremsstrahlung continuum shape and the slope of the RRC is ~1.5 keV. On the other hand, the ionization temperature derived from the observed intensity ratios between the RRC and Kα lines of iron is ~2.7 keV. These results indicate that the plasma is in a highly overionized state. Volume emission measures independently determined from the fluxes of the thermal and RRC components are consistent with each other, suggesting the same origin of these components.

X-Ray Spectrum of a Peculiar Supernova Remnant, G 359.1-0.5

We present Suzaku results of a supernova remnant (SNR), G 359.1� 0.5, in the direction of the Galactic center region. From the SNR, we have found prominent K-shell lines of highly ionized Si and S ions, together with unusual structures at 2.5–3.0 and 3.1–3.6 keV. No canonical SNR plasma model, in either ionization equilibrium or under-ionization, can explain the structures. The energies and shapes of the structures are similar to those of the radiative transitions of free electrons to the K-shell of He-like Si and S ions (radiative recombination continuum: RRC). The presence of strong RRC structures indicates that the plasma is in over-ionization. In fact, the observed spectrum has been well-fitted with an over-ionized plasma model. The best-fit electron temperature of 0.29 keV is far smaller than the ionization temperature of 0.77 keV, which means that G 359.1� 0.5 is in an extreme condition of over-ionization. We report some cautions on the physical parameters, and comment about possible origins for the over-ionized plasma.

Evolution of Supernova Remnants Expanding out of the Dense Circumstellar Matter into the Rarefied Interstellar Medium

We carry out 3D-hydrodynamical calculations for the interaction of expanding supernova ejecta with the dense circumstellar matter (CSM) and the rarefied interstellar medium (ISM) outside. The CSM is composed of the stellar wind matter from the progenitor in its pre-supernova phase, and assumed to be axially symmetric: more matter around the equator than in the polar direction driven by rotation of the progenitor. Because of high density of the CSM, the ionization state of the shock-heated ejecta quickly becomes equilibrium with the electron temperature. When the blast wave breaks out of the CSM into the rarefied ISM, the shocked ejecta cools rapidly due to adiabatic expansion, and hence an over-ionized/recombining plasma would be left. The ejecta is reheated by the second reverse shock due to the interaction with the ISM. We calculate the emission measure of the supernova remnant (SNR) along the line of sight, and find that the over-ionized plasma appears to be bar-like with wings in the edge-on (equatorial view), while shell-like in the face-on (polar view) geometry with respect to the rotation axis. The hot gas heated by the blast wave exists in the outermost region of the SNR with a nearly complete shell, but the X-rays therefrom are too faint to be observable. Thus, depending on the viewing angle, the SNR of the over-ionized plasma would exhibit center-filled morphology in X-rays, like W49B, a mixed-morphology SNR. The bar-like structure is swept out by the second reverse shock and disappears eventually, and then the SNR becomes shell-like in both the equatorial and polar views in the later phase of the evolution.

X-ray emission from the remnant of a carbon deflagration supernova: SN 1572 (TYCHO)

A spherically symmetric hydrodynamic code is used to study the evolution of a young supernova remnant on the basis of a carbon deflagration model for type Ia supernovae. The nonequilibrium X-ray emission has been determined for the elemental composition of the model. The discrepancy between the derived intensity of the Fe D-alpha line blend and the observed value is eliminated by assuming that the stratification of the elemental composition in the supernova ejecta is partially removed by mixing. 59 references.

DISCOVERY OF ENHANCED RADIATIVE RECOMBINATION CONTINUA OF He-LIKE IRON AND CALCIUM FROM IC 443 AND ITS IMPLICATIONS

We present deep observations of the Galactic supernova remnant IC 443 with the Suzaku X-ray satellite. We find prominent K-shell lines from iron and nickel, together with a triangle residual at 8-10 keV, which corresponds to the energy of the radiative recombination continuum (RRC) of He-like iron. In addition, the wavy residuals have been seen at ∼5.1 and ∼5.5 keV. We confirm that the residuals show the first enhanced RRCs of He- and H-like calcium found in supernova remnants. These facts provide robust evidence for the recombining plasma. We reproduce the plasma in the 3.7-10 keV band using a recombining plasma model at the electron temperature 0.65 keV. The recombination parameter n {sub e} t (n {sub e} is electron density and t is elapsed time after formation of a recombining plasma) and abundances of iron and nickel are strongly correlated, and hence the errors are large. On the other hand, the ratio of nickel to iron relative to the solar abundances is well constrained to 11{sub −3}{sup +4} (1σ). A possibility is that the large abundance ratio is a result of an asymmetric explosion of the progenitor star.

Evidence for Resonance Line Scattering in the Suzaku X-Ray Spectrum of the Cygnus Loop

We present an analysis of the Suzaku observation of the northeastern rim of the Cygnus Loop supernova remnant. The high detection efficiency together with the high spectral resolution of the Suzaku X-ray CCD camera enables us to detect highly-ionized C and N emission lines from the Cygnus Loop. Given the significant plasma structure within the Suzaku field of view, we selected the softest region based on ROSAT observations. The Suzaku spectral data are well characterized by a two-component non-equilibrium ionization model with different best-fit values for both the electron temperature and ionization timescale. Abundances of C to Fe are all depleted t ot ypically 0.23 times solar with the exception of O. The abundance of O is relatively depleted by an additional factor of two compared with other heavy elements. We found that the resonance-line-scattering optical depth for the intense resonance lines of O is significant and, whereas the optical depth for other resonance lines is not as significant, it still needs to be taken into account for accurate abundance determination.

Core Structure of Intracluster Gas: Isothermal Hydrostatic Equilibrium

We investigated the core structure of intracluster hot gas based on the so-called β-model. We found that the virial temperature, Tvir, of a cluster may be represented better by βTX than TX ,w hereβ is a parameter obtained from the X-ray surface brightness, and TX is the emission-weighted mean temperature of the gas. For 121 clusters observed by ROSAT and ASCA, the luminosity–temperature relation, LX–βTX, is found to be less steep than LX–TX .T he clusters can be classified into large and small core groups, and the latter exhibits no significant correlation between the core and virial radii. We examined the properties of the small core clusters, and found that they have a shorter cooling time than the Hubble time, while no significant correlation was found with the presence of cD galaxies or asymmetry in the surface brightness. We carried out hydrodynamical calculations to simulate the β-model and confirmed the above results: Tvir � βT for the gas temperature, T, and the gas core formed due to a central cD galaxy is too small to account for the cores of the smaller core clusters.

Quasi-hydrostatic intracluster gas under radiative cooling

Quasi-hydrostatic cooling of the intracluster gas is studied. In the quasi-hydrostatic model, work done by gravity on the inflow gas with dP ¬= 0, where P is the gas pressure, is taken into account in the thermal balance. The gas flows in from the outer part so as to compensate the pressure loss of the gas undergoing radiative cooling, but the mass flow is so moderate and smooth that the gas is considered to be quasi-hydrostatic. The temperature of the cooling gas decreases toward the cluster center, but, unlike cooling flows with dP = 0, approaches a constant temperature of ∼1/3 the temperature of the non-cooling ambient gas. This does not mean that gravitational work cancels out radiative cooling, but means that the temperature of the cooling gas appears to approach a constant value toward the cluster center if the gas maintains the quasi-hydrostatic balance. We discuss the mass flow in quasi-hydrostatic cooling, and compare it with the standard isobaric cooling flow model. We also discuss the implication of M for the standard cooling flow model.

NON-LTE CURVES OF GROWTH OF X-RAY ABSORPTION LINES

Non-LTE line formation is studied in order to obtain the curve of growth of absorption K lines in X-ray wavelengths. We solve radiation transfer coupling with the statistical balance of atomic level populations. Our formulation covers level populations from the coronal limit through LTE, and can describe line formation from emission through absorption regimes consistently, depending on the optical thickness. It also seamlessly covers the classical division of lines into absorption and scattering categories. Using the Voigt function for the line profile, we calculate the absorption equivalent width and find that the curve of growth of X-ray lines deviates from the typical curve of optical or longer wavelength lines, although the deviation becomes small when the radiation field of the absorber is highly anisotropic. This is due to extremely large rates of spontaneous emission for X-ray lines, and photoexcitation is followed immediately by spontaneous emission. The level populations are far from their LTE values, and the absorption lines are dominated by scattering; while in optical lines, quasi-LTE can be attained and most of photons absorbed are destroyed (thermalized). Our concepts will be helpful for understanding line formation and in the quantitative study of absorption-line spectra of intervening gas with weak thermal coupling.

Discovery of an X-ray burst source XB 1715-321

A new X-ray burst source, XB 1715-321, was discovered with the Hakucho satellite. Three type I X-ray bursts were observed from this source in 1979. The error region for XB 1715-321, about 0.3 square degrees in area, includes a persistent X-ray source MX/2S 1715-321 which has been suspected to be the source of several fast transient events. Those bursts detected by Hakucho are characterized by a slow rise time (5--10 s) and a relatively long burst interval.