Hiroya Yamaguchi

Discriminating the Progenitor Type of Supernova Remnants with Iron K-Shell Emission

Supernova remnants (SNRs) retain crucial information about both their parent explosion and circumstellar material left behind by their progenitor. However, the complexity of the interaction between supernova ejecta and ambient medium often blurs this information, and it is not uncommon for the basic progenitor type (Ia or core-collapse) of well-studied remnants to remain uncertain. Here we present a powerful new observational diagnostic to discriminate between progenitor types and constrain the ambient medium density of SNRs using solely Fe K-shell X-ray emission. We analyze all extant Suzaku observations of SNRs and detect Fe Kα emission from 23 young or middle-aged remnants, including five first detections (IC 443, G292.0+1.8, G337.2-0.7, N49, and N63A). The Fe Kα centroids clearly separate progenitor types, with the Fe-rich ejecta in Type Ia remnants being significantly less ionized than in core-collapse SNRs. Within each progenitor group, the Fe Kα luminosity and centroid are well correlated, with more luminous objects having more highly ionized Fe. Our results indicate that there is a strong connection between explosion type and ambient medium density, and suggest that Type Ia supernova progenitors do not substantially modify their surroundings at radii of up to several parsecs. We also detect a K-shell radiative recombination continuum of Fe in W49B and IC 443, implying a strong circumstellar interaction in the early evolutionary phases of these core-collapse remnants.

A Chandrasekhar mass progenitor for the Type Ia supernova remnant 3C 397 from the enhanced abundances of nickel and manganese

Despite decades of intense efforts, many fundamental aspects of Type Ia supernovae (SNe Ia) remain elusive. One of the major open questions is whether the mass of an exploding white dwarf (WD) is close to the Chandrasekhar limit. Here, we report the detection of strong K-shell emission from stable Fe-peak elements in the Suzaku X-ray spectrum of the Type Ia supernova remnant (SNR) 3C 397. The high Ni/Fe and Mn/Fe mass ratios (0.11–0.24 and 0.018–0.033, respectively) in the hot plasma component that dominates the K-shell emission lines indicate a degree of neutronization in the supernova ejecta that can only be achieved by electron capture in the dense cores of exploding WDs with a near-Chandrasekhar mass. This suggests a single-degenerate origin for 3C 397, since Chandrasekhar mass progenitors are expected naturally if the WD accretes mass slowly from a companion. Together with other results supporting the double-degenerate scenario, our work adds to the mounting evidence that both progenitor channels make a significant contribution to the SN Ia rate in star-forming galaxies.

New Evidence for Efficient Collisionless Heating of Electrons at the Reverse Shock of a Young Supernova Remnant

Although collisionless shocks are ubiquitous in astrophysics, certain key aspects of them are not well understood. In particular, the process known as collisionless electron heating, whereby electrons are rapidly energized at the shock front, is one of the main open issues in shock physics. Here we present the first clear evidence for efficient collisionless electron heating at the reverse shock of Tycho’s supernova remnant (SNR), revealed by Fe-K diagnostics using high-quality X-ray data obtained by the Suzaku satellite. We detect K� (3p!1s) fluorescence emission from low-ionization Fe ejecta excited by energetic thermal electrons at the reverse shock front, which peaks at a smaller radius than Fe-K� (2p!1s) emission dominated by a relatively highly-ionized component. Comparison with our hydrodynamical simulations implies instantaneous electron heating to a temperature 1000 times higher than expected from Coulomb collisions alone. The unique environment of the reverse shock, which is propagating with a high Mach number into rarefied ejecta with a low magnetic field strength, puts strong constraints on the physical mechanism responsible for this heating, and favors a crossshock potential created by charge deflection at the shock front. Our sensitive observation also reveals that the reverse shock radius of this SNR is about 10% smaller than the previous measurement using

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

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 � 1 keV to � 0.5 keV, possibly due to adiabatic expansion (rarefaction), occurred � 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.

Asymmetric Ejecta Distribution in SN 1006

We present the results from deep X-ray observations ({approx}400 ks in total) of SN 1006 with Suzaku. The thermal spectrum from the entire supernova remnant (SNR) exhibits prominent emission lines of O, Ne, Mg, Si, S, Ar, Ca, and Fe. The observed abundance pattern in the ejecta components is in good agreement with that predicted by a standard model of Type Ia supernovae (SNe). The spatially resolved analysis reveals that the distribution of the O-burning and incomplete Si-burning products (Si, S, and Ar) is asymmetric, while that of the C-burning products (O, Ne, and Mg) is relatively uniform in the SNR interior. The peak position of the former is clearly shifted by 5 ({approx}3.2 pc at the distance of 2.2 kpc) to the southeast (SE) from the SNRs geometric center. Using the SNR age of {approx}1000 yr, we constrain that the velocity asymmetry (in projection) of the ejecta is {approx}3100 km s{sup -1}. The Fe abundance is also significantly higher in the SE region than in the northwest. Given that the non-uniformity is observed only in the heavier elements (Si through Fe), we argue that SN 1006 originates from an asymmetric explosion, as is expected from recent multidimensional simulations ofmorexa0» Type Ia SNe, although we cannot eliminate the possibility that inhomogeneous ambient medium had induced the apparent non-uniformity. Possible evidence for the Cr-K-shell line and line broadening in the Fe-K-shell emission is also found.«xa0less

Prospect of studying hard X- and gamma-rays from type Ia supernovae

We perform multi-dimensional, time-dependent radiation transfer simulations for hard X-ray and γ-ray emissions, following radioactive decays of 56Ni and 56Co, for two-dimensional delayed-detonation models of Type Ia supernovae (SNe Ia). The synthetic spectra and light curves are compared with the sensitivities of current and future observatories for an exposure time of 106 s. The non-detection of the γ-ray signal from SN 2011fe at 6.4 Mpc by SPI on board INTEGRAL places an upper limit on the mass of 56Ni of 1.0 M ☉, independently from observations in any other wavelengths. Signals from the newly formed radioactive species have not yet been convincingly measured from any SN Ia, but future X-ray and γ-ray missions are expected to deepen the observable horizon to provide high energy emission data for a significant SN Ia sample. We predict that the hard X-ray detectors on board NuStar (launched in 2012) or ASTRO-H (scheduled for launch in 2014) will reach to SNe Ia at ~15 Mpc, i.e., one SN every few years. Furthermore, according to the present results, the soft γ-ray detector on board ASTRO-H will be able to detect the 158xa0keV line emission up to ~25 Mpc, i.e., a few SNe Ia per year. Proposed next-generation γ-ray missions, e.g., GRIPS, could reach to SNe Ia at ~20-35 Mpc by MeV observations. Those would provide new diagnostics and strong constraints on explosion models, detecting rather directly the main energy source of supernova light.

UNRAVELING THE ORIGIN OF OVERIONIZED PLASMA IN THE GALACTIC SUPERNOVA REMNANT W49B

Recent observations have shown several supernova remnants (SNRs) have overionized plasmas, 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 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 argon. Using these results, we find that W49B is overionized in the west, with a gradient of overionization increasing 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.

Suzaku View of the Supernova Remnant RCW 86 : X-Ray Studies of Newly-Discovered Fe-Rich Ejecta

We report on results of imaging and spectral analysis of the supernova remnant (SNR) RCW 86 observed with Suzaku. The SNR is known to exhibit K-shell emission of low ionized Fe, possibly originating from supernova ejecta. We revealed the global distribution of the Fe-rich plasma in the entire remnant, for the first time; the Fe-K emission was clearly detected from the west, north, and south regions, in addition to the X-ray brighter shells of southwest and northeast, where the presence of the Fe-rich ejecta has already been reported. The spectrum of each region is well represented by a three-component model consisting of low- and high-temperature thermal plasmas and a non-thermal emission. The lower-temperature component, with elemental abundances of near the solar values, likely originates from the forward shocked interstellar medium, while the Fe-rich ejecta is described by the hotter plasma. From the morphologies of the forward and reverse shocks in the west region, the total ejecta mass is estimated to be 1-2M_sun for the typical explosion energy of ~ 1 x 10^{51} erg. The integrated flux of the Fe-K emission from the entire SNR roughly corresponds to a total Fe mass of about 1M_sun. Both of these estimates suggest a Type Ia supernova origin of this SNR. We also find possible evidence of an Fe-rich clump located beyond the forward-shock front in the north rim, which is reminiscent of ejecta knots observed in the Tycho and Vela SNRs.

ELEMENTAL ABUNDANCES IN THE POSSIBLE TYPE Ia SUPERNOVA REMNANT G344.7-0.1

Recent studies on the Galactic supernova remnant (SNR) G344.7-0.1 have commonly claimed its origin to be a core-collapse supernova (SN) explosion, based on its highly asymmetric morphology and/or proximity to a star-forming region. In this paper, however, we present an X-ray spectroscopic study of this SNR using Suzaku, which is supportive of a Type Ia origin. Strong K-shell emission from lowly ionized Fe has clearly been detected, and its origin is determined, for the first time, to be the Fe-rich SN ejecta. The abundance pattern is highly consistent with that expected for a somewhat-evolved Type Ia SNR. It is suggested, therefore, that the X-ray point-like source CXOU J170357.8-414302 located at the SNRs geometrical center is not associated with the SNR but is likely to be a foreground object. Our result further indicates that G344.7-0.1 is the first possible Type Ia SNR categorized as a member of the so-called mixed-morphology class. In addition, we have detected emission from He-like Al at {approx}1.6 keV, the first clear detection of this element in the spectrum of an extended X-ray source. The possible enhancement of the Al/Mg abundance ratio from the solar value suggests that the ambient interstellar medium has a relatively high metallicitymorexa0» (not less than 10% of the solar value). We also report the marginal detection of Cr and Mn, although the measured fluxes of these lines have large statistical and systematic uncertainties.«xa0less

New insights into SNR evolution revealed by the discovery of recombining plasmas

Abstract We report the discovery of recombining plasmas in three supernova remnants (SNRs) with the Suzaku X-ray astronomy satellite. During SNR’s evolution, the expanding supernova ejecta and the ambient matter are compressed and heated by the reverse and forward shocks to form an X-ray emitting hot plasma. Since ionization proceeds slowly compared to shock heating, most young or middle-aged SNRs have ionizing (underionized) plasmas. Owing to high sensitivity of Suzaku, however, we have detected radiative recombination continua (RRCs) from the SNRs IC 443, W49B, and G359.1–0.5. The presence of the strong RRC is the definitive evidence that the plasma is recombining (overionized). As a possible origin of the overionization, an interaction between the ejecta and dense circumstellar matter is proposed; the highly ionized gas was made at the initial phase of the SNR evolution in dense regions, and subsequent rapid adiabatic expansion caused sudden cooling of the electrons. The analysis on the full X-ray band spectrum of IC 443, which is newly presented in this paper, provides a consistent picture with this scenario. We also comment on the implications from the fact that all the SNRs having recombining plasmas are correlated with the mixed-morphology class.