have different origins although it can not be concluded in the present.
be compatible with both SN models if the SN environment has a low density of . 0.03 cm−3 (Fe-core collapse SN) or .0.1 −3 (EC SN) for the uniform density, or a progenitor wind density somewhat less than that provided by a mass loss rate of 10−5 M⊙ yr−1at 20 km s−1 for the wind environment.
N132D: N132D is the X-ray brightest SNR in the Large Magellanic Cloud (LMC) with an age estimated to be∼2500 yr [197]. High-velocity ejecta were first detected and studied in optical wavelengths in N132D [e.g.,34,122,123] Optical/UV spectra from the Hubble Space Telescope show strong emission of C/Ne-burning elements (i.e., C, O, Ne, Mg), but little emission from O-burning elements (i.e., Si, S), leading to an interpretation of a Type Ib core-collapse supernova origin for this SNR [22].
Hitomi observed N132D with a very short exposure time of only 3.7 ks [69]. Despite such a short observation, theHitomi’s SXS easily detects the line complexes from highly ionized sulfur and iron with 16–17 counts in each. The Fe-K line was measured for the first time with the high spectral resolution.
Based on the plausible assumption that the Fe-K emission is dominated by He-like ions, it is found that the material responsible for this Fe emission is highly redshifted with the velocity of∼800 km s−1 compared to the local velocity in the LMC (Figure8.2b). This indicates that the Fe emission arises from the supernova ejecta, and that these ejecta are highly asymmetric, since no blue-shifted component is found. The results also show that even with a very small number of counts, direct velocity measurements from Doppler-shifted lines detected in extended objects like supernova remnants are now possible. Thanks to the very low SXS background of∼1 event per spectral resolution element per 100 ks, such results are obtainable during short pointed or slew observations with similar instruments.
As described above, even with a few SNR observations, the Hitomi satellite was able to provide us important results for understanding the progenitor stars of core-collapse SNRs. These observations highlight the power of high-spectral-resolution imaging observations, and demonstrates the new window that has been opened with Hitomiand will be greatly widened with future missions such as XARM.
Figure 8.3: (Left) The SXS simulation for the red- and blue-shifted blobs in Tycho’s SNR. The red and blue model show the red- and blue-shifted models, respectively. The Doppler velocities are∼ ±4,800 km s−1. The line width is assumed to be 12 eV (kTSi≈1.1 MeV). (Right) Close-up view around the Si-Lyα line. Color shows difference of the ion temperature (black = 1.1 MeV, red = 500 keV, green = 2 keV).
We expect that the future high-resolution X-ray spectroscopy of SNRs will further progress our dy-namical studies of young SNRs. In chapter4and5, we detected the Doppler-shifted lines from the highly ionized ejecta in Tycho’s SNR and Kepler’s SNR, which really opened the new kinetic studies in the Type
to the gain uncertainty of the CCD camera (∼500–2,000 km s−1) in the present. The high-resolution X-ray spectroscopy of XARM’s calorimeter will be able to measure the velocities with high accuracy. Figure 8.3 left shows the simulated spectrum by the X-ray calorimeter. Using its excellent energy resolution, we can easily separate the red- and blue-shift components and measure the Doppler velocities with the small systematic uncertainty in the future.
Also, such a high-resolution X-ray spectroscopy will help us to understand the SNR shock condition.
As reviewed in section2.3.4, it takes a long time for the SNR plasma to shift to the thermal equilibrium between ions and electrons, which is generally believed. However, it has been observationally unclear because of the difficulty of the ion temperature measurements. The ion temperature appears in the thermal broadening of line emission, and theHitomi’s SXS have already succeeded to measure it in the Perseus Cluster with the high accuracy [67]. In the case of SNRs, we can also measure it easily (Figure 8.3right). This study will be also related to understanding of the particle acceleration in the SNRs (see section7.4.1).
Chapter 9
Summary and Conclusions
We performed X-ray studies of young SNRs, Tycho’s SNR, Kepler’s SNR and Cassiopeia A, using the current X-ray observatories, Chandra, Suzaku and NuSTAR. The important results are summarized as follows:
1. For the type Ia SNRs of Tycho’s SNR and Kepler’s SNR, we have investigated the kinetics of the small-scale structures. Using both of the imaging and spectroscopy byChandra, we measured the radial velocities of these SNRs for the first time.
In the case of Tycho’s SNR, we performed detailed spectral fits on 27 blobs using nonequiliubrium ionization thermal plasma models. We succeed in separating these features cleanly into redshifted, blueshifted, and low velocity clumps of ejecta. The determination of velocities is shown to be robust with respect to other spectral fit parameters that can influence line centroids, such as the ionization age parameter. We conclude that the velocities of the redshifted and blueshifted blobs are.7,800 km s−1and.5,000 s−1, respectively. The results also suggest most of ejecta have similar velocities of a few thousands km s−1 and are expanding with almost like a shell geometry.
In the case of Kepler’s SNR, we reported measurements of proper motion, radial velocity, and elemental composition for 14 compact X-ray bright knots using archivalChandradata. The highest speed knots show both large proper motions (µ∼0.11–0.14′′yr−1) and high radial velocities (v∼ 8,700–10,020 km s−1). For these knots the estimated space velocities (9,100 km s−1.v3D.10,400 km s−1) are similar to the typical Si velocity seen in SN Ia near maximum light. High speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The five knots are expanding at close to the free expansion rate (expansion indices of 0.75.m .1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through relatively low density (nH < 0.1 cm−3) regions in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for SN Ia models. Other knots show slower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock.
2. We found simultaneous decrease of Fe-K line and 4.2-6 keV continuum of Cassiopeia A with the monitoring data taken by Chandra in 2000–2013. The flux change rates in the whole remnant
eastern region where the thermal emission is considered to dominate, the variations show the largest values: −1.03±0.05 % yr−1 (4.2-6 keV band) and −0.6±0.1 % yr−1 (Fe-K line). In this region, the time evolution of the emission measure and the temperature have a decreasing trend. This could be interpreted as the adiabatic cooling with the expansion ofm= 0.66. On the other hand, in the non-thermal emission dominated regions, the variations of the 4.2–6 keV continuum show the smaller rates: −0.60±0.04 % yr−1in the southwestern region,−0.46±0.05 % yr−1 in the inner region and +0.00±0.07 % yr−1 in the forward shock region. In particular, the flux does not show significant change in the forward shock region. These results imply that a strong braking in the shock velocity has not been occurring in Cassiopeia A (<5 km s−1yr−1). All of our results support that the X-ray flux decay in the remnant is mainly caused by the thermal components.
3. We present new evidence that the bright non-thermal X-ray emission features in the interior of the young SNR Cassiopeia A are caused by inward moving shocks based on Chandra and NuSTAR observations. Several bright inward-moving filaments were identified using monitoring data taken byChandra in 2000–2014. These inward-moving shock locations are nearly coincident with hard X-ray (15–40 keV) hot spots seen byNuSTAR. From proper motion measurements, the transverse velocities were estimated to be in the range∼2,100–3,800 km s−1for a distance of 3.4 kpc. The shock velocities in the frame of the expanding ejecta reach values of∼5,100–8,700 km s−1, slightly higher than the typical speed of the forward shock. Additionally, we find flux variations (both increasing and decreasing) on timescales of a few years in some of the inward-moving shock filaments. The rapid variability timescales are consistent with an amplified magnetic field ofB∼0.5–1 mG. The high speed and low photon cut-off energy of the inward-moving shocks are shown to imply a particle diffusion coefficient that departs from the Bohm regime (k0=D0/D0,Bohm∼3–8) for the few simple physical configurations we consider in this study. The maximum electron energy at these shocks is estimated to be ∼8–11 TeV, smaller than the values of ∼15–34 TeV inferred for the forward shock. Cassiopeia A is dynamically too young for its reverse shock to appear to be moving inward in the observer frame. We propose instead that the inward-moving shocks are a consequence of the forward shock encountering a density jump of&5–8 in the surrounding material.
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