The bright non-thermal X-ray emission in the interior of Cassiopeia A has been one of the most enigmatic features of the remnant since the earliest observations by Chandra. Even as basic a fact as the type of shock (e.g., forward shock, reverse shock or something else) has remained obscure. In this paper, we put forth new evidence that the interior non-thermal emission originates in an “inward shock” through new analyses of archival Chandra andNuSTAR observations. We identified inward moving filaments in the remnant’s interior using monitoring data byChandrafrom 2000 to 2014. The inward-shock positions are spatially coincident with the most intense hard (15–40 keV band) X-ray emission seen withNuSTAR.
We measured the proper motions of the inward shocks, which equate to speeds of ∼2,100–3,800 km s−1for a distance of 3.4 kpc to the remnant. Assuming the shocks are propagating through the expanding ejecta (which is itself moving outward), we determine that the shock velocities in the frame of the ejecta could reach up to ∼5,100–8,700 km s−1, which is ∼ 1–2 times higher than that at the forward shock.
Additionally some of the inward-shock filaments showed flux variations (both increasing and decreasing) on timescales of just a few years. We find that the high shock velocity combined with a high magnetic field strength (∼0.5–1 mG) in the reverse shock region can explain the non-thermal properties well. At the same time we are able to constrain the diffusion coefficient and find that diffusion at the reverse shock is less efficient (k0&3) than that of the forward shock (k0.1.6). Expressed in terms of magnetic field turbulence, we find (as do others) that the turbulence at the forward shock approaches the Bohm limit, while at the inward shocks turbulence is less well developed.
As to the nature of the inward shocks, we propose that they are “reflection shocks” caused by the forward shock’s interaction with a density enhancement in the circumstellar medium. A density jump of a factor of &5–8 reproduces the observed inward-shock velocities. Previous works have shown evidence for a local molecular cloud on the western side of Cassiopeia A with some indications of an interaction beyween the cloud and the remnant’s shock. Further investigations into the shock-cloud interaction will be useful to deepen our understanding of particle acceleration in Cassiopeia A.
Chapter 8
Discussion: High-Resolution X-ray Spectroscopy and Future works
We here discuss the results related to the SNe and SNRs by the new Hitomi observations. The Hitomi satellite[177], was launched on 2016 February 17 (JST), is equipped with several instruments, covering a wide energy range from a few keV to 600 keV. It has four Wolter-I x-ray telescopes, two of which are the soft x-ray telescopes (SXTs[174]), covering up to∼12 keV. The SXT is composed of two independent units called SXT-I and SXT-S, which focus X-rays onto detectors of the CCD camera of the Soft X-ray Imaging system (SXI) and of the SXS calorimeter’s pixel array respectively. The angular resolution and effective area of the telescopes are ∼1.3′ and∼580 cm2, respectively[83,165,167,61,166].
TheHitomi’s SXS has the highest energy resolution (∆E≈5 eV in the 2–10 keV band [67]), which enabled us to measure the plasma state and element abundance much accurately. It really progressed our understanding of the SNe and SNRs even with a few observations. We show that a combination between the high-resolution X-ray spectroscopies and our dynamical studies of young SNRs is helpful to understand the progenitor systems, plasma state and shock physics of the SNRs. Also, we discuss the necessity of further SNR studies by the X-ray Astronomy Recovery Mission “XARM”1that has the same X-ray calorimeter as theHitomi mission.
8.1 Progenitor Systems of Type Ia SNe and SNRs
The SNe Ia produce most of iron-peak elements (chromium, manganese, iron and nickel) in the Universe.
The amount of the synthesized iron-peak elements in the SNe Ia are thought to vary with difference of the progenitor systems. Therefore, revealing the element production by each progenitor is very important to understand how the current cosmic abundance built up by the supernova nucleosynthesis. In addition, the amount of the synthesized iron-peak elements is directly related to the luminosity of the SNe Ia. The SNe Ia are used as distance indicators for the cosmic expansion measurements owing to their uniform luminosities [e.g.,149,137]. On the other hand, Phillips [138] reported that the observed peak luminosity of SNe Ia varied by a factor of 3, which causes observational uncertainties for the distance measurements.
The variety of the luminosities implies the difference of the amount of the synthesized56Ni (→56Fe) among the SNe Ia [e.g., 115]. Therefore, deep understanding of the type Ia progenitors and their productions also helps us to improve their cosmological utility. However, it is not so easy to constrain the type Ia
a
b c
Figure 8.1: The metal abundance measurements of the Perseus Cluster byHitomi [71]. (a) Comparison between the observed abundances in the Perseus Cluster and theoretical calculations for the Fe-peak elements. The magenta arrows indicate the 1σlower limit of the XMM-Newton measurements for the 44 objects [117]. The blue, green, and gray regions represent the theoretical predictions for SNe Ia from the near-MCh delayed-detonation explosion [169], sub-MCh violent merger [129], and single sub-MCh WD [211], respectively. (b) The zoom-in spectrum of the Perseus Cluster in the 5.3–6.4 keV band byHitomi, where the emission from He-like Cr and Mn are detected. The red-shifted Fe I fluorescence from the AGN is resolved as well. (c) The zoom-in spectrum of the Perseus Cluster in the 7.4–8.0 keV band by Hitomi, highlighting the Ni XXVII resonance (w) line clearly separated from the stronger Fe XXV Heβ and other emission. This enables the first accurate measurement of the Ni abundance in a galaxy cluster.
For comparison, an XMM-Newton spectrum extracted from the same spatial region is shown as the blue data points.
progenitor systems and their productions observationally.
Using the element abundance of the hot plasma that permeates galaxy clusters, we can investigate the accumulation of heavy elements produced by billions of supernovae [e.g., 124, 161, 117]. This is a useful way for understanding the SN Ia progenitor systems. The recent research by Hitomihas revealed the solar abundance ratios of the iron-peak elements in the Perseus Cluster [71] (Figure8.1a), in contrast to previous observations [117]. They succeeded to detect the weak resonance lines from He-like Cr, Mn, and Ni clearly owing to theHitomi’s excellent energy resolution (Figure 8.1b and8.1c).
The solar abundance ratios could be explained by taking into account a combination of near- and sub-Chandrasekhar-mass type Ia supernova systems (Figure8.1a). Here, the main difference of the iron-peak-element productions between the near- and sub-Chandrasekhar-mass explosions is caused by the WD core’s density during the initial phase of the explosion. If the WD mass is close toMCh≈1.4M⊙, the electron capture (p+e− →n+νe) can take place in the highly dense core (ρ&108 g cm−3), which increases the production efficiency of neutron-rich species (e.g., Ni, Mn) [e.g., 85, 26]. In contrast, the sub-MCh explosion that can best reproduce the observed properties of SNe Ia require lower masses and central densities for the primary WD, and therefore predict lower yields of these species [e.g.,211,129].
We can not explain the solar abundance ratios by only one of either the near- or sub-MCh explosions.
Thus, the results strongly support the existence of both of the explosions in the Universe.
The neutron-rich species in the SNe Ia are a good indicator for distinguishing between the near-and sub-Chnear-andrasekhar-mass explosions as shown in theHitomi observations [see also 214]. In general, the near-Chandrasekhar-mass explosions are thought to be the results of the SD scenarios since aMCh
progenitor is naturally explained by the evolution of a WD slowly accreting mass from a non-degenerate companion [e.g., 58]. Using the features of the SNe Ia, we can also discuss the progenitor systems of individual SNRs. For example, the recent observation of the Type Ia SNR 3C 397 with Suzakushowed the high Ni/Fe and Mn/Fe mass ratios, which suggests the progenitor of 3C 397 must have had a mass very close to MCh [214]. Such a study for the other remnants will help us to understand the Type Ia progenitor systems and their element productions. On the other hand, the neutronization by the electron capture in the near-MCh SNe Ia is taken place only in the innermost ∼0.2 M⊙ of ejecta [26]. Since it usually takes at least several thousand years to heat the innermost ejecta, we need to observe evolved SNRs, which have already heated all of the ejecta, for the discussion on the production of the neutron-rich elements. However, there are not so many old SNRs of SNe Ia such as 3C 397. For discussing such an evolved Type Ia SNR, G337.2–0.7 (5,000–7,000 yr [143]), G344.7–0.1 (3,000–6,000 yr [212]) and G352.7–0.1 (∼5,000 yr [52]) would be useful candidates.
In chapter 4 and 5, we showed new results on the two young type Ia SNRs, Tycho’s SNR and Kepler’s SNR. These SNRs are dynamically young, so it is difficult to discuss the progenitor systems using the neutron-rich species. On the other hand, the detailed studies of the kinematics might be useful to constrain the explosion mechanism in the future. In particular, these SNRs showed quite different expansions in our studies, which might be a key for the constraint. Tycho’s SNR showed a uniform shell-like expansion [see also 207], which suggests a uniform ambient medium of the progenitor system.
Such a uniform medium around the progenitor system will support the DD scenario for Tycho’s SNR since the the system has no complex ambient medium unlike the SD scenario. Also, the recent study on the remnant [210] showed the lack of a surrounding Str¨omgren sphere made by steadily nuclear-burning white dwarfs, which supports that the remnant’s origin is the merger of a double white dwarf binary. On the other hand, Kepler’s SNR showed highly asymmetric expansion due to the asymmetric-distributed ambient medium of the progenitor system. Also, Katsuda et al. [93] suggested that Kepler’s SNR is the
have different origins although it can not be concluded in the present.