arXiv subjects

M. Sawada

Publications and source records attributed to M. Sawada.

12 recordsLinked to original sources

arXiv2025

A XRISM/Resolve view of the dynamics in the hot gaseous atmosphere of M87

The XRISM/Resolve microcalorimeter directly measured the gas velocities in the core of the Virgo Cluster, the closest example of AGN feedback in a cluster. This proximity allows us to resolve the kinematic impact of feedback on scales down to 5 kpc. Our spectral analysis reveals a high velocity dispersion of $\sigma_v$=262 (+45 / -38) km/s near the AGN, which steeply declines to ~60 km/s between 5 and 25 kpc in the northwest direction. The observed line-of-sight bulk velocity in all regions is broadly consistent with the central galaxy, M87, with a mild trend toward blueshifted motions at larger radii. Systematic uncertainties have been carefully assessed and do not affect the measurements. The central velocities, if attributed entirely to isotropic turbulence, correspond to a transonic ICM at sub-6 kpc scales with three-dimensional Mach number 0.69 (+0.14 / -0.11) and a non-thermal pressure fraction of 21 (+7 / -5)%. Simple models of weak shocks and sound waves and calculations assuming isotropic turbulence both support the hypothesis that the velocity field reflects a mix of shock-driven expansion and turbulence. Compared to other clusters observed by XRISM to date, M87's central region stands out as the most kinematically disturbed, exhibiting both the highest velocity dispersion and the largest 3D Mach number, concentrated at the smallest physical scales.

astro-ph.GA↗

arXiv2021

Intrinsic coercivity induced by valence fluctuations in $4f$-$3d$ intermetallic magnets

Temperature dependence of magnetization curves of well homogenized samples of Ce(Co$_{1-x}$Cu$_{x}$)$_5$ ($0\le x \le 0.7$), a family of representative $4f$-$3d$ intermetallic magnets found in rare-earth permanent magnets, is measured. A remarkable enhancement of intrinsic coercivity is observed with $x=0.3$ and $x=0.4$, persisting to higher temperatures. This experimental observation is theoretically attributed to an effect of electronic correlation among $4f$-electrons. That is, an intrinsic pinning happens originating in an anomalously enhanced magnetic anisotropy energy contributed by an order of magnitude stronger charge-transfer process between $4f$-electrons and $3d$-electrons, than the conventional crystal field effects. It is demonstrated that the $4f$-$3d$ charge-transfer process depends on the direction of magnetization in the middle of a crossover of the valence state of Ce between CeCu$_5$ with robust Ce$^{3+}$ and CeCo$_5$ with the mixed valence state.

cond-mat.str-el↗

arXiv2019

Disordered quantum spin state in the stripe lattice system consisting of triangular and square tilings

Quantum fluctuations originating phase competition or geometrical frustration of spins lead to novel states such as a quantum critical point and a quantum spin liquid where the strong quantum fluctuations suppress any ordered states even at 0 K. Utilizing site-selective NMR for a quasi-two dimensional organic conductor $λ$-(STF)$_2$GaCl$_4$, we investigate the non-magnetic insulating phase of the stripe lattice system consisting of triangular and square tilings. We found development of AF spin fluctuations with decreasing temperature. Regardless of large enhancement of spin-lattice relaxation rate $1/T_1$ owing to critical slowing down below 10 K, no long-range magnetic ordering was observed down to 1.63 K two orders of magnitude less than the exchange interaction $J/k_{\rm B} \simeq$ 194 K. Moreover, $1/T_1$ saturated below 3.5 K. These results are in stark contrast to observed behaviors so far in other non-magnetic ground states discussed in terms of spin liquids, demonstrating realization of an exotic quantum state accompanying quantum criticality.

cond-mat.str-el↗

arXiv2018

Crystalline structure and XMCD studies of Co40Fe40B20 grown on Bi2Te3, BiTeI and Bi2Se3

Epitaxial films of Co40Fe40B20 (further - CoFeB) were grown on Bi2Te3(001) and Bi2Se3(001) substrates by laser molecular beam epitaxy (LMBE) technique at 200-400C. Bcc-type crystalline structure of CoFeB with (111) plane parallel to (001) plane of Bi2Te3 was observed, in contrast to polycrystalline CoFeB film formed on Bi2Se3(001) at RT using high-temperature seeding layer. Therefore, structurally ordered ferromagnetic thin films were obtained on the topological insulator surface for the first time. Using high energy electron diffraction (RHEED) 3D reciprocal space mapping, epitaxial relations of main crystallographic axes for the CoFeB/ Bi2Te3 heterostructure were revealed. MOKE and AFM measurements showed the isotropic azimuthal in-plane behavior of magnetization vector in CoFeB/ Bi2Te3, in contrast to 2nd order magnetic anisotropy seen in CoFeB/Bi2Se3. XPS measurements showed more stable behavior of CoFeB grown on Bi2Te3 to the oxidation, in compare to CoFeB grown on Bi2Se3. XAS and XMCD measurements of both concerned nanostructures allowed calculation of spin and orbital magnetic moments for Co and Fe. Additionally, crystalline structure and XMCD response of the CoFeB/BiTeI and Co55Fe45/BiTeI systems were studied, epitaxial relations of main crystallographic axes were found, and spin and orbital magnetic moments were calculated.

cond-mat.mtrl-sci↗

arXiv2017

Unusual coexistence of negative/positive charge-transfer in mixed-valence Na$_x$Ca$_{1-x}$Cr$_2$O$_4$

We have investigated the electronic structure of Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ using x-ray absorption spectroscopy together with Anderson impurity model calculations with full multiplets. We show Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ taking a novel mixed-valence electronic state in which the positive charge-transfer (CT) and the negative self-doped states coexist. While CaCr$_2$O$_4$ (one end member) exhibits a typical CT nature with strong covalent character, Na substitution causes a self-doped state with an oxygen hole. In NaCr$_2$O$_4$ (the other end member), positive CT and negative self-doped states coexist with equal weight. This unusual electronic state is in sharp contrast to the conventional mixed-valence description, in which the ground state can be described by the mixture of Cr$^{3+}$ ($3d^3$) and Cr$^{4+}$ ($3d^2$).

cond-mat.str-el↗

arXiv2015

Simulating Cherenkov Telescope Array observation of RX J1713.7-3946

We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.

astro-ph.HE↗

arXiv2014

ASTRO-H White Paper - Older Supernova Remnants and Pulsar Wind Nebulae

Most supernova remnants (SNRs) are old, in the sense that their structure has been profoundly modified by their interaction with the surrounding interstellar medium (ISM). Old SNRs are very heterogenous in terms of their appearance, reflecting differences in their evolutionary state, the environments in which SNe explode and in the explosion products. Some old SNRs are seen primarily as a result of a strong shock wave interacting with the ISM. Others, the so-called mixed-morphology SNRs, show central concentrations of emission, which may still show evidence of emission from the ejecta. Yet others, the pulsar wind nebulae (PWNe), are seen primarily as a result of emission powered by a pulsar; these SNRs often lack the detectable thermal emission from the primary shock. The underlying goal in all studies of old SNRs is to understand these differences, in terms of the SNe that created them, the nature of the ISM into which they are expanding, and the fundamental physical processes that govern their evolution. Here we identify three areas of study where ASTRO-H can make important contributions. These are constraining abundances and physical processes in mature limb-brightened SNRs, understanding the puzzling nature of mixed-morphology SNRs, and exploring the nature of PWNe. The Soft X-ray Spectrometer (SXS) on-board ASTRO-H will, as a result of its high spectral resolution, be the primary tool for addressing problems associated with old SNRs, supported by hard X-ray observations with the Hard X-ray Imager (HXI) to obtain broad band X-ray coverage.

astro-ph.HE↗

arXiv2014

ASTRO-H White Paper - Young Supernova Remnants

Thanks to the unprecedented spectral resolution and sensitivity of the Soft X-ray Spectrometer (SXS) to soft thermal X-ray emission, ASTRO-H will open a new discovery window for understanding young, ejecta-dominated, supernova remnants (SNRs). In particular we study how ASTRO-H observations will address, comprehensively, three key topics in SNR research: (1) using abundance measurements to unveil SNR progenitors, (2) using spatial and velocity distribution of the ejecta to understand supernova explosion mechanisms, (3) revealing the link between the thermal plasma state of SNRs and the efficiency of their particle acceleration.

astro-ph.HE↗

arXiv2014

ASTRO-H White Paper - New Spectral Features

This white paper addresses selected new (to X-ray astronomy) physics and data analysis issues that will impact ASTRO-H SWG observations as a result of its high-spectral-resolution X-ray microcalorimeter, the focussing hard X-ray optics and corresponding detectors, and the low background soft gamma-ray detector. We concentrate on issues of atomic and nuclear physics, including basic bound-bound and bound-free transitions as well as scattering and radiative transfer. The major topic categories include the physics of charge exchange, solar system X-ray sources, advanced spectral model, radiative transfer, and hard X-ray emission lines and sources.

astro-ph.HE↗

arXiv2013

Development of the Photomultiplier-Tube Readout System for the CTA Large Size Telescope

We have developed a prototype of the photomultiplier tube (PMT) readout system for the Cherenkov Telescope Array (CTA) Large Size Telescope (LST). Two thousand PMTs along with their readout systems are arranged on the focal plane of each telescope, with one readout system per 7-PMT cluster. The Cherenkov light pulses generated by the air showers are detected by the PMTs and amplified in a compact, low noise and wide dynamic range gain block. The output of this block is then digitized at a sampling rate of the order of GHz using the Domino Ring Sampler DRS4, an analog memory ASIC developed at Paul Scherrer Institute. The sampler has 1,024 capacitors per channel and four channels are cascaded for increased depth. After a trigger is generated in the system, the charges stored in the capacitors are digitized by an external slow sampling ADC and then transmitted via Gigabit Ethernet. An onboard FPGA controls the DRS4, trigger threshold, and Ethernet transfer. In addition, the control and monitoring of the Cockcroft-Walton circuit that provides high voltage for the 7-PMT cluster are performed by the same FPGA. A prototype named Dragon has been developed that has successfully sampled PMT signals at a rate of 2 GHz, and generated single photoelectron spectra.

astro-ph.IM↗

arXiv2010

The Suzaku Hard X-ray Survey on the Galactic Center Region

Diffuse X-rays from the Galactic center (GC) region were found to exhibit many K-shell lines from iron and nickel atoms in the 6--9 keV band. The strong emission lines seen in the spectrum are neutral iron K$α$ at 6.4~keV, He-like iron K$α$ at 6.7~keV, H-like iron Ly$α$ at 6.9~keV, and He-like iron K$β$ at 7.8~keV. Among them, the 6.4~keV emission line is a probe of non-thermal phenomena. We have detected strong 6.4~keV emission in several giant molecular clouds, some of which were newly discovered by Suzaku. All the spectra exhibit large equivalent widths of 1-2~keV and absorption columns of $2-10\times 10^{23}\{\rm H\ cm}^{-2}$. We found time variability of diffuse 6.4~keV emission in the Sgr B2 region comparing the maps and spectra obtained from 1994 to 2005 with ASCA, Chandra, XMM-Newton and Suzaku. We also report discovery of K$α$ lines of neutral argon, calcium, chrome, and manganese atoms in the Sgr~A region. We show that the equivalent width of the 6.4~keV emission line detected in X-ray faint region against the 6.4 keV-associated continuum (power-law component) is $\sim 800\ {\rm eV}$. These features are naturally explained by the X-ray reflection nebula scenario rather than the low energy cosmic-ray electrons scenario. On the other hand, a 6.4~keV clump, G~0.162$-$0.217, discovered at the south end of the Radio Arc has a small equivalent width of 6.4~keV emission line of $\sim200\ {\rm eV}$. The Radio Arc is a site of relativistic electrons. Thus, it is conceivable that the X-rays of G~0.162$-$0.217 are due to low energy cosmic-ray electrons

astro-ph.HE↗

arXiv2010

Searching for Diffuse Nonthermal X-Rays from the Superbubbles N11 and N51D in the Large Magellanic Cloud

We report on observations of the superbubbles (SBs) N11 and N51D in the Large Magellanic Cloud (LMC) with Suzaku and XMM-Newton. The interior of both SBs exhibits diffuse X-ray emission, which is well represented by thin thermal plasma models with a temperature of 0.2-0.3keV. The presence of nonthermal emission, claimed in previous works, is much less evident in our careful investigation. The 3-sigma upper limits of 2-10keV flux are 3.6*10^{-14}ergs/cm^2/s and 4.7*10^{-14}ergs/cm^2/s for N11 and N51D, respectively. The previous claims of the detection of nonthermal emission are probably due to the inaccurate estimation of the non X-ray background. We conclude that no credible nonthermal emission has been detected from the SBs in the LMC, with the exception of 30 Dor C.

astro-ph.HE↗