arXiv subjects

Makoto Sawada

Publications and source records attributed to Makoto Sawada.

At least 37 records · Page 2Linked to original sources

arXiv2025

Measuring the asymmetric expansion of the Fe ejecta of Cassiopeia A with XRISM/Resolve

The expansion structure of supernova remnants (SNRs) is important for understanding not only how heavy elements are distributed into space, but also how supernovae explode. The ejecta expansion structure of the young core-collapse SNR Cas A is investigated, with Doppler parameter mapping of the Fe-K complex by the Resolve microcalorimeter onboard the X-ray Imaging and Spectroscopy Mission, XRISM. It is found that the Fe ejecta are blueshifted in the southeast (SE) and redshifted in the northwest (NW), indicating an incomplete shell structure, similar to the intermediate mass elements (IMEs), such as Si and S. The Fe has a velocity shift of $\sim1400$ km~s$^{-1}$ in the NW and $\sim2160$ km~s$^{-1}$ in the SE region, with the error range of a few 100s km~s$^{-1}$. These values are consistent with those for the IMEs in the NW region, whereas larger than those for the IMEs in the SE region, although the large error region prevented us from concluding which component has significantly higher velocity. The line broadening is larger in the center with values of $\sim$2000--3000~km~s$^{-1}$, and smaller near the edges of the remnant. The radial profiles of the Doppler shift and broadening of the IMEs and Fe indicate that the Fe ejecta may expand asymmetrically as IME ejacta, although the large error regions do not allow us to conclude it. Moreover, we see little bulk Doppler broadening of the Fe lines in the northeastern jet region whereas the IME lines exhibit significant broadening. No such narrow lines are detected in the NW region. These findings suggest an asymmetric expansion of the ejecta potentially driven by large-scale asymmetries originating from the supernova explosion. This interpretation aligns with the large-scale asymmetries predicted by models of neutrino-driven supernova explosions.

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arXiv2025

Evidence for Charge Exchange Emission in Supernova Remnant N132D from XRISM/Resolve Observations

XRISM has delivered one of its first light observations on N132D, the X-ray brightest supernova remnant in the Large Magellanic Cloud. Utilizing 193 ks of high-resolution X-ray spectroscopy data, we conduct a comprehensive search for charge exchange emission. By incorporating a charge exchange model into our spectral analysis, we observe an improvement in the fits of two weak features at 2.41 keV and 2.63 keV. These features, with a combined significance of 99.6%, are consistent with transitions from highly ionized silicon ions in high Rydberg states, which are unique indicators of charge exchange. Our analysis constrains the charge exchange flux to no more than 4% of the total source flux within the 1.7-3.0 keV band, and places an upper limit on the charge exchange interaction velocity at 450 km/s. This result supports ongoing shock-cloud interactions within N132D and highlights the unique capabilities of XRISM to probe the complex physical processes at play.

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arXiv2025

Development of the Timing System for the X-Ray Imaging and Spectroscopy Mission

This paper describes the development, design, ground verification, and in-orbit verification, performance measurement, and calibration of the timing system for the X-Ray Imaging and Spectroscopy Mission (XRISM). The scientific goals of the mission require an absolute timing accuracy of 1.0~ms. All components of the timing system were designed and verified to be within the timing error budgets, which were assigned by component to meet the requirements. After the launch of XRISM, the timing capability of the ground-tuned timing system was verified using the millisecond pulsar PSR~B1937+21 during the commissioning period, and the timing jitter of the bus and the ground component were found to be below $15~μ$s compared to the NICER (Neutron star Interior Composition ExploreR) profile. During the performance verification and calibration period, simultaneous observations of the Crab pulsar by XRISM, NuSTAR (Nuclear Spectroscopic Telescope Array), and NICER were made to measure the absolute timing offset of the system, showing that the arrival time of the main pulse with XRISM was aligned with that of NICER and NuSTAR to within $200~μ$s. In conclusion, the absolute timing accuracy of the bus and the ground component of the XRISM timing system meets the timing error budget of $500~μ$s.

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arXiv2025

Optimization of x-ray event screening using ground and in-orbit data for the Resolve instrument onboard the XRISM satellite

The XRISM (X-Ray Imaging and Spectroscopy Mission) satellite was successfully launched and put into a low-Earth orbit on September 6, 2023 (UT). The Resolve instrument onboard XRISM hosts an x-ray microcalorimeter detector, which was designed to achieve a high-resolution ($\leq$7 eV FWHM at 6 keV), high-throughput, and non-dispersive spectroscopy over a wide energy range. It also excels in a low background with a requirement of $< 2 \times 10^{-3}$ s$^{-1}$ keV$^{-1}$ (0.3--12.0 keV), which is equivalent to only one background event per spectral bin per 100 ks exposure. Event screening to discriminate x-ray events from background is a key to meeting the requirement. We present the result of the Resolve event screening using data sets recorded on the ground and in orbit based on the heritage of the preceding x-ray microcalorimeter missions, in particular, the Soft X-ray Spectrometer (SXS) onboard ASTRO-H. We optimize and evaluate 19 screening items of three types based on (1) the event pulse shape, (2) relative arrival times among multiple events, and (3) good time intervals. We show that the initial screening, which is applied for science data products in the performance verification phase, reduces the background rate to $1.8 \times 10^{-3}$ s$^{-1}$ keV$^{-1}$ meeting the requirement. We further evaluate the additional screening utilizing the correlation among some pulse shape properties of x-ray events and show that it further reduces the background rate particularly in the $<$2 keV band. Over 0.3--12 keV, the background rate becomes $1.0 \times 10^{-3}$ s$^{-1}$ keV$^{-1}$.

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arXiv2025

High count rate effects in event processing for XRISM/Resolve x-ray microcalorimeter

The spectroscopic performance of x-ray instruments can be affected at high count rates. The effects and mitigation in the optical chain, such as x-ray attenuation filters or de-focusing mirrors, are widely discussed, but those in the signal chain are not. Using the Resolve x-ray microcalorimeter onboard the XRISM satellite, we discuss the effects observed during high count rate measurements and how these can be modeled. We focus on three instrumental effects that impact performance at high count rate: CPU limit, pile up, and electrical cross talk. High count rate data were obtained during ground testing using the flight model instrument and a calibration x-ray source. A simulated observation of GX 13+1 is presented to illustrate how to estimate these effects based on these models for observation planning. The impact of these effects on high count rate observations is discussed.

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arXiv2025

High count rate effects in event processing for XRISM/Resolve x-ray microcalorimeter: I. Ground test

The spectroscopic performance of an X-ray microcalorimeter is compromised at high count rates. In this study, we utilize the Resolve X-ray microcalorimeter onboard the XRISM satellite to examine the effects observed during high count rate measurements and propose modeling approaches to mitigate them. We specifically address the following instrumental effects that impact performance: CPU limit, pile-up, and untriggered electrical cross talk. Experimental data at high count rates were acquired during ground testing using the flight model instrument and a calibration X-ray source. In the experiment, data processing not limited by the performance of the onboard CPU was run in parallel, which cannot be done in orbit. This makes it possible to access the data degradation caused by limited CPU performance. We use these data to develop models that allow for a more accurate estimation of the aforementioned effects. To illustrate the application of these models in observation planning, we present a simulated observation of GX 13+1. Understanding and addressing these issues is crucial to enhancing the reliability and precision of X-ray spectroscopy in situations characterized by elevated count rates.

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arXiv2024

Revisiting the Suzaku spectrum of the Galactic SNR W 49 B: non-detection of iron K-shell charge exchange emission and refined ejecta mass ratios of iron-group elements

The origin of the recombining plasma in several Galactic SNRs has been debated. A plausible mechanism would be a rapid cooling in the past, either by adiabatic or conductive process. A recent spectral study of W 49 B reported a possible charge exchange (CX) emission due to collisions between the shock-heated ejecta and external cold clouds, which could be a direct support for the conduction cooling scenario. However, a potentially large systematic uncertainty in the spectral analysis has not been examined. In this paper, we revisit the Suzaku spectrum of W 49 B with taking into account the systematic uncertainties in spectral codes and instrumental gain calibration. We find that the previously reported CX flux is fully attributable to dielectronic recombination satellite lines of high-shell transitions that are missing from the present version of the spectral codes. We also report refined Fe-group ejecta mass ratios, which, in comparison to those in the literatures, show a better agreement with theoretical expectations from nucleosynthesis models, either of Type Ia explosions or spherical core-collapse explosions.

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arXiv2024

Probing Shocked Ejecta in SN 1987A with XRISM-Resolve: the effects of the gate valve closed

Supernova (SN) 1987A is widely regarded as an excellent candidate for leveraging the capabilities of the freshly launched XRISM satellite. Recent researches indicate that the X-ray emission from SN 1987A will increasingly originate from its ejecta in the years to come. In a previous study, we thoroughly examined the proficiency of XRISM-Resolve in identifying signatures of shocked ejecta in SN 1987A, synthesizing the XRISM-Resolve spectrum based on a state-of-the-art magneto-hydrodynamic simulation. However, following the satellite's launch, a technical issue arose with the XRISM instrument's gate valve, which failed to open, thereby affecting observations with the Resolve spectrometer. Here, we update our analysis, reevaluating our diagnostic approach under the assumption that the gate valve remains closed. We find that, even with the reduced instrumental capabilities, it will be possible to pinpoint the ejecta contribution through the study of the line profiles in the XRISM-Resolve spectrum of SN 1987A.

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arXiv2024

Is the low-energy tail of shock-accelerated protons responsible for over-ionized plasma in supernova remnants?

Over-ionized, recombining plasma is an emerging class of X-ray bright supernova remnants (SNRs). This unique thermal state where the ionization temperature ($T_{\rm z}$) is significantly higher than the electron temperature ($T_{\rm e}$) is not expected from the standard evolution model assuming a point explosion in a uniform interstellar medium, requiring a new scenario for the dynamical and thermal evolution. A recently proposed idea attributes the over-ionization state to additional ionization contribution from the low-energy tail of shock-accelerated protons. However, this new scenario has been left untested, especially from the atomic physics point of view. We report calculation results of the proton impact ionization rates of heavy-element ions in ejecta of SNRs. We conservatively estimate the requirement for accelerated protons, and find that their relative number density to thermal electrons needs to be higher than $5~(T_{\rm e}/{\rm 1~keV})\%$ in order to explain the observed over-ionization degree at $T_{\rm z}/T_{\rm e} \ge 2$ for K-shell emission. We conclude that the proton ionization scenario is not feasible because such a high abundance of accelerated protons is prohibited by the injection fraction from thermal to non-thermal energies, which is expected to be $\sim 1\%$ at largest.

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arXiv2024

Missing Titanium in the Asymmetric Supernova Remnant W49B

The progenitor of the W49B supernova remnant is still under debate. One of the candidates is a jet-driven core-collapse supernova. In such a highly asymmetric explosion, a strong $α$-rich freezeout is expected in local high entropy regions, which should enrich elements synthesized by the capture of $α$-particles such as $^{44}$Ti and $^{48}$Cr (decaying to $^{44}$Ca and $^{48}$Ti, respectively). In the present work, in order to infer the progenitor of the W49B remnant, we constrain the amount of stable Ti ($^{48}$Ti) synthesized, using the {\it Suzaku} observation. We found no firm evidence for the Ti line and set the upper limit of $M_{\rm Ti}/M_{\rm Fe} < 8.2 \times$ 10$^{-4}$ (99\% limit using Xspec) and $M_{\rm Ti}/M_{\rm Fe} < 1.9 \times$ 10$^{-3}$ (99\% limit using SPEX), and thus excluded almost all hypernova/jet-driven supernova models. Our results, as complemented by some previous studies, suggest that a Type Ia supernova from a near-$M_{\rm Ch}$ (Chandrasekhar mass) white dwarf is the most favorable candidate for the origin of W49B. Future observations with X-ray calorimeter missions, such as XRISM, will give us a stronger constraint on the progenitor.

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arXiv2023

Probing Shocked Ejecta in SN 1987A: A novel diagnostic approach using XRISM-Resolve

Supernova (SN) 1987A is one of the best candidates to exploit the capabilities of the freshly launched XRISM satellite. This celestial object offers the unique opportunity to study the evolution of a SN into a young supernova remnant. To date, the X-ray emission has been dominated by the shocked circumstellar medium (CSM), with no shocked ejecta firmly detected. However, recent studies provide compelling evidence that in the forthcoming years the X-ray emission from SN 1987A will increasingly stem from the ejecta. Our aim is to assess the proficiency of XRISM-Resolve high resolution spectrometer in pinpointing signatures of the shocked ejecta in SN 1987A. Taking advantage of a self consistent state-of-art magneto-hydrodynamic simulation that describes the evolution from SN 1987A to its remnant, we synthesized the XRISM-Resolve spectrum of SN 1987A, as it would be collected in the allocated observation during the performance verification phase, which is foreseen for 2024. Our predictions clearly show the leading role of shocked ejecta in shaping the profile of the emission lines. The Doppler broadening associated with the bulk motion along the line of sight of the rapidly expanding ejecta is shown to increase the line widths well above the values observed so far. The quantitative comparison between our synthetic spectra and the XRISM spectra will enable us to establish a strong connection between the broadened line emission and the freshly shocked ejecta. This, in turn, will allow us to retrieve the ejecta dynamics and chemical composition from the X-ray emission.

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arXiv2022

X-ray spectra of the Fe-L complex III: systematic uncertainties in the atomic data

There has been a growing request from the X-ray astronomy community for a quantitative estimate of systematic uncertainties originating from the atomic data used in plasma codes. Though there have been several studies looking into atomic data uncertainties using theoretical calculations, in general, there is no commonly accepted solution for this task. We present a new approach for estimating uncertainties in the line emissivities for the current models of collisional plasma, mainly based upon dedicated analysis of observed high resolution spectra of stellar coronae and galaxy clusters. We find that the systematic uncertainties of the observed lines consistently show anti-correlation with the model line fluxes, after properly accounting for the additional uncertainties from the ion concentration calculation. The strong lines in the spectra are in general better reproduced, indicating that the atomic data and modeling of the main transitions are more accurate than those for the minor ones. This underlying anti-correlation is found to be roughly independent on source properties, line positions, ion species, and the line formation processes. We further apply our method to the simulated XRISM and Athena observations of collisional plasma sources and discuss the impact of uncertainties on the interpretation of these spectra. The typical uncertainties are 1-2% on temperature and 3-20% on abundances of O, Ne, Fe, Mg, and Ni.

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arXiv2021

Detailed Design of the Science Operations for the XRISM mission

XRISM is an X-ray astronomical mission by the JAXA, NASA, ESA and other international participants, that is planned for launch in 2022 (Japanese fiscal year), to quickly restore high-resolution X-ray spectroscopy of astrophysical objects. To enhance the scientific outputs of the mission, the Science Operations Team (SOT) is structured independently from the instrument teams and the Mission Operations Team. The responsibilities of the SOT are divided into four categories: 1) guest observer program and data distributions, 2) distribution of analysis software and the calibration database, 3) guest observer support activities, and 4) performance verification and optimization activities. As the first step, lessons on the science operations learned from past Japanese X-ray missions are reviewed, and 15 kinds of lessons are identified. Among them, a) the importance of early preparation of the operations from the ground stage, b) construction of an independent team for science operations separate from the instrument development, and c) operations with well-defined duties by appointed members are recognized as key lessons. Then, the team structure and the task division between the mission and science operations are defined; the tasks are shared among Japan, US, and Europe and are performed by three centers, the SOC, SDC, and ESAC, respectively. The SOC is designed to perform tasks close to the spacecraft operations, such as spacecraft planning, quick-look health checks, pre-pipeline processing, etc., and the SDC covers tasks regarding data calibration processing, maintenance of analysis tools, etc. The data-archive and user-support activities are covered both by the SOC and SDC. Finally, the science-operations tasks and tools are defined and prepared before launch.

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arXiv2020

X-ray spectra of the Fe-L complex II: atomic data constraints from EBIT experiment and X-ray grating observations of Capella

The Hitomi results for the Perseus cluster have shown that accurate atomic models are essential to the success of X-ray spectroscopic missions, and just as important as knowledge on instrumental calibration and astrophysical modeling. Preparing the models requires a multifaceted approach, including theoretical calculations, laboratory measurements, and calibration using real observations. In a previous paper, we presented a calculation of the electron impact cross sections on the transitions forming the Fe-L complex. In the present work, we systematically test the calculation against cross sections of ions measured in an electron beam ion trap experiment. A two-dimensional analysis in the electron beam energies and X-ray photon energies is utilized to disentangle radiative channels following dielectronic recombination, direct electron-impact excitation, and resonant excitation processes in the experimental data. The data calibrated through laboratory measurements are further fed into global modeling of the Chandra grating spectrum of Capella. We investigate and compare the fit quality, as well as sensitivity of the derived physical parameters to the underlying atomic data and the astrophysical plasma modeling. We further list the potential areas of disagreement between the observation and the present calculations, which in turn calls for renewed efforts in theoretical calculations and targeted laboratory measurements.

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arXiv2019

Unlocking the Capabilities of Future High-Resolution X-ray Spectroscopy Missions Through Laboratory Astrophysics

Thanks to high-resolution and non-dispersive spectrometers onboard future X-ray missions such as XRISM and Athena, we are finally poised to answer important questions about the formation and evolution of galaxies and large-scale structure. However, we currently lack an adequate understanding of many atomic processes behind the spectral features we will soon observe. Large error bars on parameters as critical as transition energies and atomic cross sections can lead to unacceptable uncertainties in the calculations of e.g., elemental abundance, velocity, and temperature. Unless we address these issues, we risk limiting the full scientific potential of these missions. Laboratory astrophysics, which comprises theoretical and experimental studies of the underlying physics behind observable astrophysical processes, is therefore central to the success of these missions.

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arXiv2019

Ionization age of iron ejecta in the Galactic Type Ia supernova remnant G306.3-0.9

We present a 190 ks observation of the Galactic supernova remnant (SNR) G306.3-0.9 with Suzaku. To study ejecta properties of this possible Type Ia SNR, the absolute energy scale at the Fe-K band was calibrated to a level of uncertainty less than 10 eV by a cross-calibration with the Hitomi microcalorimeter using the Perseus cluster spectra. This enabled us for the first time to accurately determine the ionization state of the Fe K$α$ line of this SNR. The ionization timescale ($τ$) of the Fe ejecta was measured to be $\log_{10} τ$ (cm$^{-3}$ s) $=10.24\pm0.03$, significantly smaller than previous measurements. Marginally detected K$α$ lines of Cr and Mn have consistent ionization timescales with Fe. The global spectrum was well fitted with shocked interstellar matter (ISM) and at least two ejecta components with different ionization timescales for Fe and intermediate mass elements (IME) such as S and Ar. One plausible interpretation of the one-order-of-magnitude shorter timescale of Fe than that of IME ($\log_{10} τ= 11.17\pm0.07$) is a chemically stratified structure of ejecta. By comparing the X-ray absorption column to the HI distribution decomposed along the line of sight, we refined the distance to be $\sim$20 kpc. The large ISM-to-ejecta shocked mass ratio of $\sim$100 and dynamical timescale of $\sim$6 kyr place the SNR in the late Sedov phase. These properties are consistent with a stratified ejecta structure that has survived the mixing processes expected in an evolved supernova remnant.

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arXiv2018

Discovery of recombining plasma from the faintest GeV SNR HB 21 and a possible scenario of the cosmic ray escaping from SNR shocks

We present an X-ray study of the GeV gamma-ray supernova remnant (SNR) HB 21 with Suzaku. HB 21 is interacting with molecular clouds and the faintest in the GeV band among known GeV SNRs. We discovered strong radiative recombination continua of Si and S from the center of the remnant, which provide the direct evidence of a recombining plasma (RP). The total emission can be explained with the RP and ionizing plasma components. The electron temperature and recombination timescale of the RP component were estimated as 0.17 (0.15-0.18) keV and 3.2 (2.0-4.8) $\times$ 10$^{11}$ s cm$^{-3}$, respectively. The estimated age of the RP (RP age; $\sim$ 170 kyr) is the longest among known recombining GeV SNRs, because of very low density of electrons ($\sim$ 0.05 cm$^{-3}$). We have examined dependencies of GeV spectral indices on each of RP ages and SNR diameters for nine recombining GeV SNRs. Both showed possible positive correlations, indicating that both the parameters can be good indicators of properties of accelerated protons, for instance, degree of escape from the SNR shocks. A possible scenario for a process of proton escape is introduced; interaction with molecular clouds makes weaker magnetic turbulence and cosmic-ray protons escape, simultaneously cooling down the thermal electrons and generate an RP.

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arXiv2018

The Transition from Young to Middle-aged Supernova Remnants: Thermal and Nonthermal Aspects of SNR N132D

Supernova remnants (SNRs) are the primary candidate of Galactic cosmic-ray accelerators. It is still an open issue when and how young SNRs, which typically exhibit strong synchrotron X-rays and GeV and TeV gamma-rays, undergo the state transition to middle-aged SNRs dominated by thermal X-rays and GeV gamma-rays. The SNR N132D in the Large Magellanic Cloud is an ideal target to study such a transition, exhibiting bright X-rays and gamma-rays, and with the expected age of ~2500 yrs. In this paper we present results of NuSTAR and Suzaku spectroscopy. We reveal that N132D has a nearly equilibrium plasma with a temperature of > 5 keV or a recombining plasma with a lower temperature (~1.5 keV) and a recombining timescale (net) of 8.8 (7.0--10.0)e12 cm^-3s. Together with the center filled morphology observed in the iron K line image, our results suggest that N132D is now at transition stage from a young SNR to middle-aged. We have constrained the tight upper-limit of nonthermal X-rays. Bright gamma-rays compared to faint nonthermal X-rays suggest that the gamma-rays are hadronic in origin. The spectral energy distribution from radio to gamma-rays shows a proton cut-off energy of ~30 TeV. These facts confirm that N132D is in the transition from young to middle-aged SNR. The large thermal energy of > 10^51 erg and accelerated proton energy of ~ 10^50 erg suggest the supernova explosion might have been very energetic.

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