arXiv subjects

Shinya Nakashima

Publications and source records attributed to Shinya Nakashima.

At least 19 recordsLinked to original sources

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.

astro-ph.IM↗

arXiv2019

MAXI/SSC All-sky maps from 0.7 keV to 4 keV

By accumulating data from the Solid-state Slit Camera (SSC) on board the MAXI mission for 2 years from 2009 to 2011, diffuse X-ray background maps were obtained in energies of 0.7--1.0, 1.0--2.0, and 2.0--4.0 keV. They are the first ones that were derived with a solid-state instrument, and to be compared with the previous ROSAT all sky survey result. While the SSC map in the highest energy band is dominated by point sources and the Galactic Diffuse X-ray emission, that in 0.7--1.0 keV reveals an extended X-ray structure, of which the brightness distribution is very similar to that observed with ROSAT about 20 years before. Like in the ROSAT result, the emission is dominated by a bright arc-like structure, which appears to be a part of a circle of $\sim 50^\circ$ radius centered at about $(l, b) \sim (340^\circ, 15^\circ)$. In addition, the SSC map suggests a fainter and larger ellipse, which is elongated in the north-south direction and roughly centered at the Galactic center. The spectrum of these structures is explained as thin thermal emission from a plasma, with a temperature of $\sim 0.31$ keV and an abundance of $\sim0.3$ Solar. Based on SSC observation conditions including the low Solar activity, the Solar Wind Charge Exchange signals are estimated to be negligible in the present SSC maps, as well as in the $>0.56$ keV ROSAT map. A brief discussion is given to the obtained results.

astro-ph.HE↗

arXiv2019

Comment on "An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre" (2019, Nature, 567, 34)

A recent article "An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre" (2019, Nature, 567, 34) reported the detection of chimney-like X-ray-emitting features above and below the Galactic Center from XMM-Newton observations. We note here that these features were already reported by our Suzaku papers: Nakashima et al. (2013, ApJ, 773, 20, arXiv:1310.4236) for the southern feature and Nakashima et al. (2019, ApJ, in press, arXiv:1903.02571) for the northern feature. In particular, Nakashima et al. (2013) show that the ionization state of the southern feature is not in collisional ionization equilibrium and is most likely in a recombining or over-ionized state, which suggests its origin in the Galactic Center about 0.1 Myr ago.

astro-ph.GA↗

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.

astro-ph.HE↗

arXiv2019

X-ray Observation of a Magnetized Hot Gas Outflow in the Galactic Center Region

We report the discovery of a $1^\circ$ scale X-ray plume in the northern Galactic Center (GC) region observed with Suzaku. The plume is located at ($l$, $b$) $\sim$ ($0\mbox{$.\!\!^\circ$}2$, $0\mbox{$.\!\!^\circ$}6$), east of the radio lobe reported by previous studies. No significant X-ray excesses are found inside or to the west of the radio lobe. The spectrum of the plume exhibits strong emission lines from highly ionized Mg, Si, and S that is reproduced by a thin thermal plasma model with $kT \sim 0.7$ keV and solar metallicity. There is no signature of non-equilibrium ionization. The unabsorbed surface brightness is $3\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ arcmin$^{-2}$ in the 1.5-3.0 keV band. Strong interstellar absorption in the soft X-ray band indicates that the plume is not a foreground source but is at the GC distance, giving a physical size of $\sim$100 pc, a density of 0.1 cm$^{-3}$, thermal pressure of $1\times10^{-10}$ erg cm$^{-3}$, mass of 600 $M_\odot$ and thermal energy of $7\times10^{50}$ erg. From the apparent association with a polarized radio emission, we propose that the X-ray plume is a magnetized hot gas outflow from the GC.

astro-ph.GA↗

arXiv2018

An X-ray spectroscopic search for dark matter and unidentified line signatures in the Perseus cluster with Hitomi

We present results of a search for unidentified line emission and absorption signals in the 2-12keV energy band of spectra extracted from Perseus Cluster core region observations obtained with the 5eV energy resolution Hitomi Soft X-ray Spectrometer. No significant unidentified line emission or absorption is found. Line flux upper limits (1 sigma per resolution element) vary with photon energy and assumed intrinsic width, decreasing from 100 photons cm$^{-2}$ s$^{-1}$ sr $^{-1}$ at 2keV to $<10$ cm$^{-2}$ s$^{-1}$ sr $^{-1}$ over most of the 5-10 keV energy range for a Gaussian line with Doppler broadening of 640 km/s. Limits for narrower and broader lines have a similar energy dependence and are systematically smaller and larger, respectively. These line flux limits are used to constrain the decay rate of hypothetical dark matter candidates. For the sterile neutrino decay rate, new constraints over the the mass range of 4-24 keV with mass resolution better than any previous X-ray analysis are obtained. Additionally, the accuracy of relevant thermal spectral models and atomic data are evaluated. The Perseus cluster spectra may be described by a composite of multi-temperature thermal and AGN power-law continua. Superposed on these, a few line emission signals possibly originating from unmodeled atomic processes (including Si XIV and Fe XXV) are marginally detected and tabulated. Comparisons with previous X-ray upper limits and future prospects for dark matter searches using high-energy resolution spectroscopy are discussed.

astro-ph.HE↗

arXiv2018

Suzaku X-ray observations of the mixed-morphology supernova remnant CTB 1

We present an X-ray study of the mixed-morphology supernova remnant CTB 1 (G116.9+0.2) observed with Suzaku. The 0.6-2.0 keV spectra in the northeast breakout region of CTB 1 are well represented by a collisional ionization-equilibrium plasma model with an electron temperature of ~ 0.3 keV, whereas those in the southwest inner-shell region can be reproduced by a recombining plasma model with an electron temperature of ~ 0.2 keV, an initial ionization temperature of ~ 3 keV, and an ionization parameter of ~ 9 $\times$ 10$^{11}$ cm$^{-3}$s. This is the first detection of the recombining plasma in CTB 1. The electron temperature in the inner-shell region decreases outwards, which implies that the recombining plasma is likely formed by the thermal conduction via interaction with the surrounding cold interstellar medium. The Ne abundance is almost uniform in the observed regions whereas Fe is more abundant toward the southwest of the remnant, suggesting an asymmetric ejecta distribution. We also detect a hard tail above the 2 keV band that is fitted with a power-law function with a photon index of 2-3. The flux of the hard tail in the 2-10 keV band is ~ 5 $\times$ 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ and is peaked at the center of CTB 1. Its origin is unclear but one possibility is a putative pulsar wind nebula associated with CTB 1.

astro-ph.HE↗

arXiv2018

Diffuse x-ray emission from the northern arc of loop I observed with suzaku

After discovery of the Fermi bubbles, giant structures observed in radio to X-rays have been discussed as possi- ble evidence of past activities in the Galactic Center (GC). We report here on the analysis of Suzaku data pointing around the Loop I arc. The diffuse X-ray emission was well represented by the three-component model: (1) an unabsorbed thermal plasma with kT ~ 0.1 keV either from the Local Hot Bubble (LHB) and/or solar wind charge exchange (SWCX), (2) an absorbed thermal plasma regarded as a contribution from the Loop I and the Galactic halo (GH), and (3) an absorbed power-law component representing the cosmic X-ray background. The temper- ature of the absorbed thermal plasma was clustered in a range of 0.30 +- 0.02 keV along Loop I ("ON" regions), whereas the temperature was about 20 % lower in the cavity adjacent to the bubbles and Loop I ("OFF" regions) with 0.24 +- 0.03 keV. The emission measure (EM) varied along the Galactic latitude, and was well correlated with the count rate variation as measured with the ROSAT in 0.75 keV band. Although the amount of neutral gas was not conclusive to constrain on the distance to Loop I, the observed EM values rule out a hypothesis that the structure is close to the Sun; we argue that the Loop I is a distant, kpc structure of the shock-heated GH gas. We discuss the origin of apparent mismatch in the morphologies of the Fermi bubbles and the Loop I arc, suggesting a two-step explosion process in the GC.

astro-ph.GA↗

arXiv2018

Spatial distribution of the Milky Way hot gaseous halo constrained by Suzaku X-ray observations

The formation mechanism of the hot gaseous halo associated with the Milky Way Galaxy is still under debate. We report new observational constraints on the gaseous halo using 107 lines-of-sight of the Suzaku X-ray observations at $75^{\circ} 15^{\circ}$ with a total exposure of 6.4 Ms. The gaseous halo spectra are represented by a single-temperature plasma model in collisional ionization equilibrium. The median temperature of the observed fields is 0.26 keV ($3.0\times10^6$ K) with a typical fluctuation of $\sim30$%. The emission measure varies by an order of magnitude and marginally correlates with the Galactic latitude. Despite the large scatter of the data, the emission measure distribution is roughly reproduced by a disk-like density distribution with a scale length of $\sim7$ kpc, a scale height of $\sim2$ kpc, and a total mass of $\sim5\times10^7$ $M_{\odot}$. In addition, we found that a spherical hot gas with the $β$-model profile hardly contributes to the observed X-rays but that its total mass might reach $\gtrsim10^9$ $M_{\odot}$. Combined with indirect evidence of an extended gaseous halo from other observations, the hot gaseous halo likely consists of a dense disk-like component and a rarefied spherical component; the X-ray emissions primarily come from the former but the mass is dominated by the latter. The disk-like component likely originates from stellar feedback in the Galactic disk due to the low scale height and the large scatter of the emission measures. The median [O/Fe] of $\sim0.25$ shows the contribution of the core-collapse supernovae and supports the stellar feedback origin.

astro-ph.GA↗

arXiv2018

X-ray and Gamma-ray Observations of the Fermi Bubbles and NPS/Loop I Structures

The Fermi bubbles were possibly created by large injections of energy into the Galactic Center (GC), either by an active galactic nucleus (AGN) or by nuclear starburst more than ~10 Myr ago. However, the origin of the diffuse gamma-ray emission associated with Loop I, a radio continuum loop spanning across 100 deg on the sky, is still being debated. The northern-most part of Loop I, known as the North Polar Spur (NPS), is the brightest arm and is even clearly visible in the ROSAT X-ray sky map. In this paper, we present a comprehensive review on the X-ray observations of the Fermi bubbles and their possible association with the NPS and Loop I structures. Using uniform analysis of archival Suzaku and Swift data, we show that X-ray plasma with kT ~ 0.3 keV and low metal abundance (Z ~ 0.2 Z_solar) is ubiquitous in both the bubbles and Loop I and is naturally interpreted as weakly shock-heated Galactic halo gas. However, the observed asymmetry of the X-ray-emitting gas above and below the GC has still not been resolved; it cannot be fully explained by the inclination of the axis of the Fermi bubbles to the Galactic disk normal. We argue that the NPS and Loop I may be asymmetric remnants of a large explosion that occurred before the event that created the Fermi bubbles, and that the soft gamma-ray emission from Loop I may be due to either pi^0 decay of accelerated protons or electron bremsstrahlung.

astro-ph.HE↗

arXiv2016

X-ray Performance of Back-Side Illuminated Type of Kyoto's X-ray Astronomical SOI Pixel Sensor, XRPIX

We have been developing X-ray SOI pixel Sensors, called "XRPIX", for future X-ray astronomy satellites that enable us to observe in the wide energy band of 0.5-40 keV. Since XRPIXs have the circuitry layer with a thickness of about 8 μm in the front side of the sensor, it is impossible to detect low energy X-rays with a front-illuminated type. So, we have been developing back-illuminated type of XRPIX with a less 1 μm dead layer in the back-side, which enables the sensitivity to reach 0.5 keV. We produced two types of back-side illuminated (BI) XRPIXs, one of which is produced in "Pizza process" which LBNL developed and the other is processed in the ion implantation and laser annealing. We irradiated both of the BI-XRPIXs with soft X-ray and investigate soft X-ray performance of them. We report results from soft X-ray evaluation test of the device.

astro-ph.IM↗

arXiv2016

Improvement of Spectroscopic Performance using a Charge-sensitive Amplifier Circuit for an X-Ray Astronomical SOI Pixel Detector

We have been developing monolithic active pixel sensors series, named "XRPIX," based on the silicon-on-insulator (SOI) pixel technology, for future X-ray astronomical satellites. The XRPIX series offers high coincidence time resolution ({\rm \sim}1 {\rm μ}s), superior readout time ({\rm \sim}10 {\rm μ}s), and a wide energy range (0.5--40 keV). In the previous study, we successfully demonstrated X-ray detection by event-driven readout of XRPIX2b. We here report recent improvements in spectroscopic performance. We successfully increased the gain and reduced the readout noise in XRPIX2b by decreasing the parasitic capacitance of the sense-node originated in the buried p-well (BPW). On the other hand, we found significant tail structures in the spectral response due to the loss of the charge collection efficiency when a small BPW is employed. Thus, we increased the gain in XRPIX3b by introducing in-pixel charge sensitive amplifiers instead of having even smaller BPW. We finally achieved the readout noise of 35 e{\rm ^{-}} (rms) and the energy resolution of 320 eV (FWHM) at 6 keV without significant loss of the charge collection efficiency.

astro-ph.IM↗

arXiv2015

Investigation of the Kyoto's X-ray Astronomical SOIPIXs with Double-SOI Wafer for Reduction of Cross-talks

We have been developing X-ray SOIPIXs, "XRPIX", for future X-ray astronomy satellites. XRPIX is equipped with a function of "event-driven readout", which allows us to readout signal hit pixels only and realizes a high time resolution ($\sim10μ{\rm s}$). The current version of XRPIX suffers a problem that the readout noise in the event-driven readout mode is higher than that in the the frame readout mode, in which all the pixels are read out serially. Previous studies have clarified that the problem is caused by the cross-talks between buried P-wells (BPW) in the sensor layer and in-pixel circuits in the circuit layer. Thus, we developed new XRPIX having a Double SOI wafer (DSOI), which has an additional silicon layer (middle silicon) working as an electrical shield between the BPW and the in-pixel circuits. After adjusting the voltage applied to the middle silicon, we confirmed the reduction of the cross-talk by observing the analog waveform of the pixel circuit. We also successfully detected $^{241}$Am X-rays with XRPIX.

physics.ins-det↗

arXiv2015

Metal Enrichment in the Fermi Bubbles as a Probe of Their Origin

The Fermi bubbles are gigantic gamma-ray structures in our Galaxy. The physical origin of the bubbles is still under debate. The leading scenarios can be divided into two categories. One is the nuclear star forming activity similar to extragalactic starburst galaxies and the other is the past active galactic nucleus (AGN) like activity of the Galactic center supermassive black hole. In this paper, we propose that metal abundance measurements will provide an important clue to probe their origin. Based on a simple spherically symmetric bubble model, we find that the generated metallicity and abundance pattern of the bubbles' gas strongly depend on assumed star formation or AGN activities. Star formation scenarios predict higher metallicities and abundance ratios of [O/Fe] and [Ne/Fe] than AGN scenarios do because of supernovae ejecta. Furthermore, the resultant abundance depends on the gamma-ray emission process because different mass injection histories are required for the different gamma-ray emission processes due to the acceleration and cooling time scales of non-thermal particles. Future X-ray missions such as ASTRO-H and Athena will give a clue to probe the origin of the bubbles through abundance measurements with their high energy resolution instruments.

astro-ph.HE↗

arXiv2014

Discovery of Recombining Plasma in the Supernova Remnant 3C 391

Recent X-ray study of middle-aged supernova remnants (SNRs) reveals strong radiative recombination continua (RRCs) associated with overionized plasmas, of which the origin still remains uncertain. We report our discovery of an RRC in the middle-aged SNR 3C 391. If the X-ray spectrum is fitted with a two-temperature plasma model in collisional ionization equilibrium (CIE), residuals of Si XIV Ly alpha line at 2.006 keV, S XVI Ly alpha line at 2.623 keV and the edge of RRC of Si XIII at 2.666 keV are found. The X-ray spectrum is better described by a composite model consisting of a CIE plasma and a recombining plasma (RP). The abundance pattern suggests that the RP is associated to the ejecta from a core-collapse supernova with a progenitor star of 15 solar mass. There is no significant difference of the recombining plasma parameters between the southeast region and the northwest region surrounded by dense molecular clouds. We also find a hint of Fe I K alpha line at 6.4 keV (~2.4 sigma detection) from the southeast region of the SNR.

astro-ph.HE↗

arXiv2014

X-ray Filament with a Strong 6.7 keV Line in the Galactic Center Region

An elongated X-ray source with a strong K-shell line from He-like iron (Fe XXVI) is found at (RA, Dec)_{J2000.0}=(17h44m00s.0, -29D13'40''.9) in the Galactic center region. The position coincides with the X-ray thread, G359.55+0.16, which is aligned with the radio non-thermal filament. The X-ray spectrum is well fitted with an absorbed thin thermal plasma (apec) model. The best-fit temperature, metal abundance, and column density are 4.1^{+2.7}_{-1.8} keV, 0.58^{+0.41}_{-0.32} solar, and 6.1^{+2.5}_{-1.3}x10^{22} cm^{-2}, respectively. These values are similar to those of the largely extended Galactic center X-ray emission.

astro-ph.HE↗

arXiv2014

Development and Performance of Kyoto's X-ray Astronomical SOI pixel (SOIPIX) sensor

We have been developing monolithic active pixel sensors, known as Kyoto's X-ray SOIPIXs, based on the CMOS SOI (silicon-on-insulator) technology for next-generation X-ray astronomy satellites. The event trigger output function implemented in each pixel offers microsecond time resolution and enables reduction of the non-X-ray background that dominates the high X-ray energy band above 5--10 keV. A fully depleted SOI with a thick depletion layer and back illumination offers wide band coverage of 0.3--40 keV. Here, we report recent progress in the X-ray SOIPIX development. In this study, we achieved an energy resolution of 300~eV (FWHM) at 6~keV and a read-out noise of 33~e- (rms) in the frame readout mode, which allows us to clearly resolve Mn-K$α$ and K$β$. Moreover, we produced a fully depleted layer with a thickness of $500~{\rm μm}$. The event-driven readout mode has already been successfully demonstrated.

astro-ph.IM↗

arXiv2014

Abundances in the Ejecta of Core Collapse Supernova Remnants, G350.1-0.3 and G349.7+0.2

We present Suzaku results of the two Galactic supernova remnants (SNRs), G350.1-0.3 and G349.7+0.2. We find Al and Ni K alpha lines from both the SNRs for the first time, in addition to previously detected K-shell lines of Mg, Si, S, Ar, Ca and Fe. The spectra are well described by two optically thin thermal plasmas: a low-temperature (low-kT) plasma in collisional ionization equilibrium and a high-temperature (high-kT) plasma in non-equilibrium ionization. Since the low-kT plasma has solar metal abundances, it is thought to be of interstellar medium origin. The high-kT plasma has super-solar abundances, hence it is likely to be of ejecta origin. The abundance patterns of the ejecta components are similar to those of core-collapse supernovae with the progenitor mass of ~15-25 M_solar for G350.1-0.3 and ~35-40 M_solar for G349.7+0.2. We find extremely high abundances of Ni compared to Fe (Z_Ni/Z_Fe ~8). Based on the measured column densities between the SNRs and the near sky background, we propose that G350.1-0.3 and G349.7+0.2 are located at the distance of 9+/-3 kpc and 12+/-5 kpc, respectively. Then the ejecta masses are estimated to be ~13 M_solar and ~24 M_solar for G350.1-0.3 and G349.7+0.2, respectively. These values are consistent with the progenitor mass of ~15-25 M_solar and ~35-40 M_solar for G350.1-0.3 and G349.7+0.2, respectively.

astro-ph.HE↗