arXiv subjects

Masayoshi Nobukawa

Publications and source records attributed to Masayoshi Nobukawa.

At least 73 records · Page 4Linked to original sources

arXiv2018

Origin of the low-temperature plasma in the Galactic center X-ray emission

The Galactic Center X-ray Emission (GCXE) is composed of high temperature (7 keV) and low temperature (1 keV) plasmas (HTP and LTP, respectively). The global structure of the HTP is roughly uniform over the Galactic center (GC) region, and the origin of the HTP has been extensively studied. On the other hand, the LTP is more clumpy, and the origin has not been studied in detail. In the S XV He alpha line map, a pair of horn-like soft diffuse sources are seen at the symmetric positions with respect to Sagittarius A^{star}. The X-ray spectra of the pair are well represented by an absorbed thin thermal plasma model of a temperature and N_H of 0.6-0.7 keV and 4x10^{22} cm^{-2}, respectively. The N_H values indicate that the pair are located near at the GC. Then the dynamical time scales of the pair are 10^{5} yr. The Si and S abundances and the surface brightnesses in the S XV He alpha line band are 0.7-1.2 and 0.6-1.3 solar, and (2.0-2.4)x10^{-15} erg s^{-1} cm^{-2} arcmin^{-2}, respectively. The temperature, abundances, and surface brightness are similar to those of the LTP in the GCXE, while the abundances are far larger than those of known point sources, typically coronal active stars and RS CVn-type active binaries. Based on these results, possible origin of the LTP is discussed.

astro-ph.HE↗

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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arXiv2018

Soft X-ray Imager aboard Hitomi (ASTRO-H)

The Soft X-ray Imager (SXI) is an imaging spectrometer using charge-coupled devices (CCDs) aboard the Hitomi X-ray observatory. The SXI sensor has four CCDs with an imaging area size of $31~{\rm mm} \times 31~{\rm mm}$ arranged in a $2 \times 2$ array. Combined with the X-ray mirror, the Soft X-ray Telescope, the SXI detects X-rays between $0.4~{\rm keV}$ and $12~{\rm keV}$ and covers a $38^{\prime} \times 38^{\prime}$ field-of-view. The CCDs are P-channel fully-depleted, back-illumination type with a depletion layer thickness of $200~μ{\rm m}$. Low operation temperature down to $-120~^\circ{\rm C}$ as well as charge injection is employed to reduce the charge transfer inefficiency of the CCDs. The functionality and performance of the SXI are verified in on-ground tests. The energy resolution measured is $161$-$170~{\rm eV}$ in full width at half maximum for $5.9~{\rm keV}$ X-rays. In the tests, we found that the CTI of some regions are significantly higher. A method is developed to properly treat the position-dependent CTI. Another problem we found is pinholes in the Al coating on the incident surface of the CCDs for optical light blocking. The Al thickness of the contamination blocking filter is increased in order to sufficiently block optical light.

astro-ph.IM↗

arXiv2018

Evidence for a neutral iron line generated by MeV protons from supernova remnants interacting with molecular clouds

Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. When low-energy cosmic-ray protons (LECRp) collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line (Fe I K$α$) at 6.40keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the $6^{\circ} \lesssim l \lesssim 40^{\circ}, |b| < 1^{\circ}$ region. All these SNRs interact with molecular clouds. We discover Fe I K$α$ line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe I K$α$ line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be $\gtrsim$ 10-100 eV cm$^{-3}$, which is more than 10 times higher than that in the ambient interstellar medium.

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arXiv2017

Discovery of 6.4 keV line and soft X-ray emissions from G323.7$-$1.0 with Suzaku

In this paper, the Suzaku X-ray data of the Galactic Supernova Remnant (SNR) candidate G323.7$-$1.0 are analyzed to search for X-ray emission. Spatially-extended enhancements in the 6.4 keV line and in soft X-rays are found inside or on the radio shell. The soft X-ray enhancement would be the hottest part of the shell-like X-ray emission along the radio shell. The 6.4 keV line enhancement is detected at a significance level of $4.1 σ$. The lower limit of the equivalent width (EW) is 1.2 keV. The energy centroid of the 6.4 keV line is $6.40 \pm 0.04$ keV, indicating that the iron is less ionized than the Ne-like state. If the 6.4 keV line originates from ionizing non-equilibrium thermal plasma, presence of iron-rich ejecta in a low-ionization state is required, which is disfavored by the relatively old age of the SNR. The 6.4 keV line enhancement would be due to K-shell ionization of iron atoms in a dense interstellar medium by high-energy particles. Since there is no irradiating X-ray source, the origin of the 6.4 keV line enhancement is not likely the photoionization. The large EW can only be explained by K-shell ionization due to cosmic-ray protons with an energy of $\sim 10$ MeV, which might be generated by the shock acceleration in G323.7$-$1.0.

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arXiv2017

Localized Recombining Plasma in G166.0+4.3: A Supernova Remnant with an Unusual Morphology

We observed the Galactic mixed-morphology supernova remnant G166.0+4.3 with Suzaku. The X-ray spectrum in the western part of the remnant is well represented by a one-component ionizing plasma model. The spectrum in the northeastern region can be explained by two components. One is the Fe-rich component with the electron temperature $kT_e = 0.87_{-0.03}^{+0.02}$ keV. The other is the recombining plasma component of lighter elements with $kT_e = 0.46\pm0.03$ keV, the initial temperature $kT_{init} = 3$ keV (fixed) and the ionization parameter $n_et = (6.1_{-0.4}^{+0.5}) \times 10^{11} \rm cm^{-3} s$. As the formation process of the recombining plasma, two scenarios, the rarefaction and thermal conduction, are considered. The former would not be favored since we found the recombining plasma only in the northeastern region whereas the latter would explain the origin of the RP. In the latter scenario, an RP is anticipated in a part of the remnant where blast waves are in contact with cool dense gas. The emission measure suggests higher ambient gas density in the northeastern region. The morphology of the radio shell and a GeV gamma-ray emission also suggest a molecular cloud in the region.

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arXiv2017

Origin of the Galactic Diffuse X-ray Emission: Iron K-shell Line Diagnostics

This paper reports detailed K-shell line profiles of iron (Fe) and nickel (Ni) of the Galactic Center X-ray Emission (GCXE), Galactic Bulge X-ray Emission (GBXE), Galactic Ridge X-ray Emission (GRXE), magnetic Cataclysmic Variables (mCVs), non-magnetic Cataclysmic Variables (non-mCVs), and coronally Active Binaries (ABs). For the study of the origin of the GCXE, GBXE, and GRXE, the spectral analysis is focused on equivalent widths of the Fe K$α$, Fe He$α$, and Fe Ly$α$ lines. The global spectrum of the GBXE is reproduced by a combination of the mCVs, non-mCVs, and ABs spectra. On the other hand, the GRXE spectrum shows significant data excesses at the Fe K$α$ and Fe He$α$ line energies. This means that additional components other than mCVs, non-mCVs, and ABs are required, which have symbiotic phenomena of cold gas and very high-temperature plasma. The GCXE spectrum shows larger excesses than those found in the GRXE spectrum at all the K-shell lines of iron and nickel. Among them the largest ones are the Fe K$α$, Fe He$α$, Fe Ly$α$, and Fe Ly$β$ lines. Together with the fact that the scale heights of the Fe K$α$, Fe He$α$, and Fe Ly$α$ lines are similar to that of the central molecular zone (CMZ), the excess components would be related to high-energy activity in the extreme envelopment of the CMZ.

astro-ph.HE↗

arXiv2016

Scale heights and equivalent widths of the iron K-shell lines in the Galactic diffuse X-ray emission

This paper reports the analysis of the X-ray spectra of the Galactic diffuse X-ray emission (GDXE) in the Suzaku archive. The fluxes of the Fe I K alpha (6.4 keV), Fe XXV,He alpha (6.7 keV) and Fe XXVI Ly alpha (6.97 keV) lines are separately determined. From the latitude distributions, we confirm that the GDXE is decomposed into the Galactic center (GCXE), the Galactic bulge (GBXE) and the Galactic ridge (GRXE) X-ray emissions. The scale heights (SHs) of the Fe XXV He alpha line of the GCXE, GBXE and GRXE are determined to be ~40, ~310 and ~140 pc, while those of the Fe I K alpha line are ~30, ~160 and ~70 pc, respectively. The mean equivalent widths (EWs) of the sum of the Fe XXV He alpha and Fe XXVI Ly alpha lines are ~750 eV, ~600 eV and ~550 eV, while those of the Fe I K alpha line are ~150~eV, ~60~eV and ~100~eV for the GCXE, GBXE and GRXE, respectively. The origin of the GBXE, GRXE and GCXE is separately discussed based on the new results of the SHs and EWs, in comparison with those of the Cataclysmic Variables (CVs), Active Binaries (ABs) and Coronal Active stars (CAs).

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arXiv2015

A Chandrasekhar Mass Progenitor for the Type Ia Supernova Remnant 3C 397 from The Enhanced Abundances of Nickel and Manganese

Despite decades of intense efforts, many fundamental aspects of Type Ia supernova (SNe Ia) remain elusive. One of the major open questions is whether the mass of the exploding white dwarf (WD) is close to the Chandrasekhar limit. Here we report the detection of strong K-shell emission from stable Fe-peak elements in the Suzaku X-ray spectrum of the Type Ia supernova remnant (SNR) 3C 397. The high Ni/Fe and Mn/Fe mass ratios (0.11-0.24 and 0.018-0.033, respectively) in the hot plasma component that dominates the K-shell emission lines indicate a degree of neutronization in the SN ejecta which can only be achieved by electron captures in the dense cores of exploding WDs with a near-Chandrasekhar mass. This suggests a single-degenerate origin for 3C 397, since Chandrasekhar mass progenitors are expected naturally if the WD accretes mass slowly from a companion. Together with other results supporting the double-degenerate scenario, our work adds to the mounting evidence that both progenitor channels make a significant contribution to the SN Ia rate in star-forming galaxies.

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.

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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.

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arXiv2014

Discriminating the Progenitor Type of Supernova Remnants with Iron K-Shell Emission

Supernova remnants (SNRs) retain crucial information about both their parent explosion and circumstellar material left behind by their progenitor. However, the complexity of the interaction between supernova ejecta and ambient medium often blurs this information, and it is not uncommon for the basic progenitor type (Ia or core-collapse) of well-studied remnants to remain uncertain. Here we present a powerful new observational diagnostic to discriminate between progenitor types and constrain the ambient medium density of SNRs solely using Fe K-shell X-ray emission. We analyze all extant Suzaku observations of SNRs and detect Fe K alpha emission from 23 young or middle-aged remnants, including five first detections (IC 443, G292.0+1.8, G337.2-0.7, N49, and N63A). The Fe K alpha centroids clearly separate progenitor types, with the Fe-rich ejecta in Type Ia remnants being significantly less ionized than in core-collapse SNRs. Within each progenitor group, the Fe K alpha luminosity and centroid are well correlated, with more luminous objects having more highly ionized Fe. Our results indicate that there is a strong connection between explosion type and ambient medium density, and suggest that Type Ia supernova progenitors do not substantially modify their surroundings at radii of up to several parsecs. We also detect a K-shell radiative recombination continuum of Fe in W49B and IC 443, implying a strong circumstellar interaction in the early evolutionary phases of these core-collapse remnants.

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arXiv2012

X-ray Echo from the Sagittarius C Complex and 500-year Activity History of Sagittarius A*

This paper presents the Suzaku results obtained for the Sagittarius (Sgr) C region using the concept of X-ray reflection nebulae (XRNe) as the echo of past flares from the super massive black hole, Sgr A*. The Sgr C complex is composed of several molecular clouds proximately located in projected distance. The X-ray spectra of Sgr C were analyzed on the basis of a view that XRNe are located inside the Galactic center plasma X-ray emission with an oval distribution around Sgr A*. We found that the XRNe are largely separated in the line-of-sight position, and are associated with molecular clouds in different velocity ranges detected by radio observations. We also applied the same analysis to the Sgr B XRNe and completed a long-term light curve for Sgr A* occurring in the past. As a new finding, we determined that Sgr A* was experiencing periods of high luminosity already 500 years ago, which is longer than the previously reported value. Our results are consistent with a scenario that Sgr A* was continuously active with sporadic flux variabilities of Lx = 1-3 x 10^39 erg s^-1 in the past 50 to 500 years. The average past luminosity was approximately 4-6 orders of magnitude higher than that presently observed. In addition, two short-term flares of 5-10 years are found. Thus, the past X-ray flare should not be a single short-term flare, but can be interpreted as multiple flares superposed on a long-term high state.

astro-ph.GA↗

arXiv2012

The K-shell Line Distribution of Heavy Elements along the Galactic Plane Observed with Suzaku

We report the global distribution of the intensities of the K-shell lines from the He-like and H-like ions of S, Ar, Ca and Fe along the Galactic plane. From the profiles, we clearly separate the Galactic center X-ray emission (GCXE) and the Galactic ridge X-ray emission (GRXE). The intensity profiles of the He-like K$α$ lines of S, Ar, Ca and Fe along the Galactic plane are approximately similar with each other, while not for the H-like Ly$α$ lines. In particular, the profiles of H-like Ly$α$ of S and Fe show remarkable contrast; a large excess of Fe and almost no excess of S lines in the GCXE compared to the GRXE. Although the prominent K-shell lines are represented by $\sim$1 keV and $\sim$7 keV temperature plasmas, these two temperatures are not equal between the GCXE and GRXE. In fact, the spectral analysis of the GCXE and GRXE revealed that the $\sim$1 keV plasma in the GCXE has lower temperature than that in the GRXE, and vice versa for the $\sim$7 keV plasma.

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arXiv2012

Evidence for Recombining Plasma in the Supernova Remnant G346.6-0.2

We present the Suzaku results of the supernova remnant (SNR) G346.6-0.2. The X-ray emission has a center-filled morphology with the size of 6' x 8' within the radio shell. Neither an ionization equilibrium nor non-equilibrium (ionizing) plasma can reproduce the spectra remaining shoulder-like residuals in the 2-4 keV band. These structures are possibly due to recombination of free electrons to the K-shell of He-like Si and S. The X-ray spectra are well fitted with a plasma model in a recombination dominant phase. We propose that the plasma was in nearly full ionized state at high temperature of 5 keV, then the plasma changed to a recombining phase due to selective cooling of electrons to lower temperature of 0.3 keV. G346.6-0.2 would be in an epoch of the recombining phase.

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arXiv2011

New Evidence for High Activity of the Super-Massive Black Hole in our Galaxy

Prominent K-shell emission lines of neutral iron (hereafter, FeI-K) and hard-continuum X-rays were found from molecular clouds (MCs) in the Sagittarius B (Sgr B) region with the two separate Suzaku observations in 2005 and 2009. The X-ray flux of FeI-K decreased in correlation to the hard-continuum flux by factor of 0.4-0.5 in 4 years, which is nearly equal to the light-travelling across the MCs. The rapid and correlated time-variability, the equivalent width of FeI-K, and the K-edge absorption depth of FeI are consistently explained by "X-ray echoes" due to the fluorescent and Thomson-scattering of an X-ray flare from an external source. The required flux of the X-ray flare depends on the distance to the MCs and the duration time. The flux, even in the minimum case, is larger than those of the brightest Galactic X-ray sources. Based on these facts, we conclude that the super-massive black hole, Sgr A*, exhibited a big-flare about a few hundred years ago and the luminosity of higher than 4x10^39 erg s^{-1}. The "X-ray echo" from Sgr B, located at a few hundred light-years from Sgr A*, now arrived at the Earth.

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arXiv2011

Global Distribution of Fe K alpha Lines in the Galactic Center Region Observed with the Suzaku Satellite

We have surveyed spatial profiles of the Fe K$α$ lines in the Galactic center diffuse X-rays (GCDX), including the transient region from the GCDX to the Galactic ridge X-ray emission (GRXE), with the Suzaku satellite. We resolved Fe K$α$ line complex into three lines of Fe \emissiontype{I}, Fe \emissiontype{XXV} and Fe \emissiontype{XXVI} K$α$, and obtained their spatial intensity profiles with the resolution of $\sim \timeform{0D.1}$. We compared the Fe \emissiontype{XXV} K$α$ profile with a stellar mass distribution (SMD) model made from near infrared observations. The intensity profile of Fe \emissiontype{XXV} K$α$ is nicely fitted with the SMD model in the GRXE region, while that in the GCDX region shows $3.8\pm0.3$ $(\timeform{0D.2}<|l|<\timeform{1D.5})$ or $19\pm6$ $(|l|<\timeform{0D.2})$ times excess over the best-fit SMD model in the GRXE region. Thus Fe \emissiontype{XXV} K$α$ in the GCDX is hardly explained by the same origin of the GRXE. In the case of point source origin, a new population with the extremely strong Fe \emissiontype{XXV} K$α$ line is required. An alternative possibility is that the majority of the GCDX is truly diffuse optically thin thermal plasma.

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arXiv2011

K-shell Emission of Neutral Iron Line from Sgr B2 Excited by Subrelativistic Protons

We investigated the emission of K$α$ iron line from the massive molecular clouds in the Galactic center (GC). We assume that at present the total flux of this emission consists of time variable component generated by primary X-ray photons ejected by Sagittarius A$^\ast$ (Sgr A$^\ast$) in the past and a relatively weak quasi-stationary component excited by impact of protons which were generated by star accretion onto the central black hole. The level of background emission was estimated from a rise of the 6.4 keV line intensity in the direction of several molecular clouds, that we interpreted as a stage when the X-ray front ejected by Sgr A$^\ast$ entered into these clouds. The 6.4 keV emission before this intensity jump we interpreted as emission generated by subrelativistic cosmic rays there. The cross-section of K$α$ vacancies produced by protons differs from that of electrons or X-rays. Therefore, we expect that this processes can be distinguished from the analysis of the equivalent width of the iron line and time variations of the width can be predicted. The line intensity from the clouds depends on their distance from Sgr A$^\ast$ and the coefficient of spacial diffusion near the Galactic center. We expect that in a few years the line intensity for the cloud G\,0.11$-$0.11 which is relatively close to Sgr A$^\ast$ will decreases to the level $\lesssim$ 10% from its present value. For the cloud Sagittarius B2 (Sgr B2) the situation is more intricate. If the diffusion coefficient $D\gtrsim 10^{27}$ cm$^2$ s$^{-1}$ then the expected stationary flux should be about 10% of its level in 2000. In the opposite case the line intensity from Sgr B2 should drop down to zero because the protons do not reach the cloud.

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