arXiv subjects

Shigeo Yamauchi

Publications and source records attributed to Shigeo Yamauchi.

At least 37 records · Page 2Linked to original sources

arXiv2019

Neutral iron line in the supernova remnant IC 443 and implications for MeV cosmic rays

We report a discovery of bright blob-like enhancements of an Fe I K$α$ line in the northwest and the middle of the supernova remnant (SNR) IC 443. The distribution of the line emission is associated with molecular clouds interacting with the shock front, and is totally different from that of the plasma. The Fe I K$α$ line has a large equivalent width. The most plausible scenario for the origin of the line emission is that the MeV protons accelerated in the shell leak into the molecular clouds and ionize the Fe atoms therein. The observed Fe I K$α$ line intensity is consistent with the prediction of a theoretical model, in which MeV protons are accelerated along with GeV and TeV protons at the SNR.

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arXiv2019

X-ray spectra of Sgr A East and diffuse X-ray background near the Galactic center

This paper reports the analysis procedure and results of simultaneous spectral fits of the Suzaku archive data for Sagittarius (Sgr) A East and the nearby Galactic center X-ray emission (GCXE). The results are that the mixed-morphology supernova remnant Sgr A East has a recombining plasma (RP) with Cr and Mn He$α$ lines, and a power-law component (PL) with an Fe I K$α$ line. The nearby GCXE has a $\sim$1.5-times larger surface brightness than the mean GCXE far from Sgr A East, although the spectral shape is almost identical. Based on these results, we interpret that the origins of the RP and the PL with the Fe I K$α$ line are past big flares of Sgr A$^*$.

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arXiv2018

New Scenario of plasma evolution in IC 443

Most of young and middle-aged supernova remnants (SNRs) exhibit an ionizing plasma (IP), an ionizing process following a shock heated SNR gas. On the other hand, significant fractions of SNRs exhibit a recombining plasma (RP). The origin and the mechanisms of the RP, however, are not yet well understood. This paper proposes a new model that the RP is followed after the IP process taken at the first epoch of the SNR evolution. Using the high quality and wide band (0.6-10 keV) spectrum of IC 443, we nicely fitted with a model of two RP (two-RP model) plus a power law (PL) with an Fe I Kalpha line component. The ionization temperature in one RP monotonously increases from Ne-Ca, while that in the other RP shows a drastic increase from Cr-Ni. Origin and mechanism of the two-RP and PL with an Fe I Kalpha line components are possibly due to a different evolution of two plasmas and ionization by the low-energy cosmic ray.

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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.

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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

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.

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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

Suzaku observations of two diffuse hard X-ray source regions, G22.0+0.0 and G23.5+0.1

G22.0+0.0 and G23.5+0.1 are diffuse hard X-ray sources discovered in the ASCA Galactic Plane Survey. We present Suzaku results of spectral analysis for these sources. G22.0+0.0 is confirmed to be a largely extended emission. The spectra were represented by a highly absorbed power-law model with a photon index of 1.7+/-0.3 and a moderately absorbed thermal emission with a temperature of 0.34^{+0.11}_{-0.08} keV. The difference in the N_{H} values between the two components suggests that the thermal component is unrelated with the power-law component and is a foreground emission located in the same line-of-sight. G23.5+0.1 is an extended source with a size of 3'.5. The spectra were fitted with an absorbed power-law model with a photon index of 2.4^{+0.5}_{-0.4}. The spatial and spectral properties show that both are candidates of old pulsar wind nebulae (PWNe). In addition to the extended sources, we analyzed spectra of three point sources found in the observed fields. Based on the spectral features, we discuss origin of the sources.

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arXiv2015

Iron emission line from the spiral galaxy M101

Archival Suzaku data of the face-on spiral galaxy M101 were analyzed. An intense emission line at 6.72^{+0.10}_{-0.12} keV was detected in the central region. This line is identified with a K-line from He-like iron, which indicates the existence of a thin thermal plasma with a temperature of several keV. The iron line luminosity within the central 5 arcmin radius region is estimated to be (2-12)x10^{37} erg s^{-1}. The origin of the iron emission line is discussed.

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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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arXiv2013

X-Ray Emission from the Galactic Supernova Remnant G12.0-0.1

We present results of the Suzaku/XIS observation around the radio supernova remnant (SNR) G12.0-0.1. No significant diffuse emission extending in or along the radio shell was observed. Instead two compact X-ray sources, Suzaku J181205-1835 and Suzaku J181210-1842, were found in or near G12.0-0.1. Suzaku J181205-1835 is located at the northwest of the radio shell of G12.0-0.1. The X-ray profile is slightly extended over the point spread function of the Suzaku telescope. The X-ray spectrum has no line-like structure and is well represented by a power-law model with a photon index of 2.2 and an absorption column of N_{H}=4.9x10^{22} cm^{-2}. The distances of Suzaku J181205-1835 and G12.0-0.1 are estimated from the absorption column and the Sigma-D relation, respectively, and are nearly the same with each other. These results suggest that Suzaku J181205-1835 is a candidate of a pulsar wind nebula associated with G12.0-0.1. From its location, Suzaku J181210-1842 would be unrelated object to G12.0-0.1. The X-ray profile is point-like and the spectrum is thin thermal emission with Fe K-lines at 6.4, 6.7, and 6.97 keV, similar to those of cataclysmic variables.

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arXiv2013

Thermal and Non-thermal X-Rays from the Galactic Supernova Remnant G348.5+0.1

We report on Suzaku results of the two distinct regions in the Galactic supernova remnant G348.5+0.1: extended thermal X-rays ("soft diffuse") at the northeast region and non-thermal X-rays (CXOU J171419.8-383023) at the northwest region. The X-ray spectrum of the soft diffuse is fitted with neither an ionization equilibrium nor a non-equilibrium (ionizing) plasma model, leaving saw-teeth residuals in the 1.5-3 keV energy band. The residual structures can be produced when free electrons are recombined to the K-shells of highly ionized Mg and Si ions. In fact, the X-ray spectrum is nicely fitted with a recombination-dominant plasma model. We propose a scenario that the plasma in nearly full ionized state at high temperature quickly changed to a recombining phase due to selective cooling of electrons to a lower temperature of ~0.5 keV. The spectrum of CXOU J171419.8-383023 is well explained by a simple power-law model with a photon index of 1.9, nearly equal to the typical value of pulsar wind nebulae. Since the distance is estimated to be the same as that of the soft diffuse, we infer that both the soft diffuse and CXOU J171419.8-383023 are associated with the same object, SNR G348.5+0.1.

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arXiv2013

X-Ray Observation of the Galactic Supernova Remnant G355.6-0.0 with Suzaku

We present results of the Galactic supernova remnant (SNR) G355.6-0.0 observed with Suzaku. We resolved the diffuse emission detected with ASCA into two objects, G355.6-0.0 and a point-like source, Suzaku J173530-3236. The X-ray emission from G355.6-0.0 exhibits a center-filled morphology within the radio shell. The X-ray spectrum is well represented by a thin thermal plasma model with enhanced metal abundances. The spatial and spectral properties imply that G355.6-0.0 is a member of the mixed-morphology SNRs. The NH value of ~ 6 x 10^{22} cm^{-2} supports that G355.6-0.0 is a distant SNR. Suzaku J173530-3236 exhibits a hard X-ray spectrum with a strong Fe emission line, similar to those of cataclysmic variables. The NH value of ~ 3 x 10^{22} cm^{-2} is smaller than that of G355.6-0.0, and hence Suzaku J173530-3236 is located at the near side of G355.6-0.0.

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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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arXiv2010

Suzaku Observation of the Anomalous X-ray Pulsar 1E 1841-045

We report the results of a Suzaku observation of the anomalous X-ray pulsar (AXP) 1E 1841-045 at a center of the supernova remnant Kes 73. We confirmed that the energy-dependent spectral models obtained by the previous separate observations were also satisfied over a wide energy range from 0.4 to ~70 keV, simultaneously. Here, the models below ~10 keV were a combination of blackbody (BB) and power-law (PL) functions or of two BBs wit h different temperatures at 0.6 - 7.0 keV (Morii et al. 2003), and that above ~20 keV was a PL function (Kuiper Hermsen Mendez 2004). The combination BB + PL + PL was found to best represent the phase-averaged spectrum. Phase-resolved spectroscopy indicated the existence of two emission regions, one with a thermal and the other with a non-thermal nature. The combination BB + BB + PL was also found to represent the phase-averaged spectrum well. However, we found that this model is physically unacceptable due to an excessively large area of the emission region of the blackbody. Nonetheless, we found that the temperatures and radii of the two blackbody components showed moderate correlations in the phase-resolved spectra. The fact that the same correlations have been observed between the phase-averaged spectra of various magnetars (Nakagawa et al. 2009) suggests that a self-similar function can approximate the intrinsic energy spectra of magnetars below ~10 keV.

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arXiv2009

A New Supernova Remnant Candidate and an Associated Outflow in the Sagittarius C Region

We present the Suzaku results on a new candidate of a supernova remnant (SNR) in the Sagittarius C region. We detected diffuse X-rays of an elliptical shape (G359.41-0.12) and a chimney-like structure (the Chimney), both of which were fitted with a thin thermal the model of kBT ~1 keV temperature. The absorption columns are same between these two structures, indicating that both are located at the same distance in the same line of sight. The narrow band image and one-dimensional profile of S XV Kalpha at 2.45 keV show that the Chimney is emanating from G359.41-0.12. Therefore, these two sources are physically connected with each other. The sum of the thermal energies of the Chimney and G359.41-0.12 is estimated to be 1.4x10^50 erg, typical for a galactic SNR. G359.41-0.12 is likely to be a new SNR candidate and the Chimney is an associated outflow.

astro-ph.GA↗

arXiv2008

X-Ray Reflection Nebulae with Large Equivalent Widths of Neutral Iron Ka Line in the Sgr C Region

This paper reports on the first results of the Suzaku observation in the Sgr C region. We detected four diffuse clumps with strong line emission at 6.4keV, Ka from neutral or low-ionized Fe. One of them, M359.38-0.00, is newly discovered with Suzaku. The X-ray spectra of the two bright clumps, M359.43-0.07 and M359.47-0.15, after subtracting the Galactic center diffuse X-ray emission (GCDX), exhibit strong Ka line from FeI with large equivalent widths (EWs) of 2.0-2.2keV and clear Kb of FeI. The GCDX in the Sgr C region is composed of the 6.4keV- and 6.7keV-associated components. These are phenomenologically decomposed by taking relations between EWs of the 6.4keV and 6.7keV lines. Then the former EWs against the associated continuum in the bright clump regions are estimated to be 2.4(+2.3_-0.7)keV. Since the two different approaches give similar large EWs of 2keV, we strongly suggest that the 6.4keV clumps in the Sgr C region are due to X-ray reflection/fluorescence (the X-ray reflection nebulae).

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