Takeshi Go Tsuru

Kyoto University, Kioto, Kyōto, Japan

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Publications (152)230.17 Total impact

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    ABSTRACT: 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$\alpha$ and K$\beta$. Moreover, we produced a fully depleted layer with a thickness of $500~{\rm \mu m}$. The event-driven readout mode has already been successfully demonstrated.
    08/2014;
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    ABSTRACT: We report the first results from high-statistics observation of the 6.4-keV line in the region of $l= +1.5^\circ$ to $+3.5^\circ$ (hereafter referred to as GC East), with the goal to uncover the origin of the Galactic ridge X-ray emission (GRXE). By comparing this data with that from the previous observations in the region $l=-1.5^\circ$ to $-3.5^\circ$ (hereafter referred to as GC West), we discovered that the 6.4-keV line is asymmetrically distributed with respect to the Galactic center, whereas the 6.7-keV line is symmetrically distributed. The distribution of the 6.4-keV line follows that of $^{13}$CO and its flux is proportional to the column density of the molecular gas. This correlation agrees with that seen between the 6.4-keV line and the cold interstellar medium (ISM) (H$_{\rm I}$ $+$ H$_2$) in the region $|l|>4^\circ$. This result suggests that the 6.4-keV emission is diffuse fluorescence from the cold ISM not only in GC East and West but also in the entire Galactic plane. This observational result suggests that the surface brightness of the 6.4-keV line is proportional to the column density of the cold ISM in the entire Galactic plane. For the ionizing particles, we consider X-rays and low energy cosmic-ray protons and electrons .
    08/2014;
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    ABSTRACT: 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.
    03/2014;
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    ABSTRACT: We present deep observations of the Galactic supernova remnant IC 443 with the Suzaku X-ray satellite. We find prominent K-shell lines from iron and nickel, together with a triangle residual at 8-10 keV, which corresponds to the energy of the radiative recombination continuum (RRC) of He-like iron. In addition, the wavy residuals have been seen at ~5.1 and ~5.5 keV. We confirm that the residuals show the first enhanced RRCs of He- and H-like calcium found in supernova remnants. These facts provide robust evidence for the recombining plasma. We reproduce the plasma in the 3.7-10 keV band using a recombining plasma model at the electron temperature 0.65 keV. The recombination parameter n et (n e is electron density and t is elapsed time after formation of a recombining plasma) and abundances of iron and nickel are strongly correlated, and hence the errors are large. On the other hand, the ratio of nickel to iron relative to the solar abundances is well constrained to 11 (1σ). A possibility is that the large abundance ratio is a result of an asymmetric explosion of the progenitor star.
    The Astrophysical Journal 03/2014; 784(1):74. · 6.73 Impact Factor
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    ABSTRACT: We report the detection of a distant star-forming galaxy, ALMA J010748.3─173028, which is identified by a 13σ emission line at 99.75 GHz (SΔv = 3.1 Jy km s─1), behind the nearby merging galaxies VV114 using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3. We also find an 880 μm counterpart with ALMA Band 7 (S 880μm = 11.2 mJy). A careful comparison of the intensities of the line and the continuum suggests that the line is a redshifted 12CO transition. A photometric redshift analysis using the infrared to radio data favors a CO redshift of z = 2.467, although z = 3.622 is acceptable. We also find a hard X-ray counterpart, suggesting the presence of a luminous (L X ~ 1044 erg s─1) active galactic nucleus obscured by a large hydrogen column (N H ~ 2 × 1023 cm─2 if z = 2.47). A cosmological simulation shows that the chance detection rate of a CO-emitting galaxy at z > 1 with >=1 Jy km s─1 is ~10─3 per single ALMA field of view and 7.5 GHz bandwidth at 99.75 GHz. This demonstrates that ALMA has sufficient sensitivity to find an emission-line galaxy such as ALMA J010748.3─173028 even by chance, although the likelihood of stumbling across such a source is not high.
    01/2014; 781(2).
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    ABSTRACT: We report the detection of a distant star-forming galaxy, ALMA J010748.3-173028, which is identified by a 13-sigma emission line at 99.75 GHz (SdV = 3.1 Jy km/s), behind the nearby merging galaxies VV114 using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3. We also find an 880-um counterpart with ALMA Band 7 (S_880um = 11.2 mJy). A careful comparison of the intensities of the line and the continuum suggests that the line is a redshifted 12CO transition. A photometric redshift analysis using the infrared to radio data favors a CO redshift of z = 2.467, although z = 3.622 is acceptable. We also find a hard X-ray counterpart, suggesting the presence of a luminous (L_X ~ 10^44 erg/s) active galactic nucleus obscured by a large hydrogen column (N_H ~ 2 x 10^23 cm^-2 if z = 2.47). A cosmological simulation shows that the chance detection rate of a CO-emitting galaxy at z > 1 with \ge 1 Jy km/s is ~ 10^-3 per single ALMA field of view and 7.5-GHz bandwidth at 99.75 GHz. This demonstrates that ALMA has sufficient sensitivity to find an emission-line galaxy such as ALMA J010748.3-173028 even by chance, although the likelihood of stumbling across such a source is not high.
    12/2013;
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    ABSTRACT: We have been developing active pixel sensors based on silicon-on-insulator technology for future X-ray astronomy missions. Recently we fabricated the new prototype named "XRPIX2", and investigated its spectroscopic performance. For comparison and evaluation of different chip designs, XRPIX2 consists of 3 pixel types: Small Pixel, Large Pixel 1, and Large Pixel 2. In Small Pixel, we found that the gains of the 68% pixels are within 1.4% of the mean value, and the energy resolution is 656 eV (FWHM) for 8 keV X-rays, which is the best spectroscopic performance in our development. The pixel pitch of Large Pixel 1 and Large Pixel 2 is twice as large as that of Small Pixel. Charge sharing events are successfully reduced for Large Pixel 1. Moreover Large Pixel 2 has multiple nodes for charge collection in a pixel. We confirmed that the multi-nodes structure is effective to increase charge collection efficiency.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 12/2013; · 1.14 Impact Factor
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    ABSTRACT: We report Suzaku results for soft X-ray emission to the south of the Galactic center (GC). The emission (hereafter "GC South") has an angular size of ~42' x 16' centered at (l, b) ~ (0.0, -1.4), and is located in the largely extended Galactic ridge X-ray emission (GRXE). The X-ray spectrum of GC South exhibits emission lines from highly ionized atoms. Although the X-ray spectrum of the GRXE can be well fitted with a plasma in collisional ionization equilibrium (CIE), that of GC South cannot be fitted with a plasma in CIE, leaving hump-like residuals at ~2.5 and 3.5 keV, which are attributable to the radiative recombination continua of the K-shells of Si and S, respectively. In fact, GC South spectrum is well fitted with a recombination-dominant plasma model; the electron temperature is 0.46 keV while atoms are highly ionized (kT = 1.6 keV) in the initial epoch, and the plasma is now in a recombining phase at a relaxation scale (plasma density x elapsed time) of 5.3 x 10^11 s cm^-3. The absorption column density of GC South is consistent with that toward the GC region. Thus GC South is likely to be located in the GC region (~8 kpc distance). The size of the plasma, the mean density, and the thermal energy are estimated to be 97 pc x 37 pc, 0.16 cm^-3, and 1.6 x 10^51 erg, respectively. We discuss possible origins of the recombination-dominant plasma as a relic of past activity in the GC region.
    The Astrophysical Journal 10/2013; 773(1). · 6.73 Impact Factor
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    ABSTRACT: We have been developing a monolithic active pixel sensor with the silicon-on-insulator (SOI) CMOS technology for use in future X-ray astronomical satellite missions. This sensor is called XRPIX. Our objective is to replace the X-ray CCD, which is currently the standard detector in the field, with the developed XRPIX, which offers high coincidence time resolution (~ 50 ns), superior hit-position readout time (~ 10 μs), and wide bandpass (0.5-40 keV), in addition to having comparable performance in terms of imaging spectroscopy. In our previous study, we built a prototype sensor called XRPIX1 and confirmed its basic X-ray imaging spectroscopy performance in a mode that read out the entire area (all pixels). The next step is to realize a high-speed, intelligent readout for X-ray detection. XRPIX1 comprises a trigger circuit for each pixel, so as to detect an X-ray photon injection; this system is capable of direct access to selected pixels to read out the signal amplitude. We describe the design of the trigger circuitry system and report on the first resolved X-ray spectra obtained in the trigger-driven readout mode.
    IEEE Transactions on Nuclear Science 04/2013; 60(2):586-591. · 1.22 Impact Factor
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    ABSTRACT: We have been developing monolithic active pixel sensors with 0.2 μm Silicon-On-Insulator (SOI) CMOS technology, called SOIPIX, for high-speed wide-band X-ray imaging spectroscopy on future astronomical satellites. In this work, we investigate a revised chip (XRPIX1b) for soft X-rays used in frontside illumination. The Al Kα line at 1.5 keV is successfully detected and energy resolution of 188 eV (FWHM) is achieved from a single pixel at this energy. The responsivity is improved to 6 μV/electron and the readout noise is 18 electrons rms. Data from 3 ×3 pixels irradiated with 6.4 keV (Fe Kα) X-rays demonstrates that the circuitry crosstalk between adjacent pixels is less than 0.5%.
    IEEE Transactions on Nuclear Science 02/2013; 60(1):465-469. · 1.22 Impact Factor
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    ABSTRACT: 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.
    Publications- Astronomical Society of Japan 11/2012; · 2.44 Impact Factor
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    ABSTRACT: The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV. These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3-2 keV with high spectral resolution of Delta E < 7 eV, enabled by a micro-calorimeter array located in the focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5-80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4-12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera type soft gamma-ray detector, sensitive in the 40-600 keV band. The simultaneous broad bandpass, coupled with high spectral resolution, will enable the pursuit of a wide variety of important science themes.
    10/2012;
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    ABSTRACT: We observed the supergiant fast X-ray transient AX J1841.0-0536 with Suzaku in 2011 March. Many short flares with time-scales of a few hundred seconds and a large flare with a peak flux (1.0-10 keV) of ≳; 2 × 10-10 erg s-1 cm-2 were detected. The broad-band (1.0-40 keV) spectrum was fitted with a cut-off power-law continuum plus a Kα line from a neutral iron absorbed by partial covering gas. In addition, a broad absorption feature was found in a high-energy band. The photon index of the power-law is 1.01 ± 0.12 in a low-luminosity state, and decreases (becomes hard) with increasing luminosity. On the other hand, the absorption column density and the iron line equivalent width are nearly constant. The covering fraction does not vary significantly, except for the full coverage epoch at the end of the large flare. We thus propose that the X-ray emission is due to subsequent in-falls of many small blobs.
    Publications- Astronomical Society of Japan 10/2012; · 2.44 Impact Factor
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    ABSTRACT: 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$\alpha$ lines of S, Ar, Ca and Fe along the Galactic plane are approximately similar with each other, while not for the H-like Ly$\alpha$ lines. In particular, the profiles of H-like Ly$\alpha$ 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.
    Publications- Astronomical Society of Japan 09/2012; · 2.44 Impact Factor
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    ABSTRACT: We report new features of the typical mixed-morphology (MM) supernova remnant (SNR) W44. In the X-ray spectra obtained with Suzaku, radiative recombination continua (RRCs) of highly ionized atoms are detected for the first time. The spectra are well reproduced by a thermal plasma in a recombining phase. The best-fit parameters suggest that the electron temperature of the shock-heated matters cooled down rapidly from $\sim1$,keV to $\sim 0.5$,keV, possibly due to adiabatic expansion (rarefaction) occurred $\sim20,000$ years ago. We also discover hard X-ray emission which shows an arc-like structure spatially-correlated with a radio continuum filament. The surface brightness distribution shows a clear anti-correlation with $^{12}$CO (J=2-1) emission from a molecular cloud observed with NANTEN2. While the hard X-ray is most likely due to a synchrotron enhancement in the vicinity of the cloud, no current model can quantitatively predict the observed flux.
    Publications- Astronomical Society of Japan 08/2012; · 2.44 Impact Factor
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    ABSTRACT: The high energy activity in the inner few degrees of the Galactic center is traced by diffuse radio, X-ray and gamma-ray emission. The physical relationship between different components of diffuse gas emitting at multiple wavelengths is a focus of this work. We first present radio continuum observations using Green Bank Telescope and model the nonthermal spectrum in terms of a broken power-law distribution of GeV electrons emitting synchrotron radiation. We show that the emission detected by Fermi is primarily due to nonthermal bremsstrahlung produced by the population of synchrotron emitting electrons in the GeV energy range interacting with neutral gas. The extrapolation of the electron population measured from radio data to low and high energies can also explain the origin of FeI 6.4 keV line and diffuse TeV emission, as observed with Suzaku, XMM-Newton, Chandra and the H.E.S.S. observatories. The inferred physical quantities from modeling multi-wavelength emission in the context of bremsstrahlung emission from the inner 300x120 parsecs of the Galactic center are constrained to have the cosmic ray ionization rate 1-10x10^{-15} s^-1, molecular gas heating rate elevating the gas temperature to 75-200K, fractional ionization of molecular gas 10^{-6} to 10^{-5}, large scale magnetic field 10-20 micro Gauss, the density of diffuse and dense molecular gas 100 and 10^3 cm^{-3} over 300pc and 50pc pathlengths, and the variability of FeI Kalpha 6.4 keV line emission on yearly time scales. Important implications of our study are that GeV electrons emitting in radio can explain the GeV gamma-rays detected by Fermi and that the cosmic ray irradiation model, like the model of the X-ray irradiation triggered by past activity of Sgr A*, can also explain the origin of the variable 6.4 keV emission from Galactic center molecular clouds.
    The Astrophysical Journal 06/2012; 762(1). · 6.73 Impact Factor
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    ABSTRACT: We have been developing a monolithic active pixel sensor with the 0.2 μm Silicon-On-Insulator (SOI) CMOS technology, called SOIPIX, for the wide-band X-ray imaging spectroscopy on future astronomical satellites. SOIPIX includes a thin CMOS-readout-array layer and a thick high-resistivity Si-sensor layer stacked vertically on a single chip. This arrangement allows for fast and intelligent readout circuitries on-chip, providing advantages over the charge-coupled device (CCD). We have designed and built a new SOIPIX prototype XRPIX1 for X-ray detection. XRPIX1 implements a correlated double sampling (CDS) readout circuit in each pixel to suppress the reset noise. We obtained an energy resolution of full width at half maximum of 1.2 keV (5.4%) at 22 keV with a chip having a 147 μm sensor depletion at a back bias of 100 V cooled to -50°C. Moreover, XRPIX1 offers intra-pixel hit trigger (timing) and two-dimensional hit-pattern (position) outputs. We also confirmed the trigger capability by irradiating a single pixel with laser light.
    IEEE Transactions on Nuclear Science 11/2011; · 1.22 Impact Factor
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    ABSTRACT: We analyze the diffuse Fe I K-alpha line generated in the diffuse interstellar molecular hydrogen by primary photons or subrelativistic protons injected by Sagittarius (Sgr) A*. We showed that unlike emission from compact molecular clouds, this emission can be permanently observed in the directions of the Galactic center. We conclude that the diffuse emission of 6.4 keV line observed at present is probably due to Fe I K-alpha vacancy production by primary photons if the X-ray luminosity of Sgr A* was about Lx ~ 10^39-10^40 erg/s. In principle these data can also be described in the framework of the model when the 6.4 keV line emission is generated by subrelativistic protons generated by accretion onto the central black hole but in this case extreme parameters of this model are necessary.
    Publications- Astronomical Society of Japan 09/2011; · 2.44 Impact Factor
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    ABSTRACT: 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.
    The Astrophysical Journal Letters 09/2011; 739(2). · 6.35 Impact Factor
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    ABSTRACT: Soft X-ray Imager (SXI) is a CCD camera onboard the ASTRO-H satellite which is scheduled to be launched in 2014. The SXI camera contains four CCD chips, each with an imaing aread of 31mmx31 mm, arrayed in mosaic, which cover the whole FOV area of 38'x38'. The SXI CCD of which model name is HPK Pch-NeXT4 is a P-channel type, back-illuminated, fully depleted device with a thickness of 200mum. We have developed an engineering model of the SXI camera body with coolers, and analog electronics for them. Combined with the bread board digital electronics, we succeeded in operation the whole the SXI system. The CCDs are cooled down to -120°C with this system, and X-rays from 55Fe sources are detected. Although optimization of the system is in progress, the energy resolution of typical 200 eV and best 156 eV (FWHM) at 5.9 keV are obtained. The readout noise is 10 e- to 15 e-, and to be improved its goal value of 5 e-. On-going function tests and environment tests reveal some issues to be solved until the producntion of the SXI flight model in 2012.
    Proc SPIE 09/2011;

Publication Stats

1k Citations
230.17 Total Impact Points

Institutions

  • 2001–2014
    • Kyoto University
      • • Division of Physics and Astronomy
      • • Department of Physics II
      Kioto, Kyōto, Japan
  • 2002
    • The University of Tokyo
      • Department of Astronomy
      Edo, Tōkyō, Japan