Junko Hiraga

The University of Tokyo, Tōkyō, Japan

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Publications (82)111.79 Total impact

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    ABSTRACT: A largely extended X-ray emission was discovered around the pulsar PSR J1357-6429 with the Suzaku deep observations. The pulsar, whose characteristic age is 7.3 kyr, is located within the TeV gamma-ray source HESS J1356-645. The extended emission is found to have a 1sigma X-ray size of ~4 arcmin, or ~3 pc at 2.4 kpc, with a small offset from the pulsar. Its X-ray spectrum is well reproduced by a simple power-law model with a photon index of 1.70+0.07-0.06. No significant spatial variation was found for the X-ray photon index as a function of distance from the pulsar. We conclude that the extended emission is associated to the pulsar wind nebula of PSR J1357-6429. This is a new sample of largely extended nebulae around middle-aged pulsars. We discuss the evolution of this PWN according to the relic PWN scenario.
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    ABSTRACT: We report on Suzaku observations of large-scale X-ray structures possibly related with the Fermi Bubbles obtained in 2013 with a total duration of ~ 80 ks. The observed regions were the: (i) northern cap (N-cap; l ~ 0 deg, 45 deg < b < 55 deg) seen in the Mid-band (1.7-4.0 keV) map recently provided by MAXI-SSC and (ii) southeast claw (SE-claw; l ~ 10 deg, -20 deg < b < -10 deg) seen in the ROSAT all-sky map and MAXI-SSC Low-band (0.7-1.7 keV) map. In each region, we detected diffuse X-ray emissions which are represented by a three component plasma model consisting of an unabsorbed thermal component (kT ~ 0.1 keV) from the Local Bubble, absorbed kT = 0.30+/-0.05 keV emission representing the Galactic Halo, and a power-law component due to the isotropic cosmic X-ray background radiation. The emission measure of the GH component in the SE-claw shows an excess by a factor of ~ 2.5 over the surrounding emission at 2 deg away. We also found a broad excess in the 1.7-4.0 keV count rates across the N-cap after compiling other archival data from Suzaku and Swift. The spectral stacking analysis of the N-cap data indicates the presence of another thermal component with kT = 0.70 (+0.22,-0.11) keV. The temperature of kT ~ 0.3 keV of the Galactic Halo is higher than the ubiquitous value of kT ~ 0.2 keV near the Fermi Bubbles, and can be even higher (~ 0.7 keV). We discuss our findings in the context of bubble-halo interaction.
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    ABSTRACT: Ages of the magnetar 1E 2259+586 and the associated supernova remnant CTB 109 were studied. Analyzing the Suzaku data of CTB 109, its age was estimated to be ∼ 14 kyr, which is much younger than the measured characteristic age of 1E 2259+586, 230 kyr. This reconfirms the previously reported age discrepancy of this magnetar/remnant association, and suggests that the characteristic ages of magnetars are generally over-estimated as compared to their true ages. This discrepancy is thought to arise because the former are calculated without considering decay of the magnetic fields. This novel view is supported independently by much stronger Galactic-plane concentration of magnetars than other pulsars. The process of magnetic field decay in magnetars is mathematically modeled. It is implied that magnetars are much younger objects than previously considered, and can dominate new-born neutron stars.
    Publications- Astronomical Society of Japan 01/2015; DOI:10.1093/pasj/psu135 · 2.01 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 developed by the Institute of Space and Astronautical Science (ISAS), with a planned launch in 2015. The ASTRO-H mission is equipped with a suite of sensitive instruments with the highest energy resolution ever achieved at E > 3 keV and a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. The simultaneous broad band pass, coupled with the high spectral resolution of Delta E < 7 eV of the micro-calorimeter, will enable a wide variety of important science themes to be pursued. ASTRO-H is expected to provide breakthrough results in scientific areas as diverse as the large-scale structure of the Universe and its evolution, the behavior of matter in the gravitational strong field regime, the physical conditions in sites of cosmic-ray acceleration, and the distribution of dark matter in galaxy clusters at different redshifts.
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    ABSTRACT: Most supernova remnants (SNRs) are old, in the sense that their structure has been profoundly modified by their interaction with the surrounding interstellar medium (ISM). Old SNRs are very heterogenous in terms of their appearance, reflecting differences in their evolutionary state, the environments in which SNe explode and in the explosion products. Some old SNRs are seen primarily as a result of a strong shock wave interacting with the ISM. Others, the so-called mixed-morphology SNRs, show central concentrations of emission, which may still show evidence of emission from the ejecta. Yet others, the pulsar wind nebulae (PWNe), are seen primarily as a result of emission powered by a pulsar; these SNRs often lack the detectable thermal emission from the primary shock. The underlying goal in all studies of old SNRs is to understand these differences, in terms of the SNe that created them, the nature of the ISM into which they are expanding, and the fundamental physical processes that govern their evolution. Here we identify three areas of study where ASTRO-H can make important contributions. These are constraining abundances and physical processes in mature limb-brightened SNRs, understanding the puzzling nature of mixed-morphology SNRs, and exploring the nature of PWNe. The Soft X-ray Spectrometer (SXS) on-board ASTRO-H will, as a result of its high spectral resolution, be the primary tool for addressing problems associated with old SNRs, supported by hard X-ray observations with the Hard X-ray Imager (HXI) to obtain broad band X-ray coverage.
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    ABSTRACT: Thanks to the unprecedented spectral resolution and sensitivity of the Soft X-ray Spectrometer (SXS) to soft thermal X-ray emission, ASTRO-H will open a new discovery window for understanding young, ejecta-dominated, supernova remnants (SNRs). In particular we study how ASTRO-H observations will address, comprehensively, three key topics in SNR research: (1) using abundance measurements to unveil SNR progenitors, (2) using spatial and velocity distribution of the ejecta to understand supernova explosion mechanisms, (3) revealing the link between the thermal plasma state of SNRs and the efficiency of their particle acceleration.
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    ABSTRACT: We discuss the prospects for a progress to be brought by ASTRO-H in the understanding of the physics of particle acceleration in astrophysical environments. Particular emphasis will be put on the synergy with gamma-ray astronomy, in the context of the rapid developments of recent years. Selected topics include: shock acceleration in supernova remnants (SNRs) and in clusters of galaxies, and the extreme particle acceleration seen in gamma-ray binaries. Since the hydrodynamics and thermal properties of shocks in these objects are covered in other white papers, we focus on the aspects related to the process of particle acceleration. In the case of SNRs, we emphasize the importance of SXS and HXI observations of the X-ray emission of young SNRs dominated by synchrotron radiation, particularly SNR RX J1713.7-3946. We argue that the HXI observations of young SNRs, as a byproduct of SXS observations dedicated for studies of the shock dynamics and nucleosynthesis, will provide powerful constraints on shock acceleration theories. Also, we discuss gamma-ray binary systems, where extreme particle acceleration is inferred regardless of the nature (a neutron star or a black hole) of the compact object. Finally, for galaxy clusters, we propose searches for hard X-ray emission of secondary electrons from interactions of ultra-high energy cosmic rays accelerated at accretion shocks. This should allow us to understand the contribution of galaxy clusters to the flux of cosmic rays above 10^18 eV.
  • Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2014; 765:269-274. DOI:10.1016/j.nima.2014.05.091 · 1.32 Impact Factor
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    ABSTRACT: Magnetars are a special type of neutron stars, considered to have extreme dipole magnetic fields reaching ~1e+11 T. The magnetar 4U 0142+61, one of prototypes of this class, was studied in broadband X-rays (0.5-70 keV) with the Suzaku observatory. In hard X-rays (15-40 keV), its 8.69 sec pulsations suffered slow phase modulations by +/-0.7 sec, with a period of ~1.5 hours. When this effect is interpreted as free precession of the neutron star, the object is inferred to deviate from spherical symmetry by ~1.6e-4 in its moments of inertia. This deformation, when ascribed to magnetic pressure, suggests a strong toroidal magnetic field, ~1e+12 T, residing inside the object. This provides one of the first observational approaches towards toroidal magnetic fields of magnetars.
    Physical Review Letters 04/2014; 112(17). DOI:10.1103/PhysRevLett.112.171102 · 7.73 Impact Factor
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    ABSTRACT: 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.
    The Astrophysical Journal Letters 03/2014; 785(2). DOI:10.1088/2041-8205/785/2/L27 · 5.60 Impact Factor
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    ABSTRACT: We present here the observational results of Galactic SNRs focusing on their thin thermal emission. Their X-ray spectra give us a good diagnostics of the plasma. Generally, they cannot be fitted by using model spectra in collisional ionization equilibrium (CIE) condition. Some of them show gradients both in the temperature and in the abundance. The X-ray spectrum of young SNRs, like the Kepler's SNR, can be well fitted by the superposition of the fore shock component and the reverse shock component, both of which show gradients in various parameters: temperature, density, ionization parameter etc. Cassiopeia-A, one of the typical young SNR, shows not point symmetric but axial symmetric both in the intensity profile and in the line center energy profile. This suggests the Doppler motion of the plasma about several thousands km/sec. The Cygnus Loop, a middle-aged SNR, also showed gradients: the shell regions show low metal abundance and low temperature while the core region shows high metal abundance and high temperature. The plasma left in the core region must reflect the fossil of the supernova explosion. Based on the metal abundance, we conclude that the Cygnus Loop originated from the type II SN. The Rosat observation on the Vela SNR, another typical middle-aged SNR, revealed that there were several debris running over the shell which was formed by the strong shock propagated in the surrounding medium. ASCA confirmed the abundance anomaly suggesting that they were the fossil of the progenitor star. The plasma both in the young and in the middle-aged SNRs are in the mixing phase of the ejecta with the interstellar matter (ISM).
    Advances in Space Research 07/2013; 25(s 3–4):539–548. DOI:10.1016/S0273-1177(99)00796-6 · 1.24 Impact Factor
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    ABSTRACT: The supernova remnant (SNR) RX J0852-4622 (Vela Jr., G266.6-1.2) is one of the most important SNRs for investigating the acceleration of multi-TeV particles and the origin of Galactic cosmic rays because of its strong synchrotron X-ray and TeV gamma-ray emission, which show a shell-like morphology similar to each other. Using the XMM-Newton archival data consisting of multiple pointing observations of the northwestern rim of the remnant, we investigate the spatial properties of the nonthermal X-ray emission as a function of distance from an outer shock wave. All X-ray spectra are well reproduced by an absorbed power-law model above 2 keV. It is found that the spectra show gradual softening from a photon index 2.56 in the rim region to 2.96 in the interior region. We show that this radial profile can be interpreted as a gradual decrease of the cutoff energy of the electron spectrum due to synchrotron cooling. By using a simple spectral evolution model that includes continuous synchrotron losses, the spectral softening can be reproduced with the magnetic field strength in the post-shock flow to less than several tens of uG. If this is a typical magnetic field in the SNR shell, gamma-ray emission would be accounted for by inverse Compton scattering of high-energy electrons that also produce the synchrotron X-ray emission. Future hard X-ray imaging observations with Nustar and ASTRO-H and TeV gamma-ray observations with the Cherenkov Telescope Array (CTA) will allow to us to explore other possible explanations of the systematic softening of the X-ray spectra.
    Astronomy and Astrophysics 02/2013; 551. DOI:10.1051/0004-6361/201220525 · 4.48 Impact Factor
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    ABSTRACT: We report the discovery of an X-ray counterpart of the unidentified very high energy gamma-ray source HESS J1427-608. In the sky field coincident with HESS J1427-608, an extended source was found in the 2-8 keV band, and was designated as Suzaku J1427-6051. Its X-ray radial profile has an extension of sigma=0.9' +/- 0.1' if approximated by a Gaussian. The spectrum was well fitted by an absorbed power-law with N_H=(1.1 +/- 0.3) times 10^23 cm^-2, Gamma=3.1 +0.6/-0.5, and the unabsorbed flux F_X=(9 +4/-2) times 10^-13 erg s^-1 cm^-2 in the 2-10 keV band. Using XMM-Newton archive data, we found seven point sources in the Suzaku source region. However, because their total flux and absorbing column densities are more than an order of magnitude lower than those of Suzaku J1427-6051, we consider that they are unrelated to the Suzaku source. Thus, Suzaku J1427-6051 is considered to be a truly diffuse source and an X-ray counterpart of HESS J1427-608. The possible nature of HESS J1427-608 is discussed based on the observational properties.
    Publications- Astronomical Society of Japan 01/2013; DOI:10.1093/pasj/65.3.61 · 2.01 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.
    Proceedings of SPIE - The International Society for Optical Engineering 10/2012; DOI:10.1117/12.926190 · 0.20 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 imaging area of 31mm× 31 mm, arrayed in mosaic, which cover the whole FOV area of 38' × 38'. The SXI CCDs are a P-channel back-illuminated (BI) type with a depletion layer thickness of 200 μm. High QE of 77% at 10 keV expected for this device is an advantage to cover an overlapping energy band with the Hard X-ray Imager (HXI) onboard ASTRO-H. Verification with engineering model of the SXI has been performed since 2011. Flight model design was fixed and its fabrication has started in 2012.
    SPIE Astronomical Telescopes + Instrumentation; 09/2012
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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.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2011; DOI:10.1117/12.893400 · 0.20 Impact Factor
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    ABSTRACT: Recent X-ray observations, including in particular those with Suzaku covering a wide energy band, have much reinforced the high-magnetic-field interpretation of magnetars. These include the discovery of clear evolution of their wide-band spectra (Enoto et al. 2010), and detection of a hard-tail component from their weaker short bursts (Nakagawa et al., in prep.). Nevertheless, the origin of magnetars remains a big mystery. To address the issue, we extend our Key Project for Suzaku AO4, and attempt to conduct comprehensive studies from the following four approaches. (1) To measure magnetic fields of MAXI-detected transient X-ray pulsars (e.g., GX 304-1 by Mihara in this WS), and investigate their magnetic field distribution in the >5e12 G range. (2) To study SuperGiant Fast X-ray Transients (SFXTs), under a hypothesis that they are somewhat aged binary magnetars. We have already obtained new results by analyzing archival Suzaku data. (3) To search SNRs and other environments for new magnetar candidates (e.g., CXOU J171405.7 by Sato et al. 2010). (4) To investigate SNRs associated with magnetars, trying to find their distinct characteristics that can be associated with the birth of mangetars.
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    ABSTRACT: Today, "Anomalous X-ray Pulsars" (AXPs) and "Soft Gamma Repeaters" (SGRs) are generally understood as magnetars, which are neutron stars with unusually high magnetic fields. Many active researches of magnetars are being performed by Suzaku, including the "AO4 Key project on Magnetars". One of the most interesting topics of magnetars is their formation. Although neutron stars, including magnetars, are believed to be a result of supernovae (SNe), it is not yet clear what kind of SNe produce magnetars. In an attempt to obtain clues to this problem, we analyzed archival Suzaku data of the SNR, CTB109, which is associated with the magnetar1E 2254+586. So far, we have not found marked differences between CTB109 and other typical SNRs. As another attempt, we suggest that X-ray flashes, being detected with MAXI with a higher rate than expected, could be associated with the formation of magnetars (e.g., Metzger et al. 2008). Then, MAXI data may be utilized to obtain new information on the magnetar formation.
    12/2010; DOI:10.1063/1.3696162
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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 by performing high-resolution, high-throughput spectroscopy with moderate angular resolution. ASTRO-H covers very wide energy range from 0.3 keV to 600 keV. ASTRO-H allows a combination of wide band X-ray spectroscopy (5-80 keV) provided by multilayer coating, focusing hard X-ray mirrors and hard X-ray imaging detectors, and high energy-resolution soft X-ray spectroscopy (0.3-12 keV) provided by thin-foil X-ray optics and a micro-calorimeter array. The mission will also carry an X-ray CCD camera as a focal plane detector for a soft X-ray telescope (0.4-12 keV) and a non-focusing soft gamma-ray detector (40-600 keV) . The micro-calorimeter system is developed by an international collaboration led by ISAS/JAXA and NASA. The simultaneous broad bandpass, coupled with high spectral resolution of Delta E ~7 eV provided by the micro-calorimeter will enable a wide variety of important science themes to be pursued. Comment: 18 pages, 12 figures, Proceedings of the SPIE Astronomical Instrumentation "Space Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ray"
    Proceedings of SPIE - The International Society for Optical Engineering 10/2010; DOI:10.1117/12.857875 · 0.20 Impact Factor
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    ABSTRACT: We are designing an X-ray CCD camera (SXI) for ASTRO-H, including many new items. We have developed the CCD, CCD-NeXT4, that is a P-channel type CCD. It has a thick depletion layer of 200mum with an imaging area of 30mm square. Since it is back-illuminated, it has a good low energy response and is robust against the impact of micro-meteorites. We will employ 4 chips to cover the area of 60mm square. A mechanical rather than peltier cooler will be employed so that we can cool the CCD to -120°C. We will also introduce an analog ASIC that is placed very close to the CCD. It performs well, having a similar noise level to that assembled by using individual parts used on SUZAKU. We also employ a modulated X-ray source (MXS), that improves the accuracy of the calibration. The SXI will have one of the largest SOmega among various satellites.
    Proceedings of SPIE - The International Society for Optical Engineering 07/2010; DOI:10.1117/12.856087 · 0.20 Impact Factor

Publication Stats

541 Citations
111.79 Total Impact Points

Institutions

  • 2012–2015
    • The University of Tokyo
      Tōkyō, Japan
  • 1998–2013
    • Osaka University
      • Department of Earth and Space Science
      Suika, Ōsaka, Japan
  • 2007–2009
    • RIKEN
      • Radiation Laboratory
      Wako, Saitama-ken, Japan
  • 2004–2006
    • Japan Aerospace Exploration Agency
      • Institute of Space and Astronautical Science (ISAS)
      Chōfu, Tōkyō, Japan
  • 2005
    • Nagoya University
      • Solar-Terrestrial Environment Laboratory
      Nagoya, Aichi, Japan