Junko Hiraga

The University of Tokyo, Edo, Tōkyō, Japan

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Publications (72)99.37 Total impact

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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).
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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). · 6.35 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. · 1.18 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; · 5.08 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; · 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: 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;
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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;
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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.
    12/2010;
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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"
    10/2010;
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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.
    Proc SPIE 07/2010;
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    ABSTRACT: We present the Suzaku detection of strong free-bound emission from the Galactic middle-aged super-nova remnant (SNR) IC 443. A previous ASCA observation revealed that the Lyα fluxes of Si and S are significantly higher than those expected from the electron temperature of the bremsstrahlung continuum (Kawasaki et al. 2002). Therefore, it was claimed that the thermal plasma in this SNR is in "overion-ization". In the XIS spectrum of IC 443, we discovered hump-like features around at 2.7 and 3.5 keV as well as the strong Lyα lines. These humps are well represented by radiative recombination continua (RRC) of H-like Si and S with the electron temperature of ∼ 0.6 keV. The ionization temperatures of Si and S determined from the intensity ratios of the RRC to He-like Kα line are ∼ 1.0 keV and ∼ 1.2 keV, respectively. We thus find firm evidence for an extremely-overionized (recombining) plasma. As the origin of the overionization, a thermal conduction scenario argued in the previous work is not favored in our new results. We propose that the highly-ionized gas were made at the initial phase of the SNR evolution in dense regions around a massive progenitor, and the low electron temperature is due to a rapid cooling by an adiabatic expansion.
    02/2010;
  • 02/2010;
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    ABSTRACT: We report the discovery of non-thermal X-rays from the ejecta fragment associated with the Vela supernova remnant (SNR). The ROSAT observation of the Vela SNR revealed six isolated fragment-like features, named shrapnels A to F, which have overrun the primary blast wave of the SNR. Given that the symmetry axis of each shrapnel's bow-shock front traces back to the SNR center, the shrapnels are suggested to be associated with the fossil material of the supernova explosion. In fact, the chemical compositions of shrapnels A, B, and D have been found to be abundant in heavy elements by recent observations. Using Suzaku and XMM-Newton, we have investigated the nature of another candidate of the ejecta fragment, shrapnel E. The bow-shock structure of the emission is clearly confirmed. We find that the trailing region of the shrapnel exhibits a thermal X-ray spectrum as that observed in the other fragments. On the other hand, the shrapnel's head is found to exhibit a featureless spectrum in the hard X-ray band of 2-8 keV. This spectrum is well represented by a power-law model with a photon index of 2-3, suggesting its synchrotron origin. Fluxes of point sources detected in the shrapnel's head are carefully estimated and are found to be less than 10% of the total flux in the 2-8 keV band. This is the first discovery of diffuse non-thermal emission from an ejecta fragment of the Vela SNR. Since the age of the Vela SNR is estimated to be more than ten thousand years, our observation demonstrates that even an old system, compared to typical young SNRs, can be a site of efficient cosmic-ray acceleration.
    01/2010; 38:2785.
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    ABSTRACT: Suzaku X-ray observations of a young supernova remnant, Cassiopeia A, were carried out. K-shell transition lines from highly ionized ions of various elements were detected, including Chromium (Cr-Kalpha at 5.61 keV). The X-ray continuum spectra were modeled in the 3.4--40 keV band, summed over the entire remnant, and were fitted with a simplest combination of the thermal bremsstrahlung and the non-thermal cut-off power-law models. The spectral fits with this assumption indicate that the continuum emission is likely to be dominated by the non-thermal emission with a cut-off energy at > 1 keV. The thermal-to-nonthermal fraction of the continuum flux in the 4-10 keV band is best estimated as ~0.1. Non-thermal-dominated continuum images in the 4--14 keV band were made. The peak of the non-thermal X-rays appears at the western part. The peak position of the TeV gamma-rays measured with HEGRA and MAGIC is also shifted at the western part with the 1-sigma confidence. Since the location of the X-ray continuum emission was known to be presumably identified with the reverse shock region, the possible keV-TeV correlations give a hint that the accelerated multi-TeV hadrons in Cassiopeia A are dominated by heavy elements in the reverse shock region. Comment: Publ. Astron. Soc. Japan 61, pp.1217-1228 (2009)
    Publications- Astronomical Society of Japan 12/2009; · 2.44 Impact Factor
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    ABSTRACT: We present the Suzaku spectroscopic study of the Galactic middle-aged supernova remnant (SNR) IC 443. The X-ray spectrum in the 1.75-6.0 keV band is described by an optically thin thermal plasma with the electron temperature of ~0.6 keV and several additional Lyman lines. We robustly detect, for the first time, strong radiative recombination continua (RRC) of H-like Si and S around at 2.7 and 3.5 keV. The ionization temperatures of Si and S determined from the intensity ratios of the RRC to He-like Kα lines are ~1.0 keV and ~1.2 keV, respectively. We thus find firm evidence for an extremely overionized (recombining) plasma. As the origin of the overionization, a thermal conduction scenario argued in previous work is not favored in our new results. We propose that the highly ionized gas was made at the initial phase of the SNR evolution in dense regions around a massive progenitor, and the low electron temperature is due to a rapid cooling by an adiabatic expansion.
    The Astrophysical Journal 10/2009; 705(1):L6. · 6.73 Impact Factor
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    ABSTRACT: We searched for evidence of line emission around 4keV from the northwestern rim of the supernova remnant RX J0852.0-4622 using Suzaku XIS data. Several papers have reported the detection of an emission line around 4.1keV from this region of the sky. This line would arise from K-band fluorescence by Sc, the immediate decay product of 44Ti. We performed spectral analysis for the entire portion of the NW rim of the remnant within the XIS field of view, as well as various regions corresponding to regions of published claims of line emission. We found no line emission around 4.1keV anywhere, and are able to set a restrictive upper limit to the X-ray flux: 1.1x10^-6 s^-1 cm^-2 for the entire field. For every region, our flux upper limit falls below that of the previously claimed detection. Therefore, we conclude that, to date, no definite X-ray line feature from Sc-K emission has been detected in the NW rim of RX J0852.0-4622. Our negative-detection supports the recent claim that RX J0852-4622 is neither young (1700--4000 yr) nor nearby(~750 pc). Comment: Published in PASJ
    Publications- Astronomical Society of Japan 06/2009; · 2.44 Impact Factor
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    ABSTRACT: The Vela supernova remnant (SNR) is a very ideal target for studying the detailed structure, since it has a quite large angular diameter (∼ 8 •). In this proceeding, we first review the previous observations of several "shrapnels", the fragments of the ejecta protruding beyond the primary blast-shock front. Then, we report the preliminary results of shrapnel B with Suzaku. The elemental abundances are found to be significantly higher than the solar values for the first time, indicating the shrapnel originates from the supernova ejecta. Finally, we discuss the scientific significance of MAXI observations of the Vela SNR. The most part of the SNR has not yet been observed by the recent satellites, since it is too large to be covered with their limited exposure times. MAXI will be the first X-ray mission allowing us to study the ejecta distribution in the entire SNR as well as to search other ejecta fragments hiding inside the shell by the projection effect. Those information must become a key to solve the mechanism of the supernova explosions.
    03/2009;
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    ABSTRACT: The shell-type supernova remnant RX J0852.0-4622 was observed with the High Energy Stereoscopic System (H.E.S.S.) of atmospheric Cerenkov telescopes between 2004 December and 2005 May for a total observation time of 33 hr, above an average gamma-ray energy threshold of 250 GeV. The angular resolution of ~0.06° (for events triggering three or four telescopes) and the large field of view of H.E.S.S. (5° diameter) are well adapted to studying the morphology of the object in very high energy gamma rays, which exhibits a remarkably thin shell very similar to the features observed in the radio range and in X-rays. The spectral analysis of the source from 300 GeV to 20 TeV is also presented. Finally, the possible origins of the very high energy gamma-ray emission (inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions) are discussed, on the basis of morphological and spectral features obtained at different wavelengths.
    The Astrophysical Journal 12/2008; 661(1):236. · 6.73 Impact Factor
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    ABSTRACT: We present results obtained from a series of observations of the supernova remnant RX J1713.7–3946 by Suzaku. Hard X-rays have been detected up to ~40 keV. The hard X-ray spectra are described by a power law with photon indices of ~3.0, which is larger than those below 10 keV. The combination of the spatially integrated XIS and HXD spectra clearly reveals a spectral cutoff which is linked to the maximum energy of accelerated electrons. The broadband coverage of Suzaku allows us to derive, for the first time, the energy spectrum of parent electrons in the cutoff region. The cutoff energy in the X-ray spectrum indicates that the electron acceleration in the remnant proceeds close to the Bohm diffusion limit. We discuss the implications of the spectral and morphological properties of the Suzaku data in the context of the origin of nonthermal emission. The Suzaku X-ray and H.E.S.S. gamma-ray data together hardly can be explained within a pure leptonic scenario. Moreover, the leptonic models require a weak magnetic field, which is inconsistent with the recently discovered X-ray filamentary structures and their short-term variability. The hadronic models with strong magnetic fields provide reasonable fits to the observed spectra, but require special arrangements of parameters to explain the lack of thermal X-ray emission. For morphology studies, we compare the X-ray and TeV gamma-ray surface brightness. We confirm the previously reported strong correlation between X-rays and TeV gamma rays. At the same time, the Suzaku data reveal a deviation from the general tendency, namely, the X-ray emission in the western rims appears brighter than expected from the average X-ray to gamma-ray ratio.
    The Astrophysical Journal 12/2008; 685(2):988. · 6.73 Impact Factor

Publication Stats

371 Citations
99.37 Total Impact Points

Institutions

  • 2014
    • The University of Tokyo
      • Department of Physics
      Edo, Tōkyō, Japan
  • 1998–2013
    • Osaka University
      • • Department of Earth and Space Science
      • • Department of Physics
      Suika, Ōsaka, Japan
  • 2007–2010
    • RIKEN
      • Radiation Laboratory
      Вако, Saitama, Japan
  • 2006
    • Japan Aerospace Exploration Agency
      • Institute of Space and Astronautical Science (ISAS)
      Chōfu, Tōkyō, Japan
  • 2001
    • University of Leicester
      • Space Research Centre
      Leicester, ENG, United Kingdom