研究者業績
基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 宇宙物理学研究系 教授総合研究大学院大学 先端学術院 宇宙科学コース 教授東京工業大学 理学院物理学系物理学コース 特定教授関西学院大学 大学院理工学研究科 客員教授
- 学位
- 理学博士(東京大学)
- J-GLOBAL ID
- 200901025041369206
- researchmap会員ID
- 1000144439
専門はＸ線天文学。特に、中性子星やブラックホールを含むX線連星の観測的研究および衛星搭載X線CCDカメラの開発研究。
受賞
1-
1996年
論文
213-
Publications of the Astronomical Society of Japan 2024年10月10日Abstract We present an initial analysis of the X-Ray Imaging and Spectroscopy Mission (XRISM) first-light observation of the supernova remnant (SNR) N 132D in the Large Magellanic Cloud. The Resolve microcalorimeter has obtained the first high-resolution spectrum in the 1.6–10 keV band, which contains K-shell emission lines of Si, S, Ar, Ca, and Fe. We find that the Si and S lines are relatively narrow, with a broadening represented by a Gaussian-like velocity dispersion of $\sigma _v \sim 450$ km s$^{-1}$. However, the Fe He$\alpha$ lines are substantially broadened with $\sigma _v \sim 1670$ km s$^{-1}$. This broadening can be explained by a combination of the thermal Doppler effect due to the high ion temperature and the kinematic Doppler effect due to the SNR expansion. Assuming that the Fe He$\alpha$ emission originates predominantly from the supernova ejecta, we estimate the reverse shock velocity at the time when the bulk of the Fe ejecta were shock heated to be $-1000 \lesssim V_{\rm rs}$ (km s$^{-1}$) $\lesssim 3300$ (in the observer frame). We also find that Fe Ly$\alpha$ emission is redshifted with a bulk velocity of $\sim 890$ km s$^{-1}$, substantially larger than the radial velocity of the local interstellar medium surrounding N 132D. These results demonstrate that high-resolution X-ray spectroscopy is capable of providing constraints on the evolutionary stage, geometry, and velocity distribution of SNRs.
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Space Telescopes and Instrumentation 2024: Optical, Infrared, and Millimeter Wave 81-81 2024年9月10日
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The Astrophysical Journal Letters 973(1) L25-L25 2024年9月1日Abstract We present an analysis of the first two XRISM/Resolve spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 4151, obtained in 2023 December. Our work focuses on the nature of the narrow Fe K α emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. The total line is found to consist of three components. Even the narrowest component of the line is resolved with evident Fe K α,1 (6.404 keV) and K α,2 (6.391 keV) contributions in a 2:1 flux ratio, fully consistent with neutral gas with negligible bulk velocity. Subject to the limitations of our models, the narrowest and intermediate-width components are consistent with emission from optically thin gas, suggesting that they arise in a disk atmosphere and/or wind. Modeling the three line components in terms of Keplerian broadening, they are readily associated with (1) the inner wall of the “torus,” (2) the innermost optical “broad-line region” (or “X-ray BLR”), and (3) a region with a radius of r ≃ 100 GM/c 2 that may signal a warp in the accretion disk. Viable alternative explanations of the broadest component include a fast-wind component and/or scattering; however, we find evidence of variability in the narrow Fe K α line complex on timescales consistent with small radii. The best-fit models are statistically superior to simple Voigt functions, but when fit with Voigt profiles the time-averaged lines are consistent with a projected velocity broadening of FWHM . Overall, the resolution and sensitivity of XRISM show that the narrow Fe K line in AGN is an effective probe of all key parts of the accretion flow, as it is currently understood. We discuss the implications of these findings for our understanding of AGN accretion, future studies with XRISM, and X-ray-based black hole mass measurements.
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray 228-228 2024年8月21日
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Cryogenics 138 103795-103795 2024年3月
MISC
200-
X-RAY AND GAMMA-RAY TELESCOPES AND INSTRUMENTS FOR ASTRONOMY, PTS 1 AND 2 4851 1071-1079 2003年We report on design updates for the XIS (X-ray Imaging Spectrometer) on-board the Astro-E2 satellite. Astro-E2 is a recovery mission of Astro-E, which was lost during launch in 2000. Astro-E2 carries a total of 5 X-ray telescopes, 4 of which have XIS sensors as their focal plane detectors. Each XIS CCD camera covers a field of view of 19 x 19 arcmin in the energy range of 0.4-12 keV. The design of the Astro-E2 XIS is basically the same as that for Astro-E, but some improvements will be implemented. These are (1) CCD charge injection capability, (2) a revised heat-sink assembly, and (3) addition of a Fe-55 radio-isotope on the door. Charge injection may be used to compensate for and calibrate radiation-induced degradation of the CCD charge transfer efficiency. This degradation is expected to become significant after a few year's operation in space. The new heat-sink assembly is expected to increase the mechanical reliability and cooling capability of the XIS sensor. The new radio-isotope on the door will provide better calibration data. We present details of these improvements and summarize the overall design of the XIS.
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PASJ : publications of the Astronomical Society of Japan 54(3) 373-386 2002年6月25日
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X-RAY AND GAMMA-RAY INSTRUMENTATION FOR ASTRONOMY XII 4497 158-165 2002年We have developed a novel architecture to process 2-dimensional digital image data with very high speed. The architecture is realized with an FPGA to extract only the X-ray signals from the raw frame data of an X-ray CCD for an astronomical use. The circuit scale is small enough to be implemented in an FPGA currently available for a space use, while the data processing speed of 10(7) pixels/sec is achieved. The architecture can be adapted in principle to a wide range of applications.
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PASJ : publications of the Astronomical Society of Japan 53(6) 1171-1177 2001年12月25日
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Astrophysical Journal, Supplement Series 131 571-591 2000年12月1日ASCA archive data of 20 low-mass X-ray binaries (LMXBs) were analyzed to study the iron K emission lines. The LMXBs analyzed here include Z sources, atoll sources, dippers, bursters, and ADC (accretion disk corona) sources. We paid particular attention to reducing systematic errors for the estimation of line parameters, and the combination of SIS and GIS, which have different energy resolutions, was found to be very useful for this purpose. We detected significant iron lines from about one-half of the sources. The line center was 6.56 keV on average, and a finite width of ∼0.5 keV (FWHM) was obtained from six sources; these properties may be common to LMXBs. On the other hand, equivalent width (EW) of the lines showed large scatter among the sources, from less than 10 to 170 eV. These line parameters do not show any correlation with the source categories or luminosities. The iron K lines from LMXBs are likely produced through the radiative recombination of photoionized plasma. We discuss the origin of the line width, which may result from the combination of the line blending, Doppler broadening, and Compton scattering.
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ASTROPHYSICAL JOURNAL 543(2) L145-L148 2000年11月The accretion-powered millisecond X-ray pulsar SAX J1808.4-3658 was observed in quiescence with ASCA in 1999 September. We detected a dim X-ray source in the Solid-State Imaging Spectrometer (SIS) data at the position consistent with SAX J1808.4-3658. The source count rate was (1.1 +/- 0.4) x 10(-3) counts s(-1) (0.5-5 keV) for a single SIS, which corresponds to (3 +/- 1) x 10(-14) ergs s(-1) cm(-2) if a power-law energy spectrum of photon index 2 with low-energy absorption corresponding to a hydrogen column density of 1.3 x 10(21) cm(-2) is assumed. The statistical quality of the data was insufficient to constrain the energy spectrum or to detect the 401 Hz coherent pulsation. We compare the data with the BeppoSAX observation also made during the quiescent state and find that the X-ray flux measured by ASCA is at least a factor of 4 smaller than that measured by BeppoSAX. We discuss the possible X-ray emission mechanisms that could explain the flux change, including the radio pulsar and the radio pulsar shock emission.
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Astrophys.J. 539 (2000) 413; Erratum-ibid. 651 (2006) 615-616 2000年3月16日We have discovered errors in the calculation of some of the Einstein<br /> coefficients in Table 5 and some plots in Figure 3. Due to the errors, the<br /> square-root section of the curves of growth of Fe xxvi Ka and Kb, and Ni xxviii<br /> Ka in Figure 3 were a few times underestimated. We correct Table 5 and re-plot<br /> Figure 3 together with the unaffected curves. The program to calculate the<br /> corrected curves of growth is available at<br /> http://www.hp.phys.titech.ac.jp/kotani/cog/index.html
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Astrophys.J.535:632,2000 535(2) 632-643 2000年1月3日Studies were made of ASCA spectra of seven ultra-luminous compact X-ray<br /> sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6<br /> (Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al.<br /> 1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano &<br /> Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al.<br /> 1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they<br /> are thought to represent a class of enigmatic X-ray sources often found in<br /> spiral galaxies. For some of them, the ASCA data are newly processed, or the<br /> published spectra are reanalyzed. For others, the published results are quoted.<br /> The ASCA spectra of all these seven sources have been described successfully<br /> with so called multi-color disk blackbody (MCD) emission arising from<br /> optically-thick standard accretion disks around black holes. Except the case of<br /> M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not<br /> to exceed the Eddington limits, the black holes are inferred to have rather<br /> high masses, up to ~100 solar masses. However, the observed innermost disk<br /> temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be<br /> compatible with the required high black-hole masses, as long as the standard<br /> accretion disks around Schwarzschild black holes are assumed. Similarly high<br /> disk temperatures are also observed from two Galactic transients with<br /> superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high<br /> disk temperature may be explained by the black hole rotation, which makes the<br /> disk get closer to the black hole, and hence hotter.
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BROAD BAND X-RAY SPECTRA OF COSMIC SOURCES 25(3-4) 669-672 2000年During the course of our X-ray studies of composite supernova remnants, we have discovered two young pulsars, The 65 ms pulsar, AX J1811.5-1926, is found to be associated with the central X-ray emission from the candidate of the historical SNR G11.2-0.3; and the 69 ms pulsar near the SNR RCW 103 is identified with a previously reported GINGA pulsator. Here, we present an observational summary of these two pulsars. (C) 2000 COSPAR. Published by Elsevier Science Ltd.
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BROAD BAND X-RAY SPECTRA OF COSMIC SOURCES 25(3-4) 391-394 2000年The ASCA results of the bursting X-ray pulsar GRO J1744-28 are reported. The observations were made twice, in February 1996 and March 1997. We detected 12 and 17 Type II bursts during the two observations, with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6-7 keV, which is most probably due to iron and maybe reproduced by the disk line model with additional broadening mechanism. The absorption column is constant ((5 - 6) x 10(22) cm(-2)), independent of the observation dates and emission phase (persistent, burst and dip). This means the source may be actually located near the galactic center (8.5kpc). (C) 2000 COSPAR. Published by Elsevier Science Ltd.
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BROAD BAND X-RAY SPECTRA OF COSMIC SOURCES 25(3-4) 375-378 2000年Energy spectra of four luminous supersoft X-ray sources (SSS) are investigated combining the ASCA SIS and the ROSAT PSPC data. We found that the energy spectra of the SSS are commonly reproduced by a blackbody continuum model superposed with sharp edge-like structures. Despite of several years intervals between the ASCA, and ROSAT observations, differences of the best-fit parameters are found to be small, This indicates that X-ray emissions from these foils SSS are rather stable. The blackbody temperature and the edge energies show large differences among the sources, which may reflect differences in the white dwarf mass. (C) 2000 COSPAR. Published by Elsevier Science Ltd.
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Astronomy and Astrophysics 343 197-201 1999年12月1日We have measured the phase differences between X-ray intensity variations in different energy bands in data taken with Ginga of the Z source GX 5-1 in its normal branch. We apply a χ2 optimal extraction technique to measure the phase-delay spectrum of the normal-branch quasi-periodic oscillations (QPO) and find a jump at 3.5 keV. X-ray intensity variations at the QPO frequency in energy channels below 3.5 keV are in phase with one another. The same is true of energy channels above 6 keV. However, intensity variations in energy channels above 6 keV lag those in channels below 3.5 keV by ∼ 150°. We see no minimum in the fractional rms amplitude of the quasi-periodic oscillations at the jump energy, in contrast to what is seen in the Z source Cyg X-2, but because of limited statistics we cannot rule out such a minimum. We discuss our findings in the context of a radiation-hydrodynamics model for the normal-branch quasi-periodic oscillations.
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ASTROPHYSICAL JOURNAL 517(1) 436-448 1999年5月We report the ASCA results of the bursting X-ray pulsar GRO J1744-28, which was observed in 1996 February and 1997 March. The source flux in the 2-10 keV band was 2.0 x 10(-8) ergs s(-1) cm(2) in 1996 and 5.0 x 10(-9) ergs s(-1) cm2 in 1997. We detected 12 and 17 type II bursts during type two observations, with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6-7 keV. The constant absorption column, (5-6) x 10(22) cm(-2), independent of the observation dates and emission phases (persistent, burst, and dir) is interpreted as an interstellar absorption. The source may be actually located near the Galactic center, at a distance of 8.5 kpc. The structure in the energy spectrum at 6 7 keV is most probably due to iron and may be reproduced by a disk line model with additional broadening mechanism.
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ASTROPHYSICAL JOURNAL 510(1) 369-378 1999年1月The X-ray binary pulsar GX 1+4 was observed with Ginga every year from 1987 to 1991 and with ASCA in 1994. During the Ginga observations, GX 1+4 was in the steady spin-down phase, although the X-ray flux was not steady. Assuming a distance of 10 kpc, the absorption-corrected X-ray luminosity decreased down to L2-20 keV = 2.7 x 10(36) ergs s(-1) in 1991, after the peak activity of L2-20 keV = 1.2 x 10(37) ergs s(-1) in 1989. On the other hand, the absorption column density showed a drastic increase over the Ginga observation series. It was less than 10(23) cm(-2) at the beginning of the series, and it reached a maximum of (1.4 +/- 0.2) x 10(24) cm(-2) in 1991, indicating a rapid accumulation of matter in the vicinity of the source. The center energy and equivalent width of the iron line were consistent with emission by isotropically distributed cold matter. The ASCA observation was performed on 1994 September 15, a month before the transition into a spin-up phase. The source brightened again to L2-20 keV = 10(37) ergs s(-1). The absorption column density was observed to decrease for the first time to (2.08 +/- 0.02) x 10(23) cm(-2). The ionization degree of iron in the absorbing matter was determined to be Fe I-Fe IV using the ratio of the line-center energy to the absorption-edge energy. The low ionization degree is consistent with an absorbing matter distribution extending similar to 10(12) cm from the source. We compared the results with optical observations and found that the optical data also supports the picture. Based on the geometrical model, possible causes of the bimodal behavior of the source are discussed.
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PASJ : publications of the Astronomical Society of Japan 51(4) 519-524 1999年
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PASJ : publications of the Astronomical Society of Japan 50(6) 667-673 1998年12月1日
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PASJ : publications of the Astronomical Society of Japan 50(6) 611-619 1998年12月1日
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PASJ : publications of the Astronomical Society of Japan 50(2) 249-255 1998年4月25日
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X-RAY TIMING AND COSMIC GAMMA RAY BURSTS 22(7) 961-964 1998年We report the discovery of the cessation of flickering in dips in the black hole candidate Cyg X-l, detected for the first time in the ASCA observation of May 9, 1995. During this observation, particularly deep dipping took place resulting in strong changes in hardness ratio corresponding to absorption of the power law spectral component. The deadtime corrected light curve with high time resolution clearly shows a dramatic decrease in the extent of flickering in the band 0.7 - 4.0 keV during dipping, but in the band 4.0 - 10.0 keV, there is relatively little change. We show that the rms flickering amplitude in the band 0.7 - 4.0 keV is proportional to the x-ray intensity in this band which changes by a factor of almost three. This is the first direct evidence that the strong low state flickering consists of fluctuations in the power law emission. (C) 1998 COSPAR. Published by Elsevier Science Ltd.
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Proceedings of SPIE - The International Society for Optical Engineering 3445 278-290 1998年 査読有り
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Publications of the Astronomical Society of Japan 49(6) 653-658 1997年12月1日Combined spectral and spatial analyses of the central region of the nearby spiral galaxy M83 shows the presence of two different emission components. One component has a spatial size of 2′.7, as defined by the rms radius of the emission region, and has an emission temperature of about 5 keV when it is approximated with a thermal-bremsstrahlung model. The other component has a much smaller radius with an upper limit of 1′.0 rms radius, and has a lower emission temperature of about 0.4 keV. The X-ray luminosities of the two components are, respectively, 2.1 × 1039 erg s-1 (soft component) and 4.8 × 1039 erg s-1 (hard component) in the 0.5-10 keV band (for the distance of 3.75 Mpc). The soft, lower-temperature component is consistent to be emission from the hot gas originating from the star-burst activity in the galaxy. On the other hand, the hard component is consistent with being a collection of binary X-ray sources in the bulge region.
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Astronomy and Astrophysics 322 857-867 1997年6月20日We present the results of an energy-dependent timing analysis of Ginga data from the black hole candidates GS 1124-68 and GX 339-4 in their Very High State (VHS). Large variations with energy of the timing properties are seen in both sources. The break frequency of the band-limited noise increases with energy, indicating considerable spectral changes on sub-second timescales. Quasi-periodic oscillations (QPOs) with high (and energy dependent) harmonic content are observed, whose peaks in the power spectra appear to he asymmetric. The fractional amplitude of the fundamental QPO peak increases with energy and the QPO harmonic content decreases with energy, indicating that the at higher energies the QPO cycle becomes more sinusoidal. During one observation of GS 1124-68, the QPO centroid frequency increases with decreasing 9-30 keV source count-rate. We found a previously unknown 6.7 Hz QPO peak, also energy dependent, in GS 1124-68, four months after the peak of the outburst. The energy dependence of the band-limited noise is interpreted in the frame of a shot-noise model with softening shots. The complex phenomenology reported here puts strong constraints on the theoretical models for spectral distribution and time variability of black hole candidates at high accretion rates.
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ASTRONOMY & ASTROPHYSICS 321(3) 776-790 1997年5月We have studied the spectral and rapid variations of GS 2023+338 using all Ginga (LAC) data available of this source. We used colour-colour and hardness-intensity diagrams to study the spectral behaviour and for the variability behaviour we used the FFT-technique to obtain power spectra, the energy dependence of the variability and the energy dependence of the phase delays. We find that the spectral behaviour is strongly influenced by changes in the local absorption. These changes also influence the variability at low photon energies and relatively long time scales (greater than or similar to 1 s). The global shape of the power spectra of CS 2023+338 is similar to that of other black hole candidates in the "low" (hard) state, however, small differences are present in the detailed power spectral shape between those sources.
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東海大学紀要. 理学部 32 125-133 1997年3月An eclipsing X-ray binary pulsar, Centaurus X-3 was observed with the ASCA satellite on Februarly 1-2 1995,during a non-eclipse phase of 0.11 to 0.81. We found that the X-ray pulse profiles observed with ASCA change in shape and amplitude according with the orbital phase. The three K_α lines at 6.4,6.67,and 6.96 keV from neutral, He-like, and H-like ironions, respectively, could be resolved in the observed energy spectrum owing to the high energy resolution of the CCD cameras on board ASCA. We found that the intensity of the 6.4 keV line changes with pulse phase. Distribution and ionization degree of matter surrounding the neutron star are discussed based on this result.
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PROCEEDINGS OF THE FOURTH COMPTON SYMPOSIUM, PTS 1 AND 2 (410) 844-848 1997年We present X-ray/gamma-ray observations of Cyg X-1 in the soft state during 1996 May-June. We analyze ASCA, RXTE and OSSE data. The spectrum consists of soft S-rag blackbody emission of an optically thick accretion disk in the vicinity of a black hole and a power law with an energy index alpha similar to 1.2-1.5 extending to at least several hundred keV. In the spectra, we find the presence of strong Compton reflection, which probably comes from the disk.
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Astrophysical Journal 489(1) 272-283 1997年Correlated variations in timing and spectral properties were observed near the peak of the outburst of Nova Muscae 1991 (GS 1124-683). Two regions with different spectral and timing characteristics can be distinguished in color-color and hardness-intensity diagrams. Two types of quasi-periodic oscillations (QPOs), each associated with regions in these diagrams, have been observed. Rapid flip-flop transitions sometimes occur between the regions. In one region the QPOs' centroid frequency is a strong, increasing function of X-ray intensity. Regions with similar characteristics and similar forms of QPOs were earlier found in GX 339-4 in its very high state. These two regions may turn out to be common and unique to black hole candidates in their very high state, even though they bear some similarities to the spectral branches in neutron star Z sources. © 1997. The American Astronomical Society. All rights reserved.
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IEEE Transactions on Nuclear Science 44(3) 847-853 1997年We have investigated the characteristics of radiation damage to charge coupled devices (CCDs) in the space environment. The X-ray astronomy satellite ASCA launched on February 20, 1993 in low Earth orbit carries CCDs specially developed for soft X-ray detection. We have traced the performance of the CCDs for 3 years. We have observed both the gradual decrease of charge transfer efficiency (CTE) and the increase of dark current. These are phenomenologically explained by the increase of charge traps due to irradiation by high energy charged particles. However, some of the effects of the radiation damage in the CCD are quite non-uniform over the chip on various scales. We discuss characteristics of the charge traps and possible origins of the non-uniformity. © 1997 IEEE.
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1996 IEEE NUCLEAR SCIENCE SYMPOSIUM - CONFERENCE RECORD, VOLS 1-3 1 261-262 1997年Charge coupled devices (CCDs) optimized for soft X-ray detection were utilized for the Solid-state Imaging Spectrometers (SIS) onboard the ASCA satellite, and have been in operation since its launch in 1993. We have been carefully monitoring tile change ill performance of SIS due to radiation damage over three years since launch. Both an increase of dark current and a decrease of charge transfer efficiency are observed, as expected from experiments ill tile laboratory, but their characteristics are somewhat different from those predicted before launch. This is the first detailed analysis of radiation damage to CCDs actually operated in the space environment and gives important information for the development of similar detectors for space applications.
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Astrophysical Journal 480 1997年1月1日We report the discovery of the cessation of flickering in dips in the black hole candidate Cygnus X-1, detected for the first time in the ASCA observation of 1995 May 9. During this observation, particularly deep dipping took place, which resulted in strong changes in hardness ratio corresponding to the absorption of the power-law spectral component. The dead time corrected light curve with high time resolution clearly shows a dramatic decrease in the extent of flickering in the 0.7-4.0 keV band during dipping, but in the 4.0-10.0 keV band, there is relatively little change. We show that the rms flickering amplitude in the 0.7-4.0 keV band is proportional to the X-ray intensity in this band, which changes by a factor of almost 3. This is direct evidence that the strong low-state flickering is intrinsic to the power-law emission; i.e., it takes place as part of the emission process. The rms amplitude is proportional to the intensity in the low-energy band, except for a possible deviation from linearity at the lower intensities. If confirmed, this nonlinearity could imply a process such as electron scattering of radiation, which will tend to smear out the fluctuations, or a process of fluctuation generation which depends on radial position in the source. Thus, timing observations during absorption dips can give information about the source region and may place constraints on its size. © 1997. The American Astronomical Society. All rights reserved.
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ASTROPHYSICAL JOURNAL 437(1) 449-457 1994年12月We have studied the iron line intensity variation of Her X-l over its characteristic periodicities such as the pulse period of 1.24 s, the orbital period of 1.7 days, and the long-term period of 35 days, with Ginga observations made from 1989 April to June. From an analysis of the pulse phase-resolved spectra obtained from the MAIN HIGH state of Her X-l, we discover that the line flux modulates from (4.3 +/- 1.9) x 10(-3) to (1.0 +/- 0.3) x 10(-2) photons cm(-2) s(-1) (the average 7.5 x 10(-3) cm(-2) s(-1)) during the pulse period. We also find that the line flux modulation is consistent with that of soft X-rays below 1 keV observed by previous authors. From the study of the orbital phase and the 35 day cycle dependences, we see the following facts: (1) the line flux observed during the high intensity state of Her X-l does not depend significantly on the orbital phase of 0.2-0.9, while the flux is almost invisible during the eclipse, (2) the pattern of the iron line flux modulation along the 35 day cycle is very similar to that of the continuum X-ray flux. With the line pulsations and the solid angle required to explain the equivalent width of the line in the MAIN HIGH state, we deduce that the extent of reprocessing site is less than 10(10) cm. Based on the similarity of the modulation pattern between the line flux and the continuum flux along the 35 day cycle, and the equivalent width change, we suggest that the line flux variation is mainly due to occultation of the reprocessing site by an accretion disk rather than to a change of reprocessing site; some line flux, very small compared with the MAIN HIGH line flux, from an additional site may be responsible for the equivalent width change.
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ASTROPHYSICAL JOURNAL 436(2) 871-874 1994年12月The discovery of quasi-periodic oscillations (QPOs) in the X-ray flux from the recurrent transient X-ray pulsar V0332+53 is reported. The centroid frequency, full width at half-maximum (FWHM), and the relative root mean square (rms) amplitude of the QPOs are (5.1 +/- 0.5) x 10(-2) Hz, (3.3 +/- 1.5) x 10(-2) Hz, and 4.8% +/- 1.2%, respectively. The beat frequency model is tested using these results and previously reported QPOs from other X-ray binary pulsars.
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ASTROPHYSICAL JOURNAL 427(1) 400-405 1994年5月We analyze data from two eclipse ingresses of Her X-1 observed with Ginga on 1989 April 30 and May 19. These observations occur, respectively, during the MAIN HIGH and SHORT HIGH states in the 35 day modulation of Her X-1 intensity. We find significant residual X-ray flux during eclipse, with a gradual decrease in flux following the occultation of the neutron star by the atmosphere of HZ Her. During the central part of the eclipse the count rate becomes nearly constant, at 0.5 mCrab in the energy range 1.7-36.8 keV. From a spectral analysis of the residual emission during the total eclipse of the central source in the MAIN HIGH state, we determine the energy spectral index, alpha = 0.8, similar to that before eclipse. A remarkable feature of the eclipse spectrum is that it does not show a significant iron line feature in contrast to massive wind-fed pulsars, such as Vela X-1 and Cen X-3. From a timing analysis of the same eclipse data, we show that there are no pulses. These results imply that the emission comes from the scattering of continuum X-rays by material in a region considerably larger than the companion star. An extended accretion disk corona may be responsible for this scattering. However, partial eclipse of an extended accretion disk corona is insufficient to account for the count rates in mid-eclipse, when known parameters of the binary system are used. Based on the present results, we suggest that scattering occurs not only in the accretion disk corona but also in the circumstellar matter surrounding the system of Her X-1/HZ Her.
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ASTROPHYSICAL JOURNAL 422(2) 799-809 1994年2月The X-ray binary pulsar Her X-1 was observed with Ginga on 1988 August 28 during the orbital phase of 0.76 to 0.85 in the main-on state of the 35 day cycle. During the observations the X-ray intensity varied by a factor of 5 or more on a time scale as short as 30 s, due mostly to the soft X-ray absorption in the pre-eclipse dip phase. From studies of pulse profiles and energy spectra, we find that there exists in the dip phase an unpulsed component which is approximately 3 % of the intensity of the nonabsorbed high level. We suggest that scattering of the source continuum by the optically thin hot corona is responsible for the unpulsed component. In the spectral analysis, we find that the high-state nonabsorbed spectra can be fitted by a power law without absorption, and the spectra observed in the different absorption states can be fitted by two power-law components with the same photon index. An iron-K emission line is required in both cases. The estimated equivalent width of the iron line varies from 0.18 to 0.55 keV in correlation with the change in the absorption column density along the line of sight. We suggest that the fluorescent iron line arises in a cool and relatively small region, like the Alfven surface, and may be partially intercepted by the optically thick gas cloud passing across the line of sight.
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ASTROPHYSICAL JOURNAL 421(2) 738-752 1994年2月Using a cross-spectral technique we investigate time delays between intensity variations of GX 5-1 in 10 X-ray spectral channels. The data were taken during a 1989 Ginga observation during which the source was in its horizontal-branch spectral state. We develop a new method to measure ''time-delay spectra'' in fixed Fourier frequency ranges and use it to determine the energy and intensity dependence of time delays in the low-frequency noise (nu < 2 Hz), the horizontal branch QPO, and the QPO second harmonic. These are the first time-delay spectra of a Z-source in its horizontal branch, and the first detection of time delays in the second harmonic. We find that 1. In the low-frequency noise, intensity variations at low energies lag those at high energies by tens of milliseconds; the lag increases with energy. 2. High-energy photons lag low-energy photons by up to 4 ms, not only at the QPO first harmonic frequency but also in the second harmonic. In both harmonics, the lag increases with energy. 3. Delays are not monotonically related to QPO frequency. The time delays are longest at the low-intensity, low-QPO frequency end of the horizontal branch, and decrease as the intensity increases and the source moves along the horizontal branch toward the normal branch vertex; they increase again near the vertex. 4. The time-delay spectra of the QPO first and second harmonic are similar but they are not identical. 5. Intensity variations in the different spectral channels are correlated with one another. We consider two mechanisms for the production of the time lags: Comptonization and evolving shots. We perform Monte Carlo simulations of Compton scattering in a homogeneous, isotropic, central corona and show that it can qualitatively explain the observed energy and time-delay spectra, but that it cannot explain the differences in the QPO first and second harmonic time-delay spectra, nor the observed dependence of the QPO fractional rms variability upon energy. We consider implications of our results for millisecond pulsar searches in low-mass X-ray binaries.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 44(6) 633-640 1992年The transient low-mass X-ray binary, EXO 0748-676, discovered with EXOSAT, is known to exhibit eclipses of a 492-s duration with a 3.82-hr period, intensity dips at pre-eclipse phases and type-I X-ray bursts. We observed this source with Ginga in 1989 March, 1990 December, 1991 January, and 1991 August and determined nine eclipse center times. Combining these eclipse center times with the previous result of the EXOSAT observations, we find that the orbital period of this source is not decaying monotonously, contrary to the previously reported suggestion. Instead, it shows a more complex behavior. A quadratic fit to the eclipse data yields a positive rate of change in orbital period (i.e., a secular growth of orbit) with an approximate rate of P(orb)/P(orb) congruent-to 0.9 x 10(-7) yr-1, although the EXOSAT observations made in 1985 do not fit this trend. A sinusoidal function gives a better fit to the observed orbital period changes with a period of about 12 yr and an amplitude of about 44 lt-s, although the period is much longer than the observation interval of about 6.5 yr. Possible mechanisms for the orbital period change, such as radiation driven mass transfer or a hierarchical tri e system, are discussed in the context of the present observations.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 43(4) L43-L50 1991年The 4.8-s X-ray binary pulsar Centaurus X-3 was observed with Ginga from 1989 March 22 to 24. In addition to coherent 4.8-s pulsations, we detected clear quasi-periodic oscillations (QPO) in the X-ray flux during a portion of the observations. The centroid frequency and the width of the QPO were 35 +/- 2 mHz and 10 +/- 5 mHz (FWHM), respectively. The centroid frequency of the QPO was stable during that portion of the observation, and no correlation was found between the QPO amplitude and the X-ray energy. Constraints on QPO models, such as the Kepler frequency and beat frequency models, are discussed on the basis of the present observations of QPO from Cen X-3.
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ASTROPHYSICAL JOURNAL 361(2) 514-526 1990年10月We examine the short time-scale variability in the X-ray flux of the quasi-periodic oscillations (QPOs) and low frequency noise (LFN) in GX 5-1. The data are drawn from a Ginga observation while the source was on the horizontal spectral branch. During the observation, the total X-ray intensity, 1.9 counts cm-2 s-1, and QPO centroid frequency, 24 Hz, were nearly stationary. We apply several new statistical tests for short time-scale variability and correlation among power spectral components to the GX 5-1 data in order to determine whether the QPOs are carried by LFN shots, as required by the beat-frequency modulated-accretion model. On time scales of ∼ 1 s, significant, but uncorrelated, QPO and LFN Fourier power fluctuations are evident. Simulations of QPOs carried by LFN shots show significant QPO-LFN correlations for low shot rates, whereas in simulations of models with shot rates greater than 100 Hz the underlying relationship is masked by the intrinsic noise bias manifested by overlapping shots. However, other tests reveal the shot nature of the LFN when applied to simulations with shot rates as high as ∼400 Hz: the mean intensity on a 1 s time scale is positively correlated with LFN power, and the intensity skewness is positive. For GX 5-1 the mean intensity on a 1 s time scale is uncorrelated with LFN power, and the intensity skewness is consistent with LFN shapes distributed symmetrically about the mean. In summary, there is no indication in the GX 5-1 data that QPOs are carried by positive shots with lifetimes of ∼ 1 s or less, whereas our simulations show that some tests would reveal the shot nature of the LFN and its relationship to the QPO if it were present. Instead, the results are consistent with a picture of QPOs produced independently of an intensity-symmetric LFN component.
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ASTROPHYSICAL JOURNAL 351(1) L13-L16 1990年3月Extensive Ginga observations of PSR 0540-69, the Crab-like 50 ms pulsar in the Large Magellanic Cloud, have been obtained as a side benefit of a pulsar search project for SN 1987A. Through a coherent pulse timing analysis of data from 46 separate days between 1987 July and 1988 October, we have obtained precise values for the pulse frequency and its first and second derivatives. From these values, a braking index of n = 2.02±0.01 is obtained for PSR 0540-69. This is the first accurate measurement of a pulsar braking index from X-ray observations and the third overall. The braking index for PSR 0540-69 is much smaller than those previously determined for the Crab pulsar (n = 2.51) and PSR 1509-58 (n = 2.83).
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Publications of the Astronomical Society of Japan 41(3) p427-440 1989年8月
所属学協会
4-
2000年1月 - 現在
共同研究・競争的資金等の研究課題
15-
日本学術振興会 科学研究費助成事業 基盤研究(C) 2019年4月 - 2023年3月
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日本学術振興会 科学研究費助成事業 新学術領域研究(研究領域提案型) 2012年6月 - 2017年3月
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日本学術振興会 科学研究費助成事業 2012年4月 - 2015年3月
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日本学術振興会 科学研究費助成事業 基盤研究(B) 2010年4月 - 2013年3月
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日本学術振興会 科学研究費助成事業 特別研究員奨励費 2005年 - 2007年
● 指導学生等の数
1-
年度2021年度(FY2021)修士課程学生数2連携大学院制度による学生数2学術特別研究員数1
● 専任大学名
1-
専任大学名総合研究大学院大学（SOKENDAI)
