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 主任研究開発員
- 研究者番号
- 90392810
- J-GLOBAL ID
- 202201019169253149
- researchmap会員ID
- R000042142
論文
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Publications of the Astronomical Society of Japan 2024年2月9日Abstract We develop a new deconvolution method to recover the precise Crab Nebula image taken by the Hitomi HXT, suppressing the artifact due to the bright Crab pulsar. We extend the Richardson–Lucy method, introducing two components corresponding to the nebula and pulsar with regularization for smoothness and flux, respectively, and performing simultaneous deconvolution of multi-pulse-phase images. The structures, including the torus and jets, seen in the deconvolved nebula image at the lowest energy band of 3.6–15 keV appear consistent with those identified in the high-resolution Chandra X-ray image. Above 15 keV, we confirm NuSTAR’s findings that the nebula size decreases in higher energy bands. We find that the north-east side of the nebula is fainter in higher energy bands. Our deconvolution method is applicable for any telescope images of faint diffuse objects containing a bright point source.
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Publications of the Astronomical Society of Japan 2019年6月24日 査読有り<jats:title>Abstract</jats:title> <jats:p>We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg2 ultra-deep layer and 5.78 deg2 deep layer in the Subaru Strategic Program over six- and four-month periods from 2016 to 2017, respectively. The ultra-deep layer reaches a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in g, r, i, z, and y bands, respectively; the deep layer is ∼0.6 mag shallower. In total, 1824 supernova candidates were identified. Based on light-curve fitting and derived light-curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at z > 1. Our unique data set doubles the number of Type Ia SNe at z > 1 and enables various time-domain analyses of Type II SNe, high-redshift superluminous SNe, variable stars, and active galactic nuclei.</jats:p>
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ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES 238(2) 2018年10月We present the third MAXI/GSC catalog in the high Galactic latitude sky (vertical bar b vertical bar > 10 degrees) based on the 7-year data from 2009 August 13 to 2016 July 31, complementary to that in the low Galactic latitude sky (vertical bar b vertical bar < 10 degrees) (Hori et al. 2018). We compile 682 sources detected at significances of s(D,4-10 keV) >= 6.5 in the 4-10 keV band. A two-dimensional image fit based on the Poisson likelihood algorithm (C-statistics) is adopted for the detections and constraints on their fluxes and positions. The 4-10 keV sensitivity reaches approximate to 0.48 mCrab, or approximate to 5.9 x 10(-12) erg cm(-2) s(-1), over half of the survey area. Compared with the 37-month Hiroi et al. (2013) catalog, which adopted a threshold of s(D,4-10) (keV) >= 7, the source number increases by a factor of similar to 1.4. The fluxes in the 3-4 keV and 10-20 keV bands are further estimated, and hardness ratios (HRs) are calculated using the 3-4 keV, 4-10 keV, 3-10 keV, and 10-20 keV band fluxes. We also make the 4-10 keV light curves in 1-year bins for all the sources and characterize their variabilities with an index based on a likelihood function and the excess variance. Possible counterparts are found from five major X-ray survey catalogs by Swift, Uhuru, RXTE, XMM-Newton, and ROSAT, as well as an X-ray galaxy cluster catalog (MCXC). Our catalog provides the fluxes, positions, detection significances, HRs, 1-year bin light curves, variability indices, and counterpart candidates.
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ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES 235(1) 2018年3月We present the first MAXI/GSC X-ray source catalog in the low-Galactic-latitude sky vertical bar b vertical bar < 10 degrees outside the Galactic center region (vertical bar b vertical bar < 5 degrees, l < 30 degrees, and l > 330 degrees) based on 7-year data from 2009 August 13 to 2016 July 31. To overcome source confusion in crowded regions, we have accurately calibrated the position-dependent shape of the point-spread function of the MAXI/GSC by analyzing onboard data. We have also taken into account the Galactic ridge X-ray emission. Using a maximum likelihood image fitting method, we have detected 221 sources with a significance threshold >6.5 sigma, 7 of which are transients only detected in 73-day time-sliced images. The faintest source has a flux of 5.2 x 10(-12) erg cm(-2) s(-1) (or an intensity of 0.43 mCrab) in the 4-10 keV band. We have identified the counterparts for about 81% of the detected sources, by cross-matching with the Swift, Uhuru, RXTE, XMM-Newton, MCXC, and ROSAT all-sky survey catalogs. Our catalog contains the source name, position and its error, flux and detection significance in the 3-4. keV, 4-10. keV, and 10-20. keV bands, hardness ratios, and information on the likely counterpart for the individual detected sources. We have obtained 73-day bin light curves of all the cataloged sources over 7 years and have calculated their periodograms. On the basis of the mean properties of time variability and spectral hardness, we suggest that the majority of the unidentified sources are low-mass X-ray binaries or blazars. Finally, we present the log N-log S relations at different Galactic longitudes and for different source populations.
MISC
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Planetary and Space Science 194 105011 2020年12月
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Publications of the Astronomical Society of Japan 72(1) 2019年12月6日<jats:title>Abstract</jats:title> <jats:p>Using a prototype of the Tomo-e Gozen wide-field CMOS mosaic camera, we acquire wide-field optical images at a cadence of $2\:$Hz and search them for transient sources of duration 1.5 to $11.5\:$s. Over the course of eight nights, our survey encompasses the equivalent of roughly two days on one square degree, to a fluence equivalent to a limiting magnitude of about $V = 15.6$ in a 1-s exposure. After examining by-eye the candidates identified by a software pipeline, we find no sources which meet all our criteria. We compute upper limits to the rate of optical transients consistent with our survey, and compare those to the rates expected and observed for representative sources of ephemeral optical light.</jats:p>
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The Astronomical Journal 158(6) 236 2019年11月20日
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Publications of the Astronomical Society of Japan 71(5) 2019年9月17日<jats:title>ABSTRACT</jats:title> <jats:p>We present a concept for an X-ray imaging system with a high angular resolution and moderate sensitivity. In this concept, a two-dimensional detector, i.e., an imager, is put at a slightly out-of-focus position of the focusing mirror, rather than just at the mirror focus, as in the standard optics, to capture miniature images of objects. In addition, a set of multi-grid masks (or a modulation collimator) is installed in front of the telescope. We find that the masks work as a coded aperture camera and that they boost the angular resolution of the focusing optics. The major advantage of this concept is that a much better angular resolution, having an order of 2–3 or more than in the conventional optics, is achievable, while a high throughput (large effective area) is maintained, which is crucial in photon-limited high-energy astronomy, because any type of mirrors, including lightweight reflective mirrors, can be employed in our concept. If the signal-to-noise ratio is sufficiently high, we estimate that angular resolutions at the diffraction limit of 4″ and 0.″4 at ∼7 keV can be achieved with a pair of masks at distances of 1 m and 100 m, respectively.</jats:p>
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The Astrophysical Journal 879(2) 64 2019年7月8日
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Publications of the Astronomical Society of Japan 71(4) 74 2019年5月30日<jats:title>Abstract</jats:title> <jats:p>We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg2 ultra-deep layer and 5.78 deg2 deep layer in the Subaru Strategic Program over six- and four-month periods from 2016 to 2017, respectively. The ultra-deep layer reaches a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in g, r, i, z, and y bands, respectively; the deep layer is ∼0.6 mag shallower. In total, 1824 supernova candidates were identified. Based on light-curve fitting and derived light-curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at z > 1. Our unique data set doubles the number of Type Ia SNe at z > 1 and enables various time-domain analyses of Type II SNe, high-redshift superluminous SNe, variable stars, and active galactic nuclei.</jats:p>
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Planetary and Space Science 165 281-292 2019年1月
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Publications of the Astronomical Society of Japan 71(1) 24 2019年1月1日<jats:title>Abstract</jats:title> <jats:p>We propose an image reconstruction method for an X-ray telescope system with an angular resolution booster proposed by Maeda et al. (2018, PASJ, submitted). The system consists of double multi-grid masks in front of an X-ray mirror and an off-focused two-dimensional imager. Because the obtained image is off-focused, an additional image reconstruction process is assumed to be included. Our image reconstruction method is an extension of the traditional Richardson–Lucy algorithm with two regularization terms, one for sparseness and the other for smoothness. Such a combination is desirable for astronomical imaging because astronomical objects have a variety in shape, from point sources to diffuse sources to mixtures of both. The performance of the system is demonstrated with simulated data for point sources and diffuse X-ray sources such as Cas A and the Crab Nebula. The image resolution is improved from a few arcmin of focused image without the booster to a few arcsec with the booster. Through the demonstration, the angular resolution booster with the image reconstruction method is shown to be feasible.</jats:p>
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High Energy, Optical, and Infrared Detectors for Astronomy VIII 10709 107091T 2018年7月20日
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Ground-based and Airborne Instrumentation for Astronomy VII 10702 107020J 2018年7月6日
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Ground-based and Airborne Instrumentation for Astronomy VII 2018年7月6日
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The Astrophysical Journal 857(1) 37 2018年4月11日
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The Astrophysical Journal 856(1) 18 2018年3月21日
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The Astrophysical Journal 853(1) 46 2018年1月22日
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Proceedings of the 14th International Symposium on Nuclei in the Cosmos (NIC2016) 2017年2月27日
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The Astrophysical Journal 835(1) 1 2017年1月16日
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Publications of the Astronomical Society of Japan 68 104 2016年10月17日<jats:title>Abstract</jats:title> <jats:p>We present an application of machine-learning (ML) techniques to source selection in the optical transient survey data with the Hyper Suprime-Cam (HSC) on the Subaru telescope. Our goal is to select real transient events accurately and in a timely manner out of a large number of false candidates, obtained by the standard difference-imaging method. We have developed the transient selector, which is based on majority voting of the three ML machines of AUC Boosting, Random Forest, and Deep Neural Networks. We applied it to our observing runs of Subaru-HSC in 2015 May and August, and proved it to be efficient in selecting optical transients. The false positive rate was 1.0% at the true positive rate of 90% in the magnitude range of 22.0–25.0 mag for the May data. For the August run, we successfully detected and reported 10 supernovae candidates within the same day as the observation. From these runs, we learned the following lessons: (1) training using artificial objects is effective in filtering false candidates out, especially for faint objects, and (2) a combination of ML by majority voting is advantageous.</jats:p>
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Publications of the Astronomical Society of Japan 68(5) 90 2016年9月23日<jats:title>Abstract</jats:title> <jats:p>Twenty-three giant flares from thirteen active stars (eight RS CVn systems, one Algol system, three dMe stars, and one young stellar object) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 1031–34 erg s−1 in the 2–20 keV band, the emission measure of 1054–57 cm−3, the e-folding time of 1 hr to 1.5 d, and the total radiative energy released during the flare of 1034–39 erg. Notably, the peak X-ray luminosity of $5^{+4}_{-2} \times 10^{33}\:$erg s−1 in the 2–20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest-ever-observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and our own data, we discovered a universal correlation of $\tau \propto L_{\rm X}^{0.2}$ for the flare duration τ and the intrinsic X-ray luminosity LX in the 0.1–100 keV band, which holds for 5 and 12 orders of magnitude in τ and LX, respectively. The MAXI/GSC sample is located at the highest ends of the correlation.</jats:p>
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SPIE Proceedings 9908 99083P 2016年8月9日
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SPIE Proceedings 2016年8月8日
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Publications of the Astronomical Society of Japan 68(SP1) S1 2016年4月21日<jats:title>Abstract</jats:title> <jats:p>Various transient phenomena on a timescale ranging from seconds to days appear at unexpected sky positions in X-rays. MAXI, Monitor of All-sky X-ray Image, on the International Space Station has been monitoring about 95% of the sky a day and has detected transient objects since 2009 August. Here, we describe quasi-real-time data processing systems of MAXI and a subsequent nova-alert system to find transient objects, and present the capabilities for the nova-alert system to detect transient events with excess fluxes from ≳80 mCrab in a single scan transit to ≳8 mCrab for 4 d, and to send prompt alert information to the world in less than 30 s after the onboard detection of a burst, making the best use of the International Space Station (ISS) real-time network. We also report on highlights of scientific results obtained with the system until the end of the first extended mission phase, 2015 March. Including 15 X-ray novae solely or independently discovered, we have reported on 177 transient phenomena, such as X-ray bursts, outbursts, and state transitions of X-ray binaries and X-ray flares from active stars and blazars, via the Astronomer's Telegram, and on 63 burst phenomena of other types via the Gamma-ray Coordinates Network. We summarize the results of these transient sources and phenomena focusing on the detections with the nova-alert system, and some new transients yet unpublished or requiring attention.</jats:p>
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Publications of the Astronomical Society of Japan 68(SP1) S13 2016年3月6日<jats:title>Abstract</jats:title> <jats:p>We analyzed continuous Monitor of All-sky X-ray Image/Gas Slit Camera (MAXI/GSC) data of the X-ray binary pulsar 4U 1626−67 from 2009 October to 2013 September, and determined the pulse period and the pulse-period derivative for every 60-d interval by the epoch folding method. The obtained periods are consistent with those provided by the Fermi/Gamma-ray Burst Monitor pulsar project. In all the 60-d intervals, the pulsar was observed to spin up, with the spin-up rate positively correlated with the 2–20 keV flux. We applied the accretion torque model proposed by Ghosh and Lamb (1979, ApJ, 234, 296) to the MAXI/GSC data, as well as the past data including both spin-up and spin-down phases. The “Ghosh & Lamb” relation was confirmed to successfully explain the observed relation between the spin-up/down rate and the flux. By comparing the model-predicted luminosity with the observed flux, the source distance was constrained as 5–13 kpc, which is consistent with that found by Chakrabarty (1998, ApJ, 492, 342). Conversely, if the source distance is assumed, the data can constrain the mass and radius of the neutron star, because the Ghosh & Lamb model depends on these parameters. We attempted this idea, and found that an assumed distance of, e.g., 10 kpc gives a mass in the range of 1.81–1.90 solar mass, and a radius of 11.4–11.5 km, although these results are still subject to considerable systematic uncertainties, other than distance.</jats:p>
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Search for soft X-ray flashes at the fireball phase of classical/recurrent novae using MAXI/GSC dataPublications of the Astronomical Society of Japan 68(SP1) S11 2016年3月4日<jats:title>Abstract</jats:title> <jats:p>We searched for precursive soft X-ray flashes (SXFs) associated with optically discovered classical or recurrent novae in the data of five years’ all-sky observations with the Gas Slit Camera (GSC) of the Monitor of All-sky X-ray Image (MAXI). We first developed a tool to measure the fluxes of point sources by fitting the event distribution with a model that incorporates the point-spread function (PSF-fit) to minimize the potential contamination from nearby sources. Then we applied the PSF-fit tool to 40 classical/recurrent novae that were discovered in optical observations from 2009 August to 2014 August. We found no precursive SXFs with significance above the 3 σ level in the energy range of 2–4 keV between td − 10 d and td, where td is the date when each nova was discovered. We obtained the upper limits for the bolometric luminosity of SXFs, and compared them with the theoretical prediction and that observed for MAXI J0158−744. This result could constrain the population of massive white dwarfs with a mass of roughly 1.40 solar mass, or larger, in binary systems.</jats:p>
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The Astrophysical Journal 787(2) 165 2014年5月15日
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The Astrophysical Journal Supplement Series 207(2) 36 2013年8月1日
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Publications of the Astronomical Society of Japan 65(3) 58 2013年6月25日<jats:title>Abstract</jats:title> <jats:p>We report on the spectral evolution of a new X-ray transient, MAXI J0556$-$ 332, observed by MAXI, Swift, and RXTE. The source was discovered on 2011 January 11 (MJD $ =$ 55572) by the MAXI Gas Slit Camera all-sky survey at ($ l$, $ b$) $ =$ (238.$ ^{\!\!\!\circ}$ 9, $-$ 25.$ ^{\!\!\!\circ}$ 2), relatively away from the Galactic plane. Swift/XRT follow-up observations identified it with a previously uncatalogued bright X-ray source, which led to optical identification. For more than one year since its appearance, MAXI J0556$-$ 332 has been X-ray active, with a 2–10 keV intensity above 30 mCrab. The MAXI/GSC data revealed rapid X-ray brightening in the first five days, and a hard-to-soft transition in the meantime. For the following $ \sim$ 70 days, the 0.5–30 keV spectra, obtained by the Swift/XRT and the RXTE/PCA on an almost daily basis, show a gradual hardening, with large flux variability. These spectra are approximated by a cutoff power-law with a photon index of 0.4–1 and a high-energy exponential cutoff at 1.5–5 keV, throughout the initial 10 months where the spectral evolution is mainly represented by a change of the cutoff energy. To be more physical, the spectra are consistently explained by thermal emission from an accretion disk plus Comptonized emission from a boundary layer around a neutron star. This supports the source identification as being a neutron-star X-ray binary. The obtained spectral parameters agree with those of neutron-star X-ray binaries in the soft state, whose luminosity is higher than 1.8 $ \times$ 10$ ^{37}$ erg s$ ^{-1}$. This suggests a source distance of $ >$ 17 kpc.</jats:p>
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Proceedings of the International Astronomical Union 8(S291) 422-424 2012年8月<jats:title>Abstract</jats:title><jats:p>We observed two magnetars, 4U 0142+61 and 1E2259+568, with the Japanese infrared satellite <jats:italic>AKARI</jats:italic> to search for the time variability at wavelengths between 2-4 μm. We significantly detected 4U0142+61 in the 4μm band, and determined flux upper limits in the other two bands. We did not detect 1E 2259+586 in any of the bands, and determined upper limits. Comparing the detection of 4U 0142+61 in the 4μm band with the <jats:italic>Spitzer</jats:italic> observation from 2005, we found the flux was reduced to be 64%. We interpret this time variability in the infrared band as an increase of the inner radius of the dust disk around the neutron star, where the increase is due to the sublimation of the dust by the large flare of neutron star itself.</jats:p>
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Publications of the Astronomical Society of Japan 64(1) 13 2012年2月25日<jats:title>Abstract</jats:title> <jats:p>We report on an X-ray spectral analysis of the black hole candidate XTE J1752$-$223 in the 2009–2010 outburst, utilizing data obtained with the MAXI/Gas Slit Camera (GSC), the Swift/XRT, and Suzaku, which work complementarily. As already reported by Nakahira et al. (2010, PASJ, 62, L27), MAXI monitored the source continuously throughout the entire outburst for about eight months. All of the MAXI/GSC energy spectra in the high/soft state, lasting for 2 months, are well represented by a multi-color disk plus power-law model. The innermost disk temperature changed from $\sim\ $0.7 keV to $\sim\ $0.4 keV and the disk flux decreased by an order of magnitude. Nevertheless, the innermost radius is constant at $\sim\ $41 $D_{3.5}$(cos $i$)$^{-{1}/{2 } }\ $km, where $D_{3.5}$ is the source distance in units of 3.5 kpc and $i$ the inclination. The multi-color disk parameters obtained with the MAXI/GSC are consistent with those with the Swift/XRT and Suzaku. The Suzaku data also suggest a possibility that the disk emission is slightly Comptonized, which could account for broad iron-K features reported previously. Assuming that the obtained innermost radius represents the innermost stable circular orbit for a non-rotating black hole, we estimate the mass of the black hole to be 5.51$\ \pm\ $0.28$\ M_{\odot}$$D_{3.5}$(cos $i$)$^{-{1}/{2 } }\ $, where the correction for the stress-free inner boundary condition and color hardening factor of 1.7 are taken into account. If the inclination is less than 49$^{\circ}$, as suggested from radio monitoring of transient jets, and the soft-to-hard transition in 2010 April occurred at 1%–4% of Eddignton luminosity, the fitting of the Suzaku spectra with a relativistic accretion-disk model derives constraints on the mass and the distance to be 3.1–55$\ M_{\odot}$ and 2.3–22 kpc, respectively. This confirms that the compact object in XTE J1752$-$223 is a black hole.</jats:p>
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Publications of the Astronomical Society of Japan 64(4) 2012年<jats:p>We report on the first observation of an X-ray outburst of a Be/X-ray binary pulsar, LS V $+$44 17/ RX J0440.9$+$4431, and the discovery of an absorption dip structure in the pulse profile. An outburst of this source was discovered by MAXI GSC in 2010 April. It was the first detection of transient activity of LS V $+$44 17 since the source was identified as a Be/X-ray binary in 1997. From the data of a follow-up RXTE observation near the peak of the outburst, we found a narrow dip structure in its pulse profile, which was clearer in the lower-energy bands. The pulse-phase-averaged energy spectra in the 3–100 keV band could be fitted with a continuum model containing a power-law function with an exponential cutoff and a blackbody component, which are modified at low energy by an absorption component. A weak iron K$\alpha$ emission line was also detected in the spectra. From the pulse-phase-resolved spectroscopy we found that the absorption column density at the dip phase was much higher than those in the other phases. The dip was not seen in subsequent RXTE observations at lower flux levels. These results suggest that the dip in the pulse profile originates from the eclipse of the radiation from the neutron star by the accretion column.</jats:p>
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Publications of the Astronomical Society of Japan 63(3) S821-S825 2011年11月25日 査読有り<jats:title>Abstract</jats:title> <jats:p>We report on the MAXI GSC X-ray monitoring of the Crab nebula and pulsar during the GeV gamma-ray flare for the period of 2010 September 18–24 (MJD 55457 $-$ 55463), detected by AGILE and Fermi-LAT. There were no significant variations on the pulse phase averaged and pulsed fluxes during the gamma-ray flare on time scales from 0.5 to 5 days. The pulse profile also showed no significant change during this period. The upper limits on the variations of the pulse phase averaged and pulsed fluxes for the period MJD 55457.5 $-$ 55462.5 in the 4–10 keV band were derived to be 1% and 19%, respectively, at the 90% confidence limit of the statistical uncertainty. The lack of variations in the pulsed component over the multi-wavelength range (radio, X-ray, hard X-ray, and gamma-ray) supports not a pulsar, but a nebular origin for the gamma-ray flare.</jats:p>
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Publications of the Astronomical Society of Japan 63(sp3) S713-S716 2011年11月25日<jats:title>Abstract</jats:title> <jats:p>We present a large X-ray flare from a nearby weak-lined T Tauri star TWA-7 detected with the Gas Slit Camera (GSC) on the Monitor of All-sky X-ray Image (MAXI). The GSC captured X-ray flaring from TWA-7 with a flux of 3 $\times$ 10 $^{-9}\ $ erg cm $^{-2}\ $ s $^{-1}$ in 2–20 keV band during the scan transit starting at 2010-09-07 18:24:30 (UT). The estimated X-ray luminosity at the scan in the energy band is 3 $\times$ 10 $^{32}\ $ erg s $^{-1}$ , indicating that the event is among the largest X-ray flares from T Tauri stars. Since MAXI GSC monitors a target only during a scan transit of about a minute per 92 min orbital cycle, the luminosity at the flare peak might have been higher than that detected. At the scan transit, we observed a high X-ray-to-bolometric luminosity ratio, log $\ L_{\rm X}/L_{\rm bol}$$=$$-$ 0.1 $^{+0.2}_{-0.3}$ ; i.e., the X-ray luminosity is comparable to the bolometric luminosity. Since TWA-7 has neither an accreting disk nor a binary companion, the observed event implies that none of those are essential to generate such big flares in T Tauri stars.</jats:p>
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Publications of the Astronomical Society of Japan 63(sp3) S937-S945 2011年11月25日<jats:title>Abstract</jats:title> <jats:p>We constructed a new X-ray (2–10 keV) luminosity function of Compton-thin active galactic nuclei (AGNs) in the local universe, using the first MAXI/GSC source catalog surveyed in the 4–10 keV band. The sample consists of 37 non-blazar AGNs at $z$$=$ 0.002–0.2, whose identification is highly ( $>\ $ 97%) complete. We confirmed the trend that the fraction of absorbed AGNs with $N_{\rm H}$$>\ $ 10 $^{22}$ cm $^{-2}$ rapidly decreases against the luminosity ( $L_{\rm X}$ ), from 0.73 $\ \pm\ $ 0.10 at $L_{\rm X}$$=$ 10 $^{42-43.5}$ erg s $^{-1}$ to 0.12 $\ \pm\ $ 0.08 at $L_{\rm X}$$=$ 10 $^{43.5-45.5}$ erg s $^{-1}$ . The obtained luminosity function was well-fitted with a smoothly connected double power-law model whose indices are $\gamma_1$$=$ 0.84 (fixed) and $\gamma_2$$=$ 2.0 $\ \pm\ $ 0.2 below and above the break luminosity, $L_{*}$$=$ 10 $^{43.3\pm0.4}$ erg s $^{-1}$ , respectively. While the result of the MAXI/GSC agrees well with that of HEAO-1 at $L_{\rm X}$$\gtrsim$ 10 $^{43.5}$ erg s $^{-1}$ , it gives a larger number density at the lower luminosity range. A comparison between our luminosity function in the 2–10 keV band and that in the 14–195 keV band obtained from the Swift/BAT survey indicates that the averaged broad-band spectra in the 2–200 keV band should depend on the luminosity, approximated by $\Gamma$$\sim$ 1.7 for $L_{\rm X}$$\lesssim$ 10 $^{44}$ erg s $^{-1}$ , while $\Gamma$$\sim$ 2.0 for $L_{\rm X}$$\gtrsim$ 10 $^{44}$ erg s $^{-1}$ . This trend was confirmed by the correlation between the luminosities in the 2–10 keV and 14–195 keV bands in our sample. We argue that there is no contradiction in the luminosity functions between above and below 10 keV once this effect is taken into account.</jats:p>
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Publications of the Astronomical Society of Japan 63(sp3) S803-S811 2011年11月25日<jats:p>We present the results of monitoring the galactic black hole candidate GX 339 $-$ 4 with the Monitor of All-sky X-ray Image (MAXI) / Gas Slit Camera in the high/soft state during an outburst in 2010. All of the spectra throughout the 8-month period were well reproduced with a model consisting of multi-color disk emission and its Comptonization component, whose fraction is $\leq\ $ 25% in the total flux. In spite of the flux variability over a factor of 3, the innermost disk radius is constant at $R_{\rm in}$$=$ 61 $\ \pm\ $ 2 km for an inclination angle of $i$$=$ 46 $^\circ$ and a distance of $d$$=$ 8 kpc. This $R_{\rm in}$ value is consistent with those of past measurements with Tenma in the high/soft state. Assuming that the disk extends to the innermost stable circular orbit of a non-spinning black hole, we estimate the black hole mass to be $M$$=$ 6.8 $\ \pm\ $ 0.2 $\ M_{\odot}$ for $i$$=$ 46 $^\circ$ and $d$$=$ 8 kpc, which is consistent with that estimated from the Suzaku observation of the previous low/hard state. Further combined with the mass function, we obtained a mass constraint of 4.3 $\ M_\odot$$<$$M$$<$ 13.3 $\ M_\odot$ for the allowed range of $d$$=$ 6–15 kpc and $i$$<$ 60 $^\circ$ . We also discuss the spin parameter of the black hole in GX 339 $-$ 4 by applying relativistic accretion disk models to the Swift/XRT data.</jats:p>
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Publications of the Astronomical Society of Japan 63(sp3) S1035-S1040 2011年11月25日<jats:title>Abstract</jats:title> <jats:p>The monitor of the all-sky X-ray image (MAXI) Gas Slit Camera (GSC) on the International Space Station (ISS) detected a gamma-ray burst (GRB) on 2009, September 26, GRB 090926B. This GRB had extremely hard spectra in the X-ray energy range. Joint spectral fitting with the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope shows that this burst had a peculiarly narrow spectral energy distribution, which can be represented by a Comptonized blackbody model. This spectrum can be interpreted as photospheric emission from a low baryon-load GRB fireball. Calculating the parameter of the fireball, we found the size of the base of the flow to be $r_0$$=$ (4.3 $\ \pm\ $ 0.9) $\times$ 10 $^{9}\ Y'^{-3/2}\ $ cm, the Lorentz factor of the plasma is $\Gamma$$=$ (110 $\ \pm\ $ 10) $\ Y'^{\ 1/4}$ , where $Y'$ is a ratio between the total fireball energy and the energy in the blackbody component of the gamma-ray emission. This $r_0$ is a factor of a few times larger, and the Lorentz factor of 110 is smaller by also factor of a few than other bursts that have blackbody components in the spectra.</jats:p>
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Publications of the Astronomical Society of Japan 63(3) S677-S689 2011年11月25日 査読有り<jats:title>Abstract</jats:title> <jats:p>We present the first unbiased source catalog of the Monitor of All-sky X-ray Image (MAXI) mission at high Galactic latitudes ( $\vert b\vert$$>$ 10 $^{\circ}$ ), produced from the first 7-month data (2009 September 1 to 2010 March 31) of the Gas Slit Camera in the 4–10 keV band. We have developed an analysis procedure to detect faint sources from the MAXI data, utilizing a maximum likelihood image fitting method, where the image response, background, and detailed observational conditions are taken into account. The catalog consists of 143 X-ray sources above the 7 $\sigma$ significance level with a limiting sensitivity of $\sim\ $ 1.5 $\times$ 10 $^{-11}\ $ erg cm $^{-2}\ $ s $^{-1}$ (1.2 mCrab) in the 4–10 keV band. Among them, we have identified 38 Galactic/LMC/SMC objects, 48 galaxy clusters, 39 Seyfert galaxies, 12 blazars, and 1 galaxy. Four other sources are confused with multiple objects, and one remains unidentified. The log $\ N$ –log $\ S$ relation of extragalactic objects is in good agreement with the HEAO-1 A-2 result, although the list of the brightest AGNs in the entire sky has significantly changed since that produced 30 yr ago.</jats:p>
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Publications of the Astronomical Society of Japan 63(sp3) S635-S644 2011年11月25日<jats:title>Abstract</jats:title> <jats:p>We report on the in-orbit performance of the Gas Slit Camera (GSC) on the MAXI (Monitor of All-sky X-ray Image) mission carried on the International Space Station (ISS). Its commissioning operation, which started on 2009 August 8, confirmed the basic performances of the effective area in the energy band of 2–30 keV, the spatial resolution of the slit-and-slat collimator and detector with 1 $^\circ\!\!\!.$ 5 FWHM, the source visibility of 40–150 seconds for each scan cycle, and the sky coverage of 85% per 92-minute orbital period and 95% per day. The gas gains and read-out amplifier gains have been stable within 1%. The background rate is consistent with the past X-ray experiments operated at a similar low-earth orbit if its relation with the geomagnetic cutoff rigidity is extrapolated to high latitude. We also present the status of the in-orbit operation and a calibration of the effective area and the energy response matrix using Crab-nebula data.</jats:p>
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Publications of the Astronomical Society of Japan 63(3) S623-S634 2011年11月25日 査読有り<jats:title>Abstract</jats:title> <jats:p>The Gas Slit Camera (GSC) is an X-ray instrument on the MAXI (Monitor of All-sky X-ray Image) mission aboard the International Space Station. It is designed to scan the entire sky every 92-minute orbital period in the 2–30 keV band and to achieve the highest sensitivity among the X-ray all-sky monitors ever flown so far. The GSC employs large-area position-sensitive proportional counters with a total detector area of 5350 cm $^2$. The on-board data processor has functions to format telemetry data as well as to control the high voltage of the proportional counters so as to protect them from particle irradiation. This paper describes the instruments, on-board data processing, telemetry data formats, and performance specifications expected from ground calibration tests.</jats:p>
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Publications of the Astronomical Society of Japan 63(sp3) S751-S757 2011年11月25日<jats:title>Abstract</jats:title> <jats:p>We report on the discovery of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of GX 304 $-$ 1, obtained by RXTE and Suzaku during major outbursts detected by MAXI in 2010. The peak intensity in August reached 600 mCrab in the 2–20 keV band, which is the highest ever observed from this source. The RXTE observations on more than twenty occasions and one Suzaku observation revealed a spectral absorption feature at around 54 keV, which is the first CRSF detection from this source. The estimated strength of the surface magnetic field, 4.7 $\times$ 10 $^{12}$ G, is one of the highest among binary X-ray pulsars from which CRSFs have ever been detected. The RXTE spectra taken during the August outburst also suggest that the CRSF energy changed over 50–54 keV, possibly in a positive correlation with the X-ray flux. The behavior is qualitatively similar to that observed from Her X-1 on long time scales, or from A 0535 $+$ 26, but different from the negative correlation observed from 4U 0115 $+$ 63 and X 0331 $+$ 53.</jats:p>
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Journal of Physics: Conference Series 302 012062 2011年7月20日
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日本物理学会講演概要集 66.1.1 106 2011年
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Physica E: Low-dimensional Systems and Nanostructures 43(3) 692-696 2011年1月
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Publications of the Astronomical Society of Japan 62(6) L55-L60 2010年12月25日<jats:title>Abstract</jats:title> <jats:p>Strong X-ray flares from the blazar Mrk 421 were detected in 2010 January and February through 7-month monitoring with the MAXI GSC. The maximum 2–10 keV flux in the January and February flares was measured to be 120$\ \pm\ $10 mCrab and 164$\ \pm\ $17 mCrab, respectively; the latter is the highest among those reported from the object. A comparison of the MAXI and Swift BAT data suggests a convex X-ray spectrum with an approximated photon index of $\Gamma$$\gtrsim$ 2. This spectrum is consistent with a picture that MAXI is observing near the synchrotron peak frequency. The source exhibited a spectral variation during these flares, slightly different from those in previous observations, in which the positive correlation between the flux and hardness was widely reported. By equating the halving decay timescale in the January flare, $t_{ m d}$$\sim$ 2.5 $\times$ 10$^{4}\ $s, to the synchrotron cooling time, the magnetic field was evaluated to be $B$$\sim$ 4.5 $\times$ 10$^{-2}\ $G ($\delta/$10)$^{-1/3}$, where $\delta$ is the jet beaming factor. Assuming that the light crossing time of the emission region is shorter than the doubling rise time, $t_{ m r}$$\lesssim$ 2 $\times$ 10$^{4}\ $s, the region size was roughly estimated as $R$$<$ 6 $\times$ 10$^{15}\ $cm ($\delta/$10). These results are consistent with values previously reported. For the February flare, the rise time, $t_{ m r}$$<$ 1.3 $\times$ 10$^{5}\ $s, gives a loose upper limit on the size as $R$$<$ 4 $\times$ 10$^{16}\ $cm $(\delta/10)$, although the longer decay time, $t_{ m d}$$\sim$ 1.4 $\times$ 10$^{5}\ $s, indicates $B$$\sim$ 1.5 $\times$ 10$^{-2}\ $G ($\delta/$10)$^{-1/3}$, which is weaker than the previous results. This could be reconciled by invoking a scenario that this flare is a superposition of unresolved events with a shorter timescale.</jats:p>
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Publications of the Astronomical Society of Japan 62(5) L27-L32 2010年10月25日<jats:title>Abstract</jats:title> <jats:p>We present the first results on the black-hole candidate XTE J1752$-$223 from the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image (MAXI) on the International Space Station. Including the onset of an outburst reported by the Proportional Counter Array aboard the Rossi X-ray Timing Explorer on 2009 October 23, MAXI / GSC has been monitoring this source approximately 10 times per day with high sensitivity in the 2–20 keV band. XTE J1752$-$223 was initially in a low / hard state during the first 3 months. An anti-correlated behavior between the 2–4 keV and 4–20 keV bands was observed around 2010 January 20, indicating that the source exhibited a spectral transition to the high / soft state. A transient radio jet may have been ejected when the source was in the intermediate state where the spectrum was roughly explained by a power-law with a photon index of 2.5–3.0. The unusually long period in the initial low / hard state implies a slow variation in the mass-accretion rate, and a dramatic soft X-ray increase may be explained by a sudden appearance of the accretion disk component with a relatively low innermost temperature (0.4–0.7 keV). Such a low temperature might suggest that the maximum accretion rate was just above the critical gas-evaporation rate required for the state transition.</jats:p>
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JAXA-SP-09-008E 400-403 2010年2月26日
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JAXA-SP-09-008E 218-219 2010年2月26日
共同研究・競争的資金等の研究課題
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