Curriculum Vitaes
Profile Information
- Affiliation
- Associate Senior Resarcher, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
- Researcher number
- 90392810
- J-GLOBAL ID
- 202201019169253149
- researchmap Member ID
- R000042142
Papers
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Publications of the Astronomical Society of Japan, Feb 9, 2024Abstract 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, 73(3) 716-727, Jun 13, 2021 Peer-reviewed
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Publications of the Astronomical Society of Japan, Jun 24, 2019 Peer-reviewed<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), Oct, 2018
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ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 235(1), Mar, 2018
Misc.
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Planetary and Space Science, 194 105011, Dec, 2020
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Publications of the Astronomical Society of Japan, 72(1), Dec 6, 2019<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, Nov 20, 2019
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Publications of the Astronomical Society of Japan, 71(5), Sep 17, 2019<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, Jul 8, 2019
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Publications of the Astronomical Society of Japan, 71(4) 74, May 30, 2019<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, Jan, 2019
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Publications of the Astronomical Society of Japan, 71(1) 24, Jan 1, 2019<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, Jul 20, 2018
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Ground-based and Airborne Instrumentation for Astronomy VII, 10702 107020J, Jul 6, 2018
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Ground-based and Airborne Instrumentation for Astronomy VII, Jul 6, 2018
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The Astrophysical Journal, 857(1) 37, Apr 11, 2018
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The Astrophysical Journal, 856(1) 18, Mar 21, 2018
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JSAP Annual Meetings Extended Abstracts, 2018.1 400-400, Mar 5, 2018
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The Astrophysical Journal, 853(1) 46, Jan 22, 2018
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Proceedings of the 14th International Symposium on Nuclei in the Cosmos (NIC2016), Feb 27, 2017
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The Astrophysical Journal, 835(1) 1, Jan 16, 2017
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Publications of the Astronomical Society of Japan, 68 104, Oct 17, 2016<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, Sep 23, 2016<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, Aug 9, 2016
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SPIE Proceedings, Aug 8, 2016
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Publications of the Astronomical Society of Japan, 68(SP1) S1, Apr 21, 2016<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, Mar 6, 2016<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, Mar 4, 2016<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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Meeting Abstracts of the Physical Society of Japan, 71 503-503, 2016
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SPIE Proceedings, 9144 91442P, Jul 28, 2014
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The Astrophysical Journal, 787(2) 165, May 15, 2014
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The Astrophysical Journal Supplement Series, 207(2) 36, Aug 1, 2013
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Publications of the Astronomical Society of Japan, 65(3) 58, Jun 25, 2013<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, Aug, 2012<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, Feb 25, 2012<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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63(3) S821-S825, Nov 25, 2011 Peer-reviewed
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63(sp3) S713-S716, Nov 25, 2011
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63(sp3) S937-S945, Nov 25, 2011
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63(sp3) S803-S811, Nov 25, 2011
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63(3) S677-S689, Nov 25, 2011 Peer-reviewed
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Journal of Physics: Conference Series, 302 012062, Jul 20, 2011
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Meeting Abstracts of the Physical Society of Japan, 66.1.1 106, 2011
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Physica E: Low-dimensional Systems and Nanostructures, 43(3) 692-696, Jan, 2011
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Publications of the Astronomical Society of Japan, 62(6) L55-L60, Dec 25, 2010<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}$, which is weaker than the previous results. This could be reconciled by invoking a scenario that this flare is a superposition of unresolved events with a shorter timescale.</jats:p>
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Publications of the Astronomical Society of Japan, 62(5) L27-L32, Oct 25, 2010<jats:title>Abstract</jats:title> <jats:p>We present the first results on the black-hole candidate XTE J1752$-$223 from the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image (MAXI) on the International Space Station. Including the onset of an outburst reported by the Proportional Counter Array aboard the Rossi X-ray Timing Explorer on 2009 October 23, MAXI / GSC has been monitoring this source approximately 10 times per day with high sensitivity in the 2–20 keV band. XTE J1752$-$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, Feb 26, 2010
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JAXA-SP-09-008E 218-219, Feb 26, 2010
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Meeting Abstracts of the Physical Society of Japan, 65.1.1 118, 2010
Research Projects
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2022 - Mar, 2025
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2022 - Mar, 2025
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科学研究費助成事業, 日本学術振興会, Apr, 2017 - Mar, 2020
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, 2011 - 2013
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, 2009 - 2010
