中平 聡志

© Copyright owned by the author(s) under the terms of the Creative Commons. We report on multi-color optical photometry in the 2015 outbursts of V404 Cygni, an X-ray transient. This system showed optical oscillations on timescales of 100 sec to 2.5 hours in these outbursts, and they were correlated with the simultaneous X-ray variations. On the basis of the ~1-min optical delays against X-rays and multi-wavelength SED analyses, the optical variability in this system was likely caused by X-ray reprocessing at the outer accretion disk. We also find that some repetitive optical oscillations can occur at mass accretion rates >10 times lower than previously thought. Although the mechanism of this kind of variations is still unclear, we suggest that the lack of sustained mass accretion, which would be induced in long-period systems, may be a key condition.

© The Author 2016. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. We report nine long X-ray bursts from neutron stars, detected with the Monitor of All-sky X-ray Image (MAXI). Some of these bursts lasted for hours, and hence are qualified as superbursts, which are prolonged thermonuclear flashes on neutron stars and are relatively rare events. MAXI observes roughly 85% of the whole sky every 92 minutes in the 2-20 keV energy band, and has detected nine bursts with a long e-folding decay time, ranging from 0.27 to 5.2 hr, since its launch in 2009 August until 2015 August. The majority of the nine events were found to originate from transient X-ray sources. The persistent luminosities of the sources, when these prolonged bursts were observed, were lower than 1% of the Eddington luminosity for five of them and lower than 20% for the rest. This trend is contrastive to the 18 superbursts observed before MAXI, all but two of which originated from bright persistent sources. The distribution of the total emitted energy, i.e., the product of e-folding time and luminosity, of these bursts clusters around 1041-1042 erg, whereas both the e-folding time and luminosity ranges for an order of magnitude. Among the nine events, two were from 4U 1850-086 during phases of relatively low persistent flux, whereas it usually exhibits standard short X-ray bursts during outbursts.

© Published under licence by IOP Publishing Ltd. The CALorimetric Electron Telescope (CALET) is a space experiment, currently under development by Japan in collaboration with Italy and the United States, which will measure the flux of cosmic-ray electrons (and positrons) up to 20 TeV energy, of gamma rays up to 10 TeV, of nuclei with Z from 1 to 40 up to 1 PeV energy, and will detect gamma-ray bursts in the 7 keV to 20 MeV energy range during a 5 year mission. These measurements are essential to investigate possible nearby astrophysical sources of high energy electrons, study the details of galactic particle propagation and search for dark matter signatures. The main detector of CALET, the Calorimeter, consists of a module to identify the particle charge, followed by a thin imaging calorimeter (3 radiation lengths) with tungsten plates interleaving scintillating fibre planes, and a thick energy measuring calorimeter (27 radiation lengths) composed of lead tungstate logs. The Calorimeter has the depth, imaging capabilities and energy resolution necessary for excellent separation between hadrons, electrons and gamma rays. The instrument is currently being prepared for launch (expected in 2015) to the International Space Station ISS, for installation on the Japanese Experiment Module - Exposure Facility (JEM-EF).

Context. 4U 1538-52, an absorbed high mass X-ray binary with an orbital period of similar to 3.73 days, shows moderate orbital intensity modulations with a low level of counts during the eclipse. Several models have been proposed to explain the accretion at different orbital phases by a spherically symmetric stellar wind from the companion. Aims. The aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. In particular, we study the folded light curve and the changes in the spectral parameters with orbital phase to analyse the stellar wind of QV Nor, the mass donor of this binary system. Methods. We used all the observations made from the Gas Slit Camera on board MAXI of 4U 1538-52 covering many orbits continuously. We obtained the good interval times for all orbital phase ranges, which were the input for extracting our data. We estimated the orbital period of the system and then folded the light curves, and we fitted the X-ray spectra with the same model for every orbital phase spectrum. We also extracted the averaged spectrum of all the MAXI data available. Results. The MAXI spectra in the 2-20 keV energy range were fitted with an absorbed Comptonisation of cool photons on hot electrons. We found a strong orbital dependence of the absorption column density but neither the fluorescence iron emission line nor low energy excess were needed to fit the MAXI spectra. The variation in the spectral parameters over the binary orbit were used to examine the mode of accretion onto the neutron star in 4U 1538-52. We deduce a best value of (M) over dot/nu(infinity) = 0.65x10(-9) M (circle dot) yr(-1)/( km s(-1)) for QV Nor.

© ESO, 2015. Context. 4U 1538-52, an absorbed high mass X-ray binary with an orbital period of ∼3.73 days, shows moderate orbital intensity modulations with a low level of counts during the eclipse. Several models have been proposed to explain the accretion at different orbital phases by a spherically symmetric stellar wind from the companion. Aims. The aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. In particular, we study the folded light curve and the changes in the spectral parameters with orbital phase to analyse the stellar wind of QV Nor, the mass donor of this binary system. Methods. We used all the observations made from the Gas Slit Camera on board MAXI of 4U 1538-52 covering many orbits continuously. We obtained the good interval times for all orbital phase ranges, which were the input for extracting our data. We estimated the orbital period of the system and then folded the light curves, and we fitted the X-ray spectra with the same model for every orbital phase spectrum. We also extracted the averaged spectrum of all the MAXI data available. Results. The MAXI spectra in the 2-20 keV energy range were fitted with an absorbed Comptonisation of cool photons on hot electrons. We found a strong orbital dependence of the absorption column density but neither the fluorescence iron emission line nor low energy excess were needed to fit the MAXI spectra. The variation in the spectral parameters over the binary orbit were used to examine the mode of accretion onto the neutron star in 4U 1538-52. We deduce a best value of M/ν<inf>∞</inf> = 0.65 × 10^-9M<inf>⊙</inf> yr^-1/(km s^-1) for QV Nor.

© Owned by the authors, published by EDP Sciences, 2015. The CALorimetric Electron Telescope (CALET) is a space experiment, currently under development by Japan in collaboration with Italy and the United States, which will measure the flux of cosmic-ray electrons (and positrons) up to 20 TeV energy, of gamma rays up to 10 TeV, of nuclei with Z from 1 to 40 up to 1 PeV energy, and will detect gamma-ray bursts in the 7 keV to 20 MeV energy range during a 5 year mission. These measurements are essential to investigate possible nearby astrophysical sources of high energy electrons, study the details of galactic particle propagation and search for dark matter signatures. The main detector of CALET, the Calorimeter, consists of a module to identify the particle charge, followed by a thin imaging calorimeter (3 radiation lengths) with tungsten plates interleaving scintillating fibre planes, and a thick energy measuring calorimeter (27 radiation lengths) composed of lead tungstate logs. The Calorimeter has the depth, imaging capabilities and energy resolution necessary for excellent separation between hadrons, electrons and gamma rays. The instrument is currently being prepared for launch (expected in 2015) to the International Space Station ISS, for installation on the Japanese Experiment Module - Exposure Facility (JEM-EF).

The Monitor of All-sky X-ray Image (MAXI) was launched on July 2009. It was equipped on the exposed facility of the Japanese experiment module "Kibo" on-board the international space station (ISS). The main purpose of the MAXI is to discover and promptly report X-ray transients as well as steady monitoring of known X-ray sources in the whole sky. The MAXI operation is approved at least until the end of March 2015. We are working on public data release of the MAXI, in order to enhance use of the MAXI data by scientists all over the world. On-demand data analysis system using a web-interface has started since November 2011. Now we are working toward permanent public MAXI data archive including MAXI specific software for scientific analysis.

© The Author 2014. We report MAXI and Swift observations of short-term spectral softenings of the Galactic black hole X-ray binary Swift J1753.5-0127 in the low/hard state. These softening events are characterized by a simultaneous increase of soft X-rays (2-4 keV) and a decrease of hard X-rays (15-50 keV) lasting for a few tens of days. The X-ray energy spectra during the softening periods can be reproduced with a model consisting of a multi-color disk blackbody and its Comptonized component. The fraction of the Comptonized component decreased from 0.30 to 0.15 when the spectrum became softer; meanwhile the inner disk temperature (Tin) increased from 0.2 to 0.45 keV. These results imply that the softening events are triggered by a short-term increase of the mass accretion rate. During the observed spectral softening events, the disk flux (Fdisk) and Tin did not obey the Fdisk ∝ (Tin )4 relation, suggesting that the inner disk radius does not reach the innermost stable circular orbit.

© 2014 Elsevier B.V. The CALorimetric Electron Telescope (CALET) space experiment, currently under development by Japan in collaboration with Italy and the United States, will measure the flux of cosmic-ray electrons (including positrons) to 20 TeV, gamma rays to 10 TeV and nuclei with Z=1 to 40 up to 1,000 TeV during a two-year mission on the International Space Station (ISS), extendable to five years. These measurements are essential to search for dark matter signatures, investigate the mechanism of cosmic-ray acceleration and propagation in the Galaxy and discover possible astrophysical sources of high-energy electrons nearby the Earth. The instrument consists of two layers of segmented plastic scintillators for the cosmic-ray charge identification (CHD), a 3 radiation length thick tungsten-scintillating fiber imaging calorimeter (IMC) and a 27 radiation length thick lead-tungstate calorimeter (TASC). CALET has sufficient depth, imaging capabilities and excellent energy resolution to allow for a clear separation between hadrons and electrons and between charged particles and gamma rays. The instrument will be launched to the ISS within 2014 Japanese Fiscal Year (by the end of March 2015) and installed on the Japanese Experiment Module-Exposed Facility (JEM-EF). In this paper, we will review the status and main science goals of the mission and describe the instrument configuration and performance.

MAXI started its operation in 2009 August. Owing to its unprecedentedly high sensitivity as an all-sky X-ray monitor and to its capability of real-time data transfer, we have detected 56 strong flares from twenty-one active stars (eleven RS CVn systems, one Algol system, seven dMe stars, one dKe star and one Young Stellar Object). These flares have large X-ray luminosity of 6 × 1030 -5 × 1033 ergs s-1 in the 2-20 keV band. The flares can be thought to be high ends among their own categories. During the flare from AT Mic on 2012 April 18th, one of the largest X-ray luminosities was recorded as a dMe star, 6 × 1032 ergs s -1 in the 2-20 keV band. It is larger than its bolometric luminosity by 4 times. The total energy emitted during the flare is 1036 ergs in the same band. Such total energy can be obtained on large flares from RS CVn system, but not on any other flares from dMe stars. In this proceeding, we report on the present situation in characteristics of hyper X-ray flares on each stellar categories. © International Astronomical Union 2014.

© The Authors. GRB 130925A was an unusual gamma ray burst (GRB), consisting of three distinct episodes of high-energy emission spanning 20 ks, making it a member of the proposed category of 'ultralong' bursts. It was also unusual in that its late-time X-ray emission observed by Swift was very soft, and showed a strong hard-to-soft spectral evolution with time. This evolution, rarely seen in GRB afterglows, can be well modelled as the dust-scattered echo of the prompt emission, with stringent limits on the contribution from the normal afterglow (i.e. external shock) emission.We consider and reject the possibility that GRB 130925A was some form of tidal disruption event, and instead show that if the circumburst density around GRB 130925A is low, the long duration of the burst and faint external shock emission are naturally explained. Indeed, we suggest that the ultralong GRBs as a class can be explained as those with low circumburst densities, such that the deceleration time (at which point the material ejected from the nascent black hole is decelerated by the circumburst medium) is 20 ks, as opposed to a few hundred seconds for the normal long GRBs. The increased deceleration radius means that more of the ejected shells can interact before reaching the external shock, naturally explaining both the increased duration of GRB 130925A, the duration of its prompt pulses, and the fainter-than-normal afterglow.

We report the results from an X-ray and near-infrared observation of the Galactic black hole binary 4U 1630-47 in the very high state (VHS), performed with Suzaku and the Infrared Survey Facility around the peak of the 2012 September-October outburst. The X-ray spectrum is approximated by a steep power law, with photon index of 3.2, identifying the source as being in the VHS. A more detailed fit shows that the X-ray continuum is well described by a multicolor disk, together with thermal and nonthermal Comptonization. The inner disk appears slightly truncated by comparison with a previous high/soft state of this source, even taking into account energetic coupling between the disk and corona, although there are uncertainties due to the dust-scattering correction. The near-infrared fluxes are higher than the extrapolated disk model, showing that there is a contribution from irradiation in the outer disk and/or the companion star at these wavelengths. Our X-ray spectra do not show the Doppler-shifted iron emission lines indicating a baryonic jet that were seen four days previously in an XMM-Newton observation, despite the source being in a similar state. There are also no significant absorption lines from highly ionized iron lines as are seen in the previous high/soft state data. We show that the increased source luminosity is not enough on its own to make the wind so highly ionized as to be undetectable. This shows that the disk wind has changed in terms of its launch radius and/or density compared to the high/soft state. © 2014. The American Astronomical Society. All rights reserved..

MAXI/GSC (Monitor of All-sky X-ray Image/Gas Slit Camera) observed 21 outbursts from Circinus X-1 between 2009 August and 2013 December. Although 14 outbursts showed ordinary gradual decays, in seven others we found a sudden drop in luminosity on a time scale of a few hours around the periastron, and then the outbursts terminated. These sudden drops started at the estimated luminosity of a few times 1037 erg s-1 and their luminosities dropped to &lsim;3 &times; 1036 erg s-1. We offer three interpretations of the sudden drop: (1) the end of the outburst during its dip, (2) the propeller effect, and (3) the stripping effect by the stellar wind of the companion star. It is difficult to explain the phenomenon by any of these interpretations alone. The interpretation of (1) is possible for only two outbursts on the assumption of rapid decay. The propeller effect (2) is expected to occur at a constant luminosity, which is incompatible with the observed facts. In the stripping effect (3), the ram pressure of a typical stellar wind is not sufficient to blowmost of the accretion disk off. In this paper, we discuss the possibility of a modified stripping effect by assuming other additional conditions such as wind clumping and disk instability. © The Author 2014.

© The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. The Monitor of All-sky X-ray Image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs), including bursts with soft spectra, such as X-ray flashes (XRFs).MAXI/GSC is sensitive to the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which are capable of detecting GRBs. Since the beginning of the MAXI operation on 2009 August 15, GSC observed 35 GRBs up to the middle of 2013. One third of them were also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N-log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range as that of MAXI/GSC, we find the possibility that many of the MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N-log S distribution suggests that the MAXI XRFs are distributed over a closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss the possibility of confusion of such galactic transients with the MAXI GRB samples.

© The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. We report on long-term monitoring of a newly discovered X-ray nova, MAXI J1910-057 (= Swift J1910.2-0546) by MAXI and Swift. The new X-ray transient was first detected on 2012 May 31 by the MAXI Gas Slit Camera (GSC) and the Swift Burst Alert Telescope (BAT) almost simultaneously. We analyzed X-ray and UV data for 270 d following the outburst onset taken by repeated MAXI scans and Swift pointing observations. The obtained X-ray light curve for the inital 90 d is roughly represented by a fast-rise and exponentialdecay profile. However, it re-brightened at ∼110 d after the onset and finally went down below both GSC and BAT detection limits after 240 d. All the X-ray energy spectra are fitted well with a model consisting of a multi-color-disk blackbody and its Comptonized hard tail. During the soft-state periods, the inner-disk radius of the best-fit model was almost constant. If the radius represents the innermost stable circular orbit of a nonspinning black hole and the soft-to-hard transitions occur at 1%-4% of the Eddington luminosity, the mass of the compact object is estimated to be > 2.9 M⊙ and the distance to be > 1.70 kpc. The inner-disk radius became larger in the hard/hard-intermediate state. This suggests that the accretion disk would be truncated. We detected an excess of the UV flux over the disk blackbody component extrapolated from the X-ray data, which can be modeled as reprocessed emission irradiated by the inner disk. We also found that the UV light curve mostly traced the X-ray curve, but a short dipping event was observed in both the UV and the X-ray bands with a 3.5 d X-ray time lag. This can be interpreted as the radial inflow of accreting matter from the outer UV region to the inner X-ray region.

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. We present our recent results from Suzaku and Swift Time-Of-Opportunity observations of the Galactic black hole transients MAXI J1305-704, H 1743-322, and 4U 1630-472. The wide-band high-quality data obtained in the low/hard state (the former two sources) and in the very high state (4U 1630-472) enable us to constrain the structure of the accretion flow in various X-ray luminosities. We find that all the spectra, including the 4U 1630-472 data, taken 4 days after the detection of the baryonic jet [2], contain strong Comptonised components and suggest disc truncation. In the H 1743-322 data, a low-frequency QPO is detected, whose frequency is found to correlate the X-ray luminosity and photon index. This may be explained by the evolution of the disc truncation radius. Considering these results, we discuss the inner disc structure of black hole binaries over a wide range of mass accretion rates.

We present the observation of an extraordinary luminous soft X-ray transient, MAXI J0158-744, by the Monitor of All-sky X-ray Image (MAXI) on 2011 November 11. This transient is characterized by a soft X-ray spectrum, a short duration (1.3 × 103 s < ΔTd < 1.10 × 10 4 s), a rapid rise (<5.5 × 103 s), and a huge peak luminosity of 2 × 1040 erg s-1 in 0.7-7.0 keV band. With Swift observations and optical spectroscopy from the Small and Moderate Aperture Research Telescope System, we confirmed that the transient is a nova explosion, on a white dwarf in a binary with a Be star, located near the Small Magellanic Cloud. An early turn-on of the super-soft X-ray source (SSS) phase (<0.44 days), the short SSS phase duration of about one month, and a 0.92 keV neon emission line found in the third MAXI scan, 1296 s after the first detection, suggest that the explosion involves a small amount of ejecta and is produced on an unusually massive O-Ne white dwarf close to, or possibly over, the Chandrasekhar limit. We propose that the huge luminosity detected with MAXI was due to the fireball phase, a direct manifestation of the ignition of the thermonuclear runaway process in a nova explosion. © 2013. The American Astronomical Society. All rights reserved.

We report the results from X-ray studies of the newly discovered black hole candidate MAXI J1305-704 based on Suzaku and Swift observations in the low/hard and high/soft states, respectively. The long Suzaku observation shows two types of clear absorption dips, both of which recur on a dip interval of 9.74 ± 0.04 hr, which we identify with the orbital period. There is also partially ionized absorption in the nondip (persistent) emission in both the high/soft state and, very unusually, the low/hard state. However, this absorption (in both states) has substantially lower ionization than that seen in other high inclination systems, where the material forms a homogeneous disk wind. Here instead the absorption is most likely associated with clumpy, compact structures associated with the dipping material, which we see uniquely in this source likely because we view it at a very large inclination angle. A large inclination angle is also favored, together with a low black hole mass, to explain the high disk temperature seen in the fairly low luminosity high/soft state, as Doppler boosting enhances the disk temperature at high inclination. The disk radius inferred from these data is significantly smaller than that of the soft component seen in the low/hard state, supporting models where the disk is truncated at low luminosities. We find, however, that the lack of variability power on timescales of ∼50 s in the Suzaku low/hard state data is difficult to explain, even with a low-mass black hole. © 2013. The American Astronomical Society. All rights reserved..

We present the luminosity dwell-time distributions during the hard states of two low-mass X-ray binaries containing a neutron star (NS), 4U 1608-52 and Aql X-1, observed with MAXI/GSC. The luminosity distributions show a steep cutoff on the low-luminosity side at 1.0 × 1036 erg s -1 in both sources. The cutoff implies a rapid luminosity decrease in their outburst decay phases and this decrease can be interpreted as being due to the propeller effect. We estimate the surface magnetic field of 4U 1608-52 to be (0.5-1.6) × 108 G and Aql X-1 to be (0.6-1.9) × 108 G from the cutoff luminosity and apply the same propeller mechanism to the similar rapid luminosity decrease observed in the transient Z source, XTE J1701-462, with RXTE/ASM. Assuming that the spin period of the NS is on the order of milliseconds, the observed cutoff luminosity implies a surface magnetic field on the order of 109 G. © 2013. The American Astronomical Society. All rights reserved..

We present a catalog of high Galactic-latitude (|b| > 10°) X-ray sources detected in the first 37 months of data of the Monitor of All-sky X-ray Image/Gas Slit Camera (MAXI/GSC). To achieve the best sensitivity, we develop a background model of the GSC that well reproduces the data based on the detailed on-board calibration. Source detection is performed through image fits with a Poisson likelihood algorithm. The catalog contains 500 objects detected with significances of s D, 4-10 keV ≥ 7 in the 4-10 keV band. The limiting sensitivity is ≈7.5 × 10-12 erg cm-2 s-1 (≈0.6 mCrab) in the 4-10 keV band for 50% of the survey area, which is the highest ever achieved in an all-sky survey mission covering this energy band. We summarize the statistical properties of the catalog and results from cross matching with the Swift/BAT 70 month catalog, the meta-catalog of X-ray detected clusters of galaxies, and the MAXI/GSC 7 month catalog. Our catalog lists the source name (2MAXI), position and its error, detection significances and fluxes in the 4-10 keV and 3-4 keV bands, the hardness ratio, and the basic information of the likely counterpart available for 296 sources. © 2013. The American Astronomical Society. All rights reserved.

We report on the spectral evolution of a new X-ray transient, MAXIJ0556-332, observed by MAXI, Swift, and RXTE. The source was discovered on 2011 January 11 (MJD = 55572) by the MAXI Gas Slit Camera allsky survey at (l , b) = (238.i9, -25.i2), 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, MAXIJ0556-332 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 ×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 × 1037 erg s-1. This suggests a source distance of >17 kpc. © 2013. Astronomical Society of Japan.

MAXI(Monitor of All-sky X-ray Image)は2009年8月から「きぼう」船外実験プラットフォーム上で観測を続けている X線全天モニタである.MAXIには2つの主要な役割があり,1つはX線突発変動天体の速報,もう1つは長期にわたる全天のX線による定常的な観測である.MAXIはISSの通信環境により,観測時間の70%はリアルタイムにデータを取得できる.MAXIは,リアルタイムデータを効率良く扱うため,MAXI-DBという地上システムを構築し,それらを利用してこれまでの3年間の観測で150件以上ものX線天体の変動現象に関する観測速報を行った.MAXIの観測データには多くの天体に関する様々な科学情報が含まれるため,データを全世界の科学者に利用しやすい形式で提供することは重要である.我々は段階的にデータ公開を実現してきており,現在までに300程度のカタログ天体の日々の画像,ライトカーブ,エネルギースペクトルデータや,ユーザーが任意に入力した位置/時刻に対応したデータ(オンデマンドデータ)を公開している. MAXIのデータは時間/空間方向で複雑に入り組んでいるため,1日毎に全データを1ファイルにアーカイブする方式では非効率で,オンデマンド公開で天体毎の解析処理の際,要求される処理時間を達成できなかった.そこで,X線データを時刻や到来方向によって整理し,幾つかのファイルに分割し,結合する方式を使ったシステムを構築した.その結果,100倍程度の効率化を達成することができた.

科学分野で扱われるデータ量は爆発的に増加しており,膨大なデータに対する機械的処理への要求は極めて高い.論文では,天体物理学におけるX線天体の X線アウトバースト現象を対象に,その観測データの類似検索を行う手法を提案する.具体的には,観測データからアウトバースト部分の自動検出を行なう.得られたデータに対し,ノイズ除去を目的とした平滑化と正規化を施した上で,動的タイムワーピング(DTW; DynamicTimeWarping)法を適用する. DTW法は,長さが異なる時系列データに対しても適用可能であるだけでなく,時間軸方向のスケーリングに対しても頑健なマッチングを行うことが距離である.さらに,X線アウトバーストの持つ特徴や,類似検索に対する要求を考慮し,DTW法の改良手法であるDerivative DTW法や,DTW法に滑り窓を適用した手法など,いくつかの新たな手法を提案する.さらに,実データとの比較によってその有効性を評価する.

The stellar wind around a compact object in luminous wind-accreting high-mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Strömgren region, leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from the Monitor of All-sky X-ray Image (MAXI), we studied the observed dependence of the photoelectric absorption as a function of the orbital phase in Vela X-1 and found that it is inconsistent with expectations for a spherically symmetric smooth wind. Taking into account previous investigations, we developed a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star, which is responsible for the observed absorption curve. © 2013 ESO.

Monitor of All-sky X-ray Image (MAXI) on board International Space Station is capable of observing gamma-ray bursts (GRBs) and sending notices of GRBs or other transient events, using real time connection to the ground. MAXI observed 32 GRBs or short X-ray transients as of the end of September 2012. Among them, eleven events were simultaneously detected by other satellites. The observed rate of the MAXI GRBs is about one event per month. This rate is comparable to a past observation with larger effective area and larger field of view. The fact indicates that MAXI has better sensitivity to observe GRBs because of low background. The distribution of the spectral hardness of MAXI GRBs is similar to the results of a past instrument, which is sensitive to similar energy range. © EAS, EDP Sciences 2013.

We analyzed the initial rising behaviors of X-ray outbursts from two transient low-mass X-ray binaries (LMXBs) containing a neutron-star (NS), Aquila X-1 (Aql X-1) and 4U 1608-52, which are continuously being monitored by MAXI/GSC in 2-20 keV, RXTE/ASM in 2-10 keV, and Swift/BAT in 15-50 keV. We found that the observed ten outbursts can be classified into two types based on the patterns of the relative intensity evolutions in the two energy bands below/above 15 keV. One type behaves as the 15-50 keV intensity achieves the maximum during the initial hard-state period, and drops greatly at the hard-to-soft state transition. On the other hand, the other type does as both the 2-15 keV and 15-50 keV intensities achieve the maximums after the transition. The former have the longer initial hard-state (9 d) than the latter (5 d). Therefore, we named them as slow-type (S-type) and fast-type (F-type), respectively. These two types also show differences in the luminosity at the hard-to-soft state transition as well as in the average luminosity before the outburst started, where the S-type are higher than the F-type in both. These results suggest that the X-ray radiation during the pre-outburst period, which heats up the accretion disk and delays the disk transition (i.e., from a geometrically thick disk to a thin one), would determine whether the following outburst becomes S-type or F-type. The luminosity when the hard-to-soft state transition occurs is higher than ̃8×1036 erg s-1 in the S-type, which corresponds to 4% of the Eddington luminosity for a 1.4M NS. © 2012 Astronomical Society of Japan.

MAXI/GSC detected a superburst from EXO 1745-248 in the globular cluster Terzan 5 on 2011 October 24. The GSC light curve shows an exponential decay with an e-folding time of 0.3 d. The spectra are consistent with the blackbody radiation, whose temperature is 2.2 keV and 1.2 keV at MJD 55858.56 and 55859.20, respectively. The fluence is 1.4 × 1042 erg in 2-20 keV, assuming an 8.7 kpc distance. The sphere radius of the blackbody and its luminosity are estimated to be 6.2 km and 1.1 × 1038ergs -1, respectively, from a spectral fitting at the flux peak. Those e-folding time, temperature, softening, fluence, and radius are typical of superbursts from low-mass X-ray binaries. The superburst was followed by an outburst 28 hours after the superburst onset. The outburst lasted for 5 d and the fluence was 4.3 × 1042 erg. The instability of the accretion disk caused by the superburst would be an explanation for the outburst, whereas the mass accretion of the matter evaporated from the surface of the companion star by the superburst would be another possibility. © 2012. Astronomical Society of Japan.

We report on the first observation of an X-ray outburst of a Be/X-ray binary pulsar, LSV+44 17/RXJ0440.9+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 LSV+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 Kα 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. © 2012. Astronomical Society of Japan.

We report on X-ray spectral and timing results of a new black hole candidate (BHC), MAXI J1659-152, with an orbital period of 2.41 hr (shortest among BHCs) in a 2010 outburst from 65 Rossi X-ray Timing Explorer (RXTE) observations and 8 simultaneous Swift and RXTE observations. According to the definitions of the spectral states in Remillard and McClintock (2006, ARA&A, 44, 49), most of the observations have been classified into the intermediate state. All of the X-ray broadband spectra can be modeled by a multi-color disk plus a power-law with an exponential cutoff or a multi-color disk plus a Comptonization component. During the initial phase of the outburst, a high-energy cutoff was visible at 30-40 keV. The innermost radius of the disk gradually decreased by a factor of more than 3 from the onset of the outburst, and reached a constant value of 35 d(10)cos i(-1/2) km, where d(10) is the distance in units of 10 kpc and i is the inclination. The type-C quasi-periodic oscillation (QPO) frequency varied from 1.6 Hz to 7.3 Hz in association with a change of the innermost radius, while the innermost radius remained constant during the type-B QPO detections at 1.6-4.1 Hz. Hence, we suggest that the origin of the type-B QPOs is different from that of type-C QPOs, the latter of which would originate from the disk truncation radius. Assuming the constant innermost radius in the latter phase of the outburst as the innermost stable circular orbit, the black hole mass in MAXI J1659-152 is estimated to be 3.6-8.0 M-circle dot for a distance of 5.3-8.6 kpc and an inclination angle of 60 degrees-75 degrees.

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 comple-mentarily. 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 ∼0.7 keV to ∼0.4 keV and the disk flux decreased by an order of magnitude. Nevertheless, the innermost radius is constant at ∼41 D 3.5 (cos i) -1/2 km, where D 3.5 is the source distance in units of 3.5kpc 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 ± 0.28 M ⊙ D 3.5 (cos -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°, 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 3.1-55M ⊙ and 2.3-22 kpc, respectively. This confirms that the compact object in XTE J1752-223 is a black hole. © 2012. Astronomical Society of Japan.

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 × 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 × 1032 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, logLX=Lbol = -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. © 2011. Astronomical Society of Japan.

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. © 2011. Astronomical Society of Japan.

We present the first unbiased source catalog of the Monitor of All-sky X-ray Image (MAXI) mission at high Galactic latitudes (|b| > 10°), 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 σ significance level with a limiting sensitivity of ∼1.5 × 10-11 erg cm-2 s-1 (1.2mCrab) 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 logN-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. © 2011. Astronomical Society of Japan.

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, GRB090926B. 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±0.9) × 10 9 Y' -3/2 cm, the Lorentz factor of the plasma is Γ = (110±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. © 2011. Astronomical Society of Japan.

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 H > 10 22 cm -2 rapidly decreases against the luminosity (L X ), from 0.73±0.10 at L X = 10 42-43.5 erg s -1 to 0.12±0.08 at L 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 γ 1 = 0.84 (fixed) and γ 2 = 2.0±0.2 below and above the break luminosity, L * = 10 43.3±0.4 erg s -1 , respectively. While the result of the MAXI/GSC agrees well with that of HEAO-1 at L X ≥ 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 Γ ∼ 1.7 for L X ≤ 10 44 erg s -1 , while Γ ∼ 2.0 for L X ≥ 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. © 2011. Astronomical Society of Japan.

We present the results of monitoring the galactic black hole candidate GX 3394 with the Monitor of Allsky 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 ≤25% in the total flux. In spite of the flux variability over a factor of 3, the innermost disk radius is constant at Rin = 61±2 km for an inclination angle of i = 46° and a distance of d = 8 kpc. This Rin value is consistent with those of past measurementswith 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±0.2M for i = 46° 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 4.3M⊙ <M < 13.3M⊙ for the allowed range of d = 6-15 kpc and i < 60°. 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. © 2011. Astronomical Society of Japan.

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.°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 readout 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. © 2011. Astronomical Society of Japan.

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 counterswith a total detector area of 5350cm2. 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. © 2011. Astronomical Society of Japan.

The CALET Gamma-ray Burst Monitor (CGBM) is the secondary scientific instrument of the CALET mission to be attached to the ISS, sensitive to X and gamma-rays from 7 keV to 20 MeV. The scientific goal of the CGBM is to search out a clue to radiation mechanisms of gamma-ray bursts (GRBs) by obtaining very broadband spectra from optical to TeV gamma-rays together with the primary instrument, the calorimeter (CAL) sensitive to GeV-TeV gamma-rays, and the star camera (ASC). The CGBM sensor consists of the hard X-ray monitor (HXM) sensitive to the 7-1000 keV range and the soft gamma-ray monitor (SGM) to 100 keV-20 MeV utilizing two LaBr3 (Ce) and one BGO scintillators. The LaBr3 crystals would be employed first for GRB observations in space. The electonics box (E-box) processing signals from the sensors, is equipped with analog circuits for a wide dynamic range, onboard GRB trigger system, and 10 Mbyte memory for GRB data accumulation. In this paper, we will describe the scientific performance and the development status of the CGBM.

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± 10mCrab and 164± 17mCrab, 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 F ≳ 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 d ∼ 2.5 × 10 4 s, to the synchrotron cooling time, the magnetic field was evaluated to be B ∼ 4.5 × 10 -2 G (?=10)δ 1=3, where ? is the jet beaming factor. Assuming that the light crossing time of the emission region is shorter than the doubling rise time, t r . 2 × 10 4 s, the region size was roughly estimated as R < 6 × 10 15 cm (δ =10). These results are consistent with values previously reported. For the February flare, the rise time, tr < 1.3 × 10 5 s, gives a loose upper limit on the size asR< 4 × 10 16 cm .δ =10/, although the longer decay time, t d ∼ 1.4 × 10 5 s, indicates B ∼ 1.5 × 10 -2 G (δ =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. © 2010. Astronomical Society of Japan.

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-lawwith 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. © 2010. Astronomical Society of Japan.

MAXI, the first astronomical payload attached to ISS JEM-EF, began operation on August 3, 2009 for monitoring all-sky X-ray images every ISS orbit (92 min). This paper reports the first results obtained during the 1-month test observations. All instruments as well as two main X-ray slit cameras, the GSC and SSC, worked as we expected. The detection limits of MAXI-GSC are about 25 mCrab and 8 mCrab for one ISS orbit (92 min) and 1-day observations, respectively. GSC covers about 76% and 96% of the entire sky for respective single-orbit and 1-day observations. MAXI has detected a transient X-ray pulsar A0535+26, an X-ray burst, and a gamma-ray burst in the first month of the test observation period. © 2010 American Institute of Physics.