arXiv subjects

K. Ebisawa

Publications and source records attributed to K. Ebisawa.

At least 37 records · Page 2Linked to original sources

arXiv2019

The CALorimetric Electron Telescope (CALET) on the International Space Station: Results from the First Two Years On Orbit

The CALorimetric Electron Telescope (CALET) is a high-energy astroparticle physics space experiment installed on the International Space Station (ISS), developed and operated by Japan in collaboration with Italy and the United States. The CALET mission goals include the investigation of possible nearby sources of high-energy electrons, of the details of galactic particle acceleration and propagation, and of potential signatures of dark matter. CALET measures the cosmic-ray electron + positron flux up to 20 TeV, gamma-rays up to 10 TeV, and nuclei with Z=1 to 40 up to 1,000 TeV for the more abundant elements during a long-term observation aboard the ISS. Starting science operation in mid-October 2015, CALET performed continuous observation without major interruption with close to 20 million triggered events over 10 GeV per month. Based on the data taken during the first two-years, we present an overview of CALET observations: uses w/o major interruption 1) Electron + positron energy spectrum, 2) Nuclei analysis, 3) Gamma-ray observation including a characterization of on-orbit performance. Results of the electromagnetic counterpart search for LIGO/Virgo gravitational wave events are discussed as well.

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arXiv2018

Search for GeV Gamma-ray Counterparts of Gravitational Wave Events by CALET

We present results on searches for gamma-ray counterparts of the LIGO/Virgo gravitational-wave events using CALorimetric Electron Telescope ({\sl CALET}) observations. The main instrument of {\sl CALET}, CALorimeter (CAL), observes gamma-rays from $\sim1$ GeV up to 10 TeV with a field of view of nearly 2 sr. In addition, the {\sl CALET} gamma-ray burst monitor (CGBM) views $\sim$3 sr and $\sim2π$ sr of the sky in the 7 keV -- 1 MeV and the 40 keV -- 20 MeV bands, respectively, by using two different crystal scintillators. The {\sl CALET} observations on the International Space Station started in October 2015, and here we report analyses of events associated with the following gravitational wave events: GW151226, GW170104, GW170608, GW170814 and GW170817. Although only upper limits on gamma-ray emission are obtained, they correspond to a luminosity of $10^{49}\sim10^{53}$ erg s$^{-1}$ in the GeV energy band depending on the distance and the assumed time duration of each event, which is approximately the order of luminosity of typical short gamma-ray bursts. This implies there will be a favorable opportunity to detect high-energy gamma-ray emission in further observations if additional gravitational wave events with favorable geometry will occur within our field-of-view. We also show the sensitivity of {\sl CALET} for gamma-ray transient events which is the order of $10^{-7}$~erg\,cm$^{-2}$\,s$^{-1}$ for an observation of 100~s duration.

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arXiv2018

Extended Measurement of the Cosmic-Ray Electron and Positron Spectrum from 11 GeV to 4.8 TeV with the Calorimetric Electron Telescope on the International Space Station

Extended results on the cosmic-ray electron + positron spectrum from 11 GeV to 4.8 TeV are presented based on observations with the Calorimetric Electron Telescope (CALET) on the International Space Station utilizing the data up to November 2017. The analysis uses the full detector acceptance at high energies, approximately doubling the statistics compared to the previous result. CALET is an all-calorimetric instrument with a total thickness of 30 $X_0$ at normal incidence and fine imaging capability, designed to achieve large proton rejection and excellent energy resolution well into the TeV energy region. The observed energy spectrum in the region below 1 TeV shows good agreement with Alpha Magnetic Spectrometer (AMS-02) data. In the energy region below $\sim$300 GeV, CALET's spectral index is found to be consistent with the AMS-02, Fermi Large Area Telescope (Fermi-LAT) and Dark Matter Particle Explorer (DAMPE), while from 300 to 600 GeV the spectrum is significantly softer than the spectra from the latter two experiments. The absolute flux of CALET is consistent with other experiments at around a few tens of GeV. However, it is lower than those of DAMPE and Fermi-LAT with the difference increasing up to several hundred GeV. The observed energy spectrum above $\sim$1 TeV suggests a flux suppression consistent within the errors with the results of DAMPE, while CALET does not observe any significant evidence for a narrow spectral feature in the energy region around 1.4 TeV. Our measured all-electron flux, including statistical errors and a detailed breakdown of the systematic errors, is tabulated in the Supplemental Material in order to allow more refined spectral analyses based on our data.

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arXiv2018

On-orbit Operations and Offline Data Processing of CALET onboard the ISS

The CALorimetric Electron Telescope (CALET), launched for installation on the International Space Station (ISS) in August, 2015, has been accumulating scientific data since October, 2015. CALET is intended to perform long-duration observations of high-energy cosmic rays onboard the ISS. CALET directly measures the cosmic-ray electron spectrum in the energy range of 1 GeV to 20 TeV with a 2% energy resolution above 30 GeV. In addition, the instrument can measure the spectrum of gamma rays well into the TeV range, and the spectra of protons and nuclei up to a PeV. In order to operate the CALET onboard ISS, JAXA Ground Support Equipment (JAXA-GSE) and the Waseda CALET Operations Center (WCOC) have been established. Scientific operations using CALET are planned at WCOC, taking into account orbital variations of geomagnetic rigidity cutoff. Scheduled command sequences are used to control the CALET observation modes on orbit. Calibration data acquisition by, for example, recording pedestal and penetrating particle events, a low-energy electron trigger mode operating at high geomagnetic latitude, a low-energy gamma-ray trigger mode operating at low geomagnetic latitude, and an ultra heavy trigger mode, are scheduled around the ISS orbit while maintaining maximum exposure to high-energy electrons and other high-energy shower events by always having the high-energy trigger mode active. The WCOC also prepares and distributes CALET flight data to collaborators in Italy and the United States. As of August 31, 2017, the total observation time is 689 days with a live time fraction of the total time of approximately 84%. Nearly 450 million events are collected with a high-energy (E>10 GeV) trigger. By combining all operation modes with the excellent-quality on-orbit data collected thus far, it is expected that a five-year observation period will provide a wealth of new and interesting results.

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arXiv2017

Energy Calibration of CALET Onboard the International Space Station

In August 2015, the CALorimetric Electron Telescope (CALET), designed for long exposure observations of high energy cosmic rays, docked with the International Space Station (ISS) and shortly thereafter began tocollect data. CALET will measure the cosmic ray electron spectrum over the energy range of 1 GeV to 20 TeV with a very high resolution of 2% above 100 GeV, based on a dedicated instrument incorporating an exceptionally thick 30 radiation-length calorimeter with both total absorption and imaging (TASC and IMC) units. Each TASC readout channel must be carefully calibrated over the extremely wide dynamic range of CALET that spans six orders of magnitude in order to obtain a degree of calibration accuracy matching the resolution of energy measurements. These calibrations consist of calculating the conversion factors between ADC units and energy deposits, ensuring linearity over each gain range, and providing a seamless transition between neighboring gain ranges. This paper describes these calibration methods in detail, along with the resulting data and associated accuracies. The results presented in this paper show that a sufficient accuracy was achieved for the calibrations of each channel in order to obtain a suitable resolution over the entire dynamic range of the electron spectrum measurement.

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arXiv2017

Energy Spectrum of Cosmic-ray Electron and Positron from 10 GeV to 3 TeV Observed with the Calorimetric Electron Telescope on the International Space Station

First results of a cosmic-ray electron + positron spectrum, from 10 GeV to 3 TeV, is presented based upon observations with the CALET instrument on the ISS starting in October, 2015. Nearly a half million electron + positron events are included in the analysis. CALET is an all-calorimetric instrument with total vertical thickness of 30 $X_0$ and a fine imaging capability designed to achieve a large proton rejection and excellent energy resolution well into the TeV energy region. The observed energy spectrum over 30 GeV can be fit with a single power law with a spectral index of -3.152 $\pm$ 0.016 (stat.+ syst.). Possible structure observed above 100 GeV requires further investigation with increased statistics and refined data analysis.

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arXiv2017

Application of data science techniques to disentangle X-ray spectral variation of super-massive black holes

We apply three data science techniques, Nonnegative Matrix Factorization (NMF), Principal Component Analysis (PCA) and Independent Component Analysis (ICA), to simulated X-ray energy spectra of a particular class of super-massive black holes. Two competing physical models, one whose variable components are additive and the other whose variable components are multiplicative, are known to successfully describe X-ray spectral variation of these super-massive black holes, within accuracy of the contemporary observation. We hope to utilize these techniques to compare the viability of the models by probing the mathematical structure of the observed spectra, while comparing advantages and disadvantages of each technique. We find that PCA is best to determine the dimensionality of a dataset, while NMF is better suited for interpreting spectral components and comparing them in terms of the physical models in question. ICA is able to reconstruct the parameters responsible for spectral variation. In addition, we find that the results of these techniques are sufficiently different that applying them to observed data may be a useful test in comparing the accuracy of the two spectral models.

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arXiv2016

CALET Upper Limits on X-ray and Gamma-ray Counterparts of GW 151226

We present upper limits in the hard X-ray and gamma-ray bands at the time of the LIGO gravitational-wave event GW 151226 derived from the CALorimetric Electron Telescope (CALET) observation. The main instrument of CALET, CALorimeter (CAL), observes gamma-rays from ~1 GeV up to 10 TeV with a field of view of ~2 sr. The CALET gamma-ray burst monitor (CGBM) views ~3 sr and ~2pi sr of the sky in the 7 keV - 1 MeV and the 40 keV - 20 MeV bands, respectively, by using two different scintillator-based instruments. The CGBM covered 32.5% and 49.1% of the GW 151226 sky localization probability in the 7 keV - 1 MeV and 40 keV - 20 MeV bands respectively. We place a 90% upper limit of 2 x 10^{-7} erg cm-2 s-1 in the 1 - 100 GeV band where CAL reaches 15% of the integrated LIGO probability (~1.1 sr). The CGBM 7 sigma upper limits are 1.0 x 10^{-6} erg cm-2 s-1 (7-500 keV) and 1.8 x 10^{-6} erg cm-2 s-1 (50-1000 keV) for one second exposure. Those upper limits correspond to the luminosity of 3-5 x 10^{49} erg s-1 which is significantly lower than typical short GRBs.

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arXiv2014

ASTRO-H White Paper - AGN Winds

In this white paper we describe the prospects for ASTRO-H for the study of outflows from active galactic nuclei. The most important breakthroughs in this field are expected to arise from the high spectral resolution and sensitivity in the Fe-K band, combined with broad-band sensitivity over the full X-ray band and spectral capabilities also at lower energies. The sensitivity in the Fe-K region allows to extend the absorption measure distribution of the outflow out to the highest ionisation states accessible, where observations with current X-ray missions indicate that most of the outflowing gas is to be found. Due to the high-resolution and sensitivity it will also be able to give the definitive proof for the existence of ultra-fast outflows, and if so, characterise their physical properties in great detail. These ultra-fast outflows carry very large amounts of energy and momentum, and are of fundamental importance for feedback studies. We show how the ASTRO-H observations in general can help to constrain numerical models for outflows. The link to reflection and emission processes is also discussed, as well as the possible relation between outflows and relativistic emission lines. Finally, we discuss the prospects for other related categories of objects like BAL quasars, partially covered sources and Compton thick outflows.

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arXiv2010

On the light-bending model of X-ray variability of MCG-6-30-15

We apply the light bending model of X-ray variability to Suzaku data of the Seyfert 1 galaxy MCG-6-30-15. We analyze the energy dependence of the root mean square (rms) variability, and discuss conditions necessary for the model to explain the characteristic decrease of the source variability around 5-8 keV. A model, where the X-ray source moves radially rather than vertically close to the disk surface, can indeed reproduce the reduced variability near the energy of the Fe Kalpha line, although the formal fit quality is poor. The model then predicts the energy spectra, which can be compared to observational data. The spectra are strongly reflection dominated, and do not provide a good fit to Suzaku spectral data of the source. The inconsistency of this result with some previous claims can be traced to our using data in a broader energy band, where effects of warm absorber in the spectrum cannot be neglected.

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arXiv2010

Suzaku Observation of GRS 1915+105: Evolution of Accretion Disc Structure during Limit-Cycle Oscillation

We present results from the Suzaku observation of the microquasar GRS 1915+105 performed during the 2005 October multiwavelength campaign. The data include both stable state (class χ) and limit-cycle oscillation (class θ). Correct interstellar absorption as well as effects of dust scattering are fully taken into account in the spectral analysis. The energy spectra in the 2-120 keV band in both states are all dominated by strong Comptonization of disk photons by an optically thick (τ~7-10) and low temperature (T_e ~2-3 keV) hybrid plasmas containing non-thermal electrons produced with 10-60% of the total power input. Absorption lines of highly ionized Fe ions detected during the oscillation indicate that a strong disk wind is developed. The ionization stage of the wind correlates with the X-ray flux, supporting the photoionization origin. The iron-K emission line shows a strong variability during the oscillation; the reflection is strongest during the dip but disappears during the flare. We interpret this as evidence for "self-shielding" that the Comptonizing corona becomes geometrically thick in the flare phase, preventing photons from irradiating the outer disk. The low-temperature and high luminosity disk emission suggests that the disk structure is similar to that in the very high state of canonical black hole binaries. The spectral variability during the oscillation is explained by the change of the disk geometry and of the physical parameters of Comptonizing corona, particularly the fractional power supplied to the acceleration of non-thermal particles.

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arXiv2009

Is the black hole in GX 339-4 really spinning rapidly?

The wide-band Suzaku spectra of the black hole binary GX 339-4, acquired in 2007 February during the Very High state, were reanalyzed. Effects of event pileup (significant within ~ 3' of the image center) and telemetry saturation of the XIS data were carefully considered. The source was detected up to ~ 300$ keV, with an unabsorbed 0.5--200 keV luminosity of ~3.8 10^{38} erg/s at 8 kpc. The spectrum can be approximated by a power-law of photon index 2.7, with a mild soft excess and a hard X-ray hump. When using the XIS data outside 2' of the image center, the Fe-K line appeared extremely broad, suggesting a high black hole spin as already reported by Miller et al. (2008) based on the Suzaku data and other CCD data. When the XIS data accumulation is further limited to >3' to avoid event pileup, the Fe-K profile becomes narrower, and there appears a marginally better solution that suggests the inner disk radius to be 5-14 times the gravitational radius (1-sigma), though a maximally spinning black hole is still allowed by the data at the 90% confidence level. Consistently, the optically-thick accretion disk is inferred to be truncated at a radius 5-32 times the gravitational radius. Thus, the Suzaku data allow an alternative explanation without invoking a rapidly spinning black hole. This inference is further supported by the disk radius measured previously in the High/Soft state.

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arXiv2008

Broad-Band Spectrum of The Black Hole Candidate IGR J17497-2821 Studied with Suzaku

The broad-band 1-300 keV Suzaku spectrum of IGR J17497-2821, the X-ray transient discovered by INTEGRAL in September 2006, is presented. Suzaku observed IGR J17497-2821 on September 25, eight days after its discovery, for a net exposure of about 53 ksec. During the Suzaku observation, IGR J17497-2821 is very bright, 2 x 10^37 erg/s at 8 kpc in the 1-300 keV range, and shows a hard spectrum, typical of black hole candidates in the low-hard state. Despite the multi-mission X-ray monitoring of the source, only with Suzaku is it possible to obtain a broad-band spectrum in the 1-300 keV range with a very high signal to noise ratio. A sum of a multi-color disc (DISKBB) and a thermal Comptonization component (COMPPS) with mild reflection is a good representation of our IGR J17497-2821 Suzaku spectrum. The spectral properties of the accretion disc as well as the cut-off energy in the spectrum at about 150 keV are clearly detected and constrained. We discuss the implications on the physical model used to interpret the data and the comparison with previous results.

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arXiv2007

Detection of a Rare Supersoft Outburst Event during a Suzaku Observation of 1E0102.2-7219

We report the detection of a transient X-ray source toward the Small Magellanic Cloud (SMC) using the X-ray Imaging Spectrometer (XIS) onboard the Suzaku telescope. The source was detected at the edge of the XIS image during a routine observation of the calibration source 1E 0102.2-7219, a supernova remnant in the SMC. We constrained the source position using ray-tracing simulations. No such transient source was found at the position in the other Suzaku observations nor in all the available archived images of other X-ray missions for the last ~28 years. The XIS spectrum can be explained by a single blackbody with a temperature of ~72 eV, and an interstellar extinction of ~4.9x10^{20} H atoms cm^{-2} consistent with the value to the SMC. An additional absorption edge at ~0.74 keV was also confirmed, which is presumably due to the absorption by helium-like oxygen ions. Assuming that the source is at the distance of the SMC, the X-ray luminosity in the 0.2-2.0 keV band is ~10^{37} erg s^{-1} and the radius of the source is ~10^{8} cm. The XIS light curve shows about a two-fold decline in X-ray flux during the 24 ks observation. Together with the archived data, the X-ray flux in the burst is at least three orders of magnitude brighter than the undetected quiescent level. All these properties are often seen among supersoft sources (SSSs). We conclude that the transient source is another example of SSS in the SMC.

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arXiv2007

Hunting the nature of IGR J17497-2821 with X-ray and NIR observations

We report on a Chandra grating observation of the recently discovered hard X-ray transient IGR J17497-2821. The observation took place about two weeks after the source discovery at a flux level of about 20 mCrab in the 0.8-8 keV range. We extracted the most precise X-ray position of IGR J17497-2821, RA=17 49 38.037, DEC= -28 21 17.37 (90% uncertainty of 0.6"). We also report on optical and near infra-red photometric follow-up observations based on this position. With the multi-wavelength information at hand, we discuss the possible nature of the source proposing that IGR J17497-2821 is a low-mass X-ray binary, most likely hosting a black hole, with a red giant K-type companion.

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arXiv2007

The INTEGRAL Galactic bulge monitoring program: the first 1.5 years

The Galactic bulge region is a rich host of variable high-energy point sources. Since 2005, February 17 we are monitoring the source activity in this region about every three days with INTEGRAL. Thanks to the large field of view, the imaging capabilities and the sensitivity at hard X-rays, we are able to present for the first time a detailed homogeneous (hard) X-ray view of a sample of 76 sources in the Galactic bulge region. We describe the successful monitoring program and show the first results for a period of about one and a half year. We focus on the short (hour), medium (month) and long-term (year) variability in the 20-60 keV and 60-150 keV bands. When available, we discuss the simultaneous observations in the 3-10 keV and 10-25 keV bands. Per visibility season we detect 32/33 sources in the 20-60 keV band and 8/9 sources in the 60-150 keV band. On average, we find per visibility season one active bright (>~100 mCrab, 20-60 keV) black-hole candidate X-ray transient and three active weaker (<~25 mCrab, 20-60 keV) neutron star X-ray transients. Most of the time a clear anti-correlation can be seen between the soft and hard X-ray emission in some of the X-ray bursters. Hard X-ray flares or outbursts in X-ray bursters, which have a duration of the order of weeks, are accompanied by soft X-ray drops. On the other hand, hard X-ray drops can be accompanied by soft X-ray flares/outbursts. We found a number of new sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGR J17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here on some of the high-energy properties of these sources. The high-energy light curves of all the sources in the field of view, and the high-energy images of the region, are made available through the WWW at http://isdc.unige.ch/Science/BULGE/.

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arXiv2006

Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC 1313

Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313, named X-1 and X-2, were observed with Suzaku on 2005 September 15. During the observation for a net exposure of 28~ks (but over a gross time span of 90~ks), both objects varied in intensity by about 50~%. The 0.4--10 keV X-ray luminosity of X-1 and X-2 was measured as $2.5 \times 10^{40}~{\rm erg~s^{-1}}$ and $5.8 \times 10^{39}~{\rm erg~s^{-1}}$, respectively, with the former the highest ever reported for this ULX. The spectrum of X-1 can be explained by a sum of a strong and variable power-law component with a high energy cutoff, and a stable multicolor blackbody with an innermost disk temperature of $\sim 0.2$ keV. These results suggest that X-1 was in a ``very high'' state, where the disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1 is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with the innermost disk temperature of 1.2--1.3 keV, and becomes flatter as the source becomes brighter. Hence X-2 is interpreted to be in a slim-disk state. These results suggest that the two ULXs have black hole masses of a few tens to a few hundreds solar masses.

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arXiv2006

Monte-Carlo Simulator and Ancillary Response Generator of Suzaku XRT/XIS System for Spatially Extended Source Analysis

We have developed a framework for the Monte-Carlo simulation of the X-Ray Telescopes (XRT) and the X-ray Imaging Spectrometers (XIS) onboard Suzaku, mainly for the scientific analysis of spatially and spectroscopically complex celestial sources. A photon-by-photon instrumental simulator is built on the ANL platform, which has been successfully used in ASCA data analysis. The simulator has a modular structure, in which the XRT simulation is based on a ray-tracing library, while the XIS simulation utilizes a spectral "Redistribution Matrix File" (RMF), generated separately by other tools. Instrumental characteristics and calibration results, e.g., XRT geometry, reflectivity, mutual alignments, thermal shield transmission, build-up of the contamination on the XIS optical blocking filters (OBF), are incorporated as completely as possible. Most of this information is available in the form of the FITS (Flexible Image Transport System) files in the standard calibration database (CALDB). This simulator can also be utilized to generate an "Ancillary Response File" (ARF), which describes the XRT response and the amount of OBF contamination. The ARF is dependent on the spatial distribution of the celestial target and the photon accumulation region on the detector, as well as observing conditions such as the observation date and satellite attitude. We describe principles of the simulator and the ARF generator, and demonstrate their performance in comparison with in-flight data.

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