Yu-Ichiro Ezoe

Improvements of the astro-E2 hard X-ray detector (HXD-II)

We summarize significant improvements which have been achieved in the development of Astro-E2 Hard X-ray Detector (HXD-II). An expanded energy range and better energy resolution have been achieved from progresses in device materials and redesigning of the front-end electronics. An improved estimation for the detector background in orbit has also been conducted based upon results from our proton irradiation experiment. The sensitivity of HXD-II can be expected to reach an order of 10/sup -6/ [cs/sup -1/ keV/sup -1/ cm/sup -2/].

Investigation of Diffuse Hard X-Ray Emission from the Massive Star-forming Region NGC 6334

Chandra ACIS-I data of the molecular cloud and H II region complex NGC 6334 were analyzed. The hard X-ray clumps detected with ASCA (Sekimoto and coworkers) were resolved into 792 point sources. After removing the point sources, an extended X-ray emission component was detected over a 5 × 9 pc2 region, with the 0.5-8 keV absorption-corrected luminosity of 2 × 1033 ergs s-1. The contribution from faint point sources to this extended emission was estimated as at most ~20%, suggesting that most of the emission is diffuse in nature. The X-ray spectrum of the diffuse emission was observed to vary from place to place. In tenuous molecular cloud regions with hydrogen column density of (0.5-1) × 1022 cm-2, the spectrum can be represented by a thermal plasma model with temperatures of several keV. The spectrum in dense cloud cores exhibits harder continuum, together with higher absorption of more than ~3 × 1022 cm-2. In some of such highly obscured regions, the spectra show extremely hard continua equivalent to a photon index of ~1, and favor a nonthermal interpretation. These results are discussed in the context of thermal and nonthermal emission, both powered by fast stellar winds from embedded young early-type stars through shock transitions.

Performance of the ASTRO-E hard X-ray detector

This paper summarizes the design and performance of the hard X-ray detector constructed for the ASTRO-E satellite. The detector utilizes the GSO/BGO well-type phoswich counters in a compound-eye configuration to achieve an extremely low background level of a few /spl times/ 10/sup -5/ counts s/sup -1/ cm/sup -2/ keV/sup -1/. The GSO scintillators installed in the BGO active shield wells are sensitive to 30-600 keV photons, while the 2-mm-thick silicon PIN diodes, placed in front of each GSO crystal, cover the 10-60 keV energy band with a spectral resolution of /spl sim/3.5-keV full-width at half-maximum. The design goals, of both low background and high energy resolution, in the hard X-ray bands were verified through the preflight calibration experiments.

Preflight performance of the Astro-E hard X-ray detector

The hard x-ray detector (HXD) is one of the three experiments of the Astro-E mission, the fifth Japanese X-ray Satellite devoted to studies of high energy phenomena in the universe in the x-ray to soft gamma-ray region. Prepared for launch at the beginning of 200 via the newly developed M-V launch vehicle of the Institute of Space and Astronomical Science, the Astro-E is to be thrown in to a near-circular orbit of 550 km altitude, with an inclination of 31 degrees. The flight model has been finished assembled this year, and we carried out various tests to verify the performance. We acquired the background spectrum at sea level, and confirmed that our system is operating effectively in reducing the background level. The HXD will observe photons in the energy range of 10-600 keV, and the calculations based on the preflight calibration suggest that the HXD will have the highest sensitivity ever achieved in this energy range. We also verified that our electronic system will maintain its performance against charged particle events expected in orbit.

Fabrication of the ASTRO-E hard-x-ray detector

The Hard X-ray Detector (HXD) is one of the three instruments on the fifth Japanese cosmic X-ray satellite ASTRO-E, scheduled for launch in January 2000. The HXD covers a wide energy range of 10-600 keV, using 16 identical GSO/BGO phoswich-counter modules, of which the low-energy efficiency is greatly improved by adding 2 m-thick silicon PIN diodes. Production of the HXD has been completed and pre-flight calibration is now in progress. The design concept of the HXD sensor, detail of the production process, and a brief summary of the measured performance is reported.

Spatially dependent response of thick and large area p-i-n diode for ASTRO-E hard X-ray detector

The ASTRO-E hard X-ray detector utilizes GSO(Gd/sub 2/SiO/sub 5/:Ce 0.5% mol)-BGO(Bi/sub 4/Ge/sub 3/O/sub 12/) well-type phoswich counters in compound-eye configuration to achieve an extremely low background level of about a few times 10/sup -5/ counts s/sup -1/ cm/sup -2/ keV/sup -1/. The GSO scintillators placed at the bottom of the BGO well observe photons in the energy range 30-600 keV. To cover the lower energy range of 10-60 keV, Si p-i-n diodes of 2 mm in thickness and 21.5/spl times/21.5 mm/sup 2/ in size were newly developed and placed in front of the GSO scintillators. The p-i-n diode exhibits complex spectral responses, including subpeak and low energy tail components. To examine the origin of these components, we measured the spatially resolved response of the p-i-n diode and confirmed that the subpeak and the low energy tail are related to the electrode structures and electric fields in the p-i-n diode, respectively.

Study of the Long-Term X-Ray Variability of a Possible Quasar RX J0957.9+6903 with ASCA

The long-term variability and spectral properties of a possible quasar, RX J0957.9+6903, were studied utilizing 16 ASCA observations spanning 5.5 years. The average 0.7–10 keV spectrum of RX J0957.9+6903 is well represented by a power-law continuum having a photon index of 1.58±0.03 and an absorption column of∼1×1021 cm−2. The 2–10 keV flux of RX J0957.9+6903 varied by a factor of four over a period of six years, around a mean of ∼ 8.8× 10−12 erg s−1 cm−2. Peak-to-peak variability within each observation was less than 25% on ∼ 1 day time scale. These properties support the classification of RX J0957.9+6903 as a quasar. The power spectrum density (PSD) was estimated in a “forward” manner over a frequency range of 10−8.2–10−4.3 Hz by utilizing the structure function method and a Monte Carlo simulation assuming a broken power-law type PSD. Then, the break frequency fb of the PSD of RX J0957.9+6903 has been constrained as 1/fb = 1600+∞ −1100 days, and the logarithmic slope of the high-frequency region of the PSD as α = −1.55± 0.2 . A comparison of the estimated PSDs is made between RX J0957.9+6903 and the M81 nucleus, observed in the same field of view.

The Discovery of Diffuse X-Ray Emission in NGC 2024, One of the Nearest Massive Star-forming Regions

We analyzed deep 75 ks Chandra ACIS-I data of NGC 2024 with the aim of searching for diffuse X-ray emission in this most nearby (415 pc) of massive star-forming regions. After removing point sources, extended emission was detected in the central circular region with a radius of 0.5 pc, and it is spatially associated with this young massive stellar cluster. Its X-ray spectrum exhibits a very hard continuum (kT > 8 keV) and shows signs of having a He-like Fe Kα line with a 0.5-7 keV absorption-corrected luminosity of 2 × 1031 ergs s-1. Undetected faint point sources, estimated from the luminosity function of the detected sources, contribute less than 10% to this emission. Hence, the emission is truly diffuse in nature. Because of the proximity of NGC 2024 and the long exposure, this discovery is one of the strongest pieces of evidence in support of the existence of diffuse X-ray emission in massive star-forming regions.

Development of focal plane x-ray detector aboard a microsatellite for monitoring supermassive blackholes

We describe the development of the focal plane detector onboard a micro-satellite aimed for observing cosmic Xray emission. Combined with an X-ray optics with focal length of approximately 40 mm, an X-ray CCD camera realizes low and stable background thanks to its capability of event classification by pulse height distribution of a event. The mission will intensively monitor a specific binary black hole to investigate periodic time variability owing to its possible binary motion. The focal plane detector adopts P-channel back-illumination type CCD. It is a miniature version of the sensors utilized in the CCD camera aboard Hitomi satellite but is upgraded in terms of the energy resolution and the prevention of visible light transmittance. We have built up an equipment for cooling and driving the device. Dark current as a function of device temperature is investigated. We see clear difference of the amount of the dark current between the imaging area and frame store area, which is probably due to the difference of the pixel size. The result indicates sufficiently low dark current can be achieved with temperature lower or equal to -80 °C. Number of pinholes in a surface aluminium layer is significantly different between devices. We identified a process with which we decrease the number of pinholes. To realize a whole instrument, we develop communication board and compact analog board.

Super DIOS: future x-ray spectroscopic mission to search for dark baryons

We are working on an updated program of the future Japanese X-ray satellite mission DIOS (Diffuse Intergalactic Oxygen Surveyor), called Super DIOS. We keep the main aim of searching for dark baryons in the form of warmhot intergalactic medium (WHIM) with high-resolution X-ray spectroscopy. The mission will detect redshifted emission lines from OVII, OVIII and other ions, leading to an overall understanding of the physical nature and spatial distribution of dark baryons as a function of cosmological timescale. We are working on the conceptual design of the satellite and onboard instruments, with a provisional launch time in the early 2030s. The major changes will be improved angular resolution of the X-ray telescope and increased number of TES calorimeter pixels. Super DIOS will have a 10-arcsecond resolution and a few tens of thousand TES pixels. Most contaminating X-ray sources will be resolved, and the level of diffuse X-ray background will be reduced after subtraction of point sources. This will give us very high sensitivity to map out the WHIM in emission. The status of the spacecraft study will be presented: the development plan of TES calorimeters, on-board cooling system, X- ray telescope, and the satellite system. The previous study results for DIOS and technical achievements reached by the Hitomi (ASTRO-H) mission provide baseline technology for Super DIOS. We will also consider large scale international collaboration for all the on-board instruments.