I. Takahashi

Development of the HXD-II wide-band all-sky monitor onboard Astro-E2

The hard X-ray detector (HXD-II) is one of the three scientific instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled to be launched in 2005. This mission is very unique in a point of having a lower background than any other past missions in the 10-600 keV range. In the HXD-II, the large and thick BGO crystals are used as active shields for particle and gamma-ray background to the main detector. They have a wide field of view of ~2pi and a large effective area of 400 cm2 even at 1 MeV. Hence, the BGO shields have been developed as a wide-band all-sky monitor (WAM) with a broadband coverage of 50-5000 keV. In this paper, overall design and performance of the HXD-II/WAM based on the results of preflight calibration tests carried out in June 2004 are described. By irradiating various radio isotopes with the WAM flight model, we verified that it had comparable capabilities with other gamma-ray burst detectors

Thermochromic Properties of Double-Doped VO2 Thin Films Prepared by a Wet Coating Method Using Polyvanadate-Based Sols Containing W and Mo or W and Ti

A mixture of metallic vanadium, tungsten and molybdenum powder was dissolved in a hydrogen peroxide solution, to yield a polyvanadate sol containing W and Mo. A thin film of V1-x-yWxMoyO2 was fabricated on a fused silica substrate by spin-coating using such a sol followed by reduction in hydrogen and successive annealing in a nitrogen atmosphere. Thin films in the system V1-x-zWxTizO2 were similarly prepared using a sol containing W and Ti. All the films obtained in this study showed a clear thermochromic switching performance with a large transmittance change in the infrared region. For V1-x-yWxMoyO2, the switching temperature (Tt) was lowered by doping at approximately Tt=Tt0 (=67?C)-ax-by, where a and b are the decrease rates of each individual dopant (about 1920 and 870?C, respectively, for 0x, y0.03). Doping of Ti had little effect on Tt, but markedly reduced hysteresis of the transition. We demonstrated that a double-doped film of V1-x-zWxTizO2 (x=0.018 and y=0.10) showed thermochromic switching almost without any hysteresis at a temperature as low as 45?C.

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/].

Thermochromic V1-xWxO2 Thin Films Prepared by Wet Coating Using Polyvanadate Solutions

Metallic vanadium powder was dissolved in a hydrogen peroxide solution, yielding a hydrosol based on a polyvanadate. A polycrystalline thin film of VO2 was fabricated on a fused silica substrate by spin-coating using this hydrosol followed by reduction in hydrogen and additional post-annealing in a nitrogen atmosphere. The thin film showed a sharp semiconductor-to-metal transition and concomitant thermochromic switching at 67° C. Tungsten-doped thin films ( V1-x Wx O2) were easily fabricated by a similar technique. The transition temperature of a doped film with x=0.015 was as low as 36° C.

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.

The XMM-Newton Examination of Energetics in the East Lobe of the Nearby Radio Galaxy Fornax A (NGC 1316)

An XMM-Newton observation of the east radio lobe of the nearby radio galaxy Fornax A is reported. The diffuse hard X-ray emission associated with the east lobe, which was initially discovered by ASCA and ROSAT, is confirmed with significant signal statistics, after strictly removing 59 sources detected within the MOS field of view. Its X-ray spectrum is described by a single power-law model, which is absorbed by a medium with a column density consistent with that toward the object. The best-fit X-ray photon index, ΓX = 1.62, agrees with the synchrotron radio index, ΓR = 1.68 ± 0.1, determined from the radio spectrum between 29.9 MHz and 5 GHz. Hence, the inverse Compton interpretation for the diffuse X-rays is justified. The X-ray flux density in the east lobe is measured to be 90 ± 21 nJy at 1 keV (including both statistical and systematic errors) with the index fixed at the radio value. This gives electron and magnetic energy densities of 3.0 × 10-13 and 6.1 × 10-14 ergs cm-3, respectively. The latter corresponds to a magnetic field strength of 1.24 μG, which is smaller than the field estimated under the minimum energy condition, 1.55 μG, although with a slightly large error. Reevaluation is also made of the ASCA result on the west lobe, to show that both lobes share a similar physical condition in terms of energetics.

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.

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.

CALET UPPER LIMITS on X-RAY and GAMMA-RAY COUNTERPARTS of GW151226

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.

X-ray Diagnostics of Thermal Conditions of the Hot Plasmas in the Centaurus Cluster

X-ray data of the Centaurus cluster, obtained with XMM-Newton for 45 ks, were analyzed. Deprojected EPIC spectra from concentric thin-shell regions were reproduced equally well by a single-phase plasma emission model, or by a two-phase model developed by ASCA, both incorporating cool (1.7-2.0 keV) and hot (~ 4 keV) plasma temperatures. However, EPIC spectra with higher statistics, accumulated over three-dimensional thick-shell regions, were reproduced better by the two-phase model than by the singe-phase one. Therefore, hot and cool plasma phases are inferred to co-exist in the cluster core region within ~ 70 kpc. The iron and silicon abundances of the plasma were reconfirmed to increase significantly toward the center, while that of oxygen was consistent with being radially constant. The implied nonsolar abundance ratios explain away the previously reported excess X-ray absorption from the central region. Although an additional cool (~ 0.7 keV) emission was detected within ~ 20 kpc of the center, the RGS data gave tight upper limits on any emission with temperatures below ~ 0.5 keV. These results are compiled into a magnetosphere model, which interprets the cool phase as confined within closed magnetic loops anchored to the cD galaxy. When combined with the so-called Rosner-Tucker-Vaiana mechanism which applies to solar coronae, this model can potentially explain basic properties of the cool phase, including its temperature and thermal stability.