Motohide Kokubun

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

In-orbit performance of the hard X-ray detector on board Suzaku

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10–600keV, energy resolutions of ∼ 4keV (FWHM) at 40keV and ∼ 11% at 511keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5–20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15–70 and 150–500keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

Evaluation of properties of YAG (Ce) ceramic scintillators

We measured basic properties of three ceramic Y/sub 3/Al/sub 5/O/sub 12/ (YAG) scintillators doped with Ce to a concentration of 0.5, 0.05, and 0.005 mol%, in comparison with a monocrystalline YAG scintillator with unknown amount of Ce doping. First, optical transparency and emission spectra were investigated. We confirmed that the transparency of the ceramics is comparable to that of the monocrystalline one (/spl sim/80%) in wavelengths longer than /spl sim/500 nm. The ceramics did not show an indication of lattice defects which is present in the monocrystalline YAG. Then the response to /spl gamma/-rays was studied using a phototube as a scintillation light detector. The 0.5 mol% sample exhibited the highest light yield (/spl sim/40% of CsI), with an energy resolution of about 7.2% at /sup 137/Cs 662 keV photoabsorption peak. The optimum Ce concentration for a /spl sim/2 mm thick ceramic YAG was determined to be /spl sim/0.1 mol%. Using the delayed coincidence method, the principal time constant of the ceramic YAGs was measured as /spl sim/80 ns. By irradiating 5.49 MeV /spl alpha/-particles, the /spl alpha/-ray to /spl gamma/-ray light yield ratio of the ceramic YAGs was found to depend negatively on the amount of Ce; namely, 0.28, 0.20, and 0.13 in the increasing order of the Ce concentration. The 200-1000 keV intrinsic background of the 0.5 mol% ceramic was /spl sim/10/sup -5/ counts/s/cm/sup 3/, indicating that it is not significantly contaminated by radioactive impurities.

Study of Nonthermal Emission from SNR RX J1713.7–3946 with Suzaku

We present results obtained from a series of observations of the supernova remnant RX J1713.7–3946 by Suzaku. Hard X-rays have been detected up to ~40 keV. The hard X-ray spectra are described by a power law with photon indices of ~3.0, which is larger than those below 10 keV. The combination of the spatially integrated XIS and HXD spectra clearly reveals a spectral cutoff which is linked to the maximum energy of accelerated electrons. The broadband coverage of Suzaku allows us to derive, for the first time, the energy spectrum of parent electrons in the cutoff region. The cutoff energy in the X-ray spectrum indicates that the electron acceleration in the remnant proceeds close to the Bohm diffusion limit. We discuss the implications of the spectral and morphological properties of the Suzaku data in the context of the origin of nonthermal emission. The Suzaku X-ray and H.E.S.S. gamma-ray data together hardly can be explained within a pure leptonic scenario. Moreover, the leptonic models require a weak magnetic field, which is inconsistent with the recently discovered X-ray filamentary structures and their short-term variability. The hadronic models with strong magnetic fields provide reasonable fits to the observed spectra, but require special arrangements of parameters to explain the lack of thermal X-ray emission. For morphology studies, we compare the X-ray and TeV gamma-ray surface brightness. We confirm the previously reported strong correlation between X-rays and TeV gamma rays. At the same time, the Suzaku data reveal a deviation from the general tendency, namely, the X-ray emission in the western rims appears brighter than expected from the average X-ray to gamma-ray ratio.

Detection of high-energy gamma rays from winter thunderclouds

A report is made on a comprehensive observation of a burstlike gamma-ray emission from thunderclouds on the Sea of Japan, during strong thunderstorms on 6 January 2007. The detected emission, lasting for approximately 40 sec, preceded cloud-to-ground lightning discharges. The burst spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung photons originating from relativistic electrons. This ground-based observation provides the first clear evidence that strong electric fields in thunderclouds can continuously accelerate electrons beyond 10 MeV prior to lightning discharges.

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

High resolution CdTe detectors for the next-generation multi-Compton gamma-ray telescope

A multi-Compton gamma-ray telescope based on high resolution semiconductor materials (Semiconductor Multi-Compton Telescope (SMCT) or Advanced Compton Telescope (ACT)) is a promising approach to achieve high sensitivity for gamma-rays with energies from several hundred keV up to several MeV. A SMCT utilizing several tens of layers of thin CdTe (Cadmium Telluride) detector is an attractive concept to obtain higher detection efficiency in comparison with Si-based SMCT. Recently we have developed high energy-resolution CdTe diode detectors. A large-area detector with dimensions of 2.15 × 2.15 cm2 with a thickness of 0.5mm shows an energy resolution of better than 3 keV (FWHM) at 60 keV. In order to extend the application of CdTe diodes to the detection of MeV gamma-rays, we have constructed a stacked detector consisting of 40 layers of large CdTe diodes. Here we report the recent progress on the high-resolution CdTe diode and describe the conceptual design of new Multi-Compton Gamma-ray telescopes based on Monte Carlo simulation. An idea of active pair production telescope is briefly described.

X-ray temperature and mass measurements to the virial radius of abell 1413 with suzaku

We present X-ray observations of the northern outskirts of the relaxed galaxy cluster A1413 with Suzaku, whose XIS instrument has the low intrinsic background needed to make measurements of these low surface brightness regions. We excise 15 point sources superimposed on the image above a flux of

High Energy Resolution Hard X-Ray and Gamma-Ray Imagers Using CdTe Diode Devices

1\times 10^{-14}

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

\fluxunit (2--10keV) using XMM-Newton and Suzaku images of the cluster. We quantify all known systematic errors as part of our analysis, and show our statistical errors encompasses them for the most part. Our results extend previous measurements with Chandra and XMM-Newton, and show a significant temperature drop to about 3keV at the virial radius,