Hirokazu Odaka

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.

The origin of ultrafast outflows in AGN : Monte Carlo simulations of the wind in PDS 456.

Ultrafast outflows (UFOs) are seen in many AGN, giving a possible mode for AGN feedback on to the host galaxy. However, the mechanism(s) for the launch and acceleration of these outflows are currently unknown, with UV line driving apparently strongly disfavoured as the material along the line of sight is so highly ionized that it has no UV transitions. We revisit this issue using the Suzaku X-ray data from PDS 456, an AGN with the most powerful UFO seen in the local Universe. We explore conditions in the wind by developing a new 3D Monte Carlo code for radiation transport. The code only handles highly ionized ions, but the data show the ionization state of the wind is high enough that this is appropriate, and this restriction makes it fast enough to explore parameter space. We reproduce the results of earlier work, confirming that the mass-loss rate in the wind is around 30 per cent of the inferred inflow rate through the outer disc. We show for the first time that UV line driving is likely to be a major contribution to the wind acceleration. The mass-loss rate in the wind matches that predicted from a purely line driven system, and this UV absorption can take place out of the line of sight. Continuum driving should also play a role as the source is close to Eddington. This predicts that the most extreme outflows will be produced from the highest mass accretion rate flows on to high-mass black holes, as observed.

New CdTe Pixel Gamma-Ray Detector with Pixelated Al Schottky Anodes

We developed a new Al Schottky CdTe pixel detector and measured its spectral performance. It has pixelated anodes made of aluminum and a common cathode made of platinum. Because of the low leakage current and the high bias voltage owing to the Schottky diode characteristic and the anode pixel configuration, a good spectral performance including a high energy resolution was achieved. When the pixel detector with a thickness of 0.75 mm was subjected to a bias voltage of 400 V and was operated at -20 °C, the full-width-half-maximum (FWHM) energy resolution of 1.1 and 1.8 keV at 59.5 and 122 keV, respectively, were successfully obtained. The spectral performance obtained with the Al Schottky CdTe pixel detector exceeded that obtained with the conventional In Schottky CdTe pixel detector, which has an In common anode and Pt pixelated cathodes, under the same operating conditions.

X-ray Diagnostics of Giant Molecular Clouds in the Galactic Center Region and Past Activity of Sgr A*

Strong iron fluorescence at 6.4 keV and hard-X-ray emissions from giant molecular clouds in the Galactic center region have been interpreted as reflections of a past outburst of the Sgr A* supermassive black hole. Careful treatment of multiple interactions of photons in a complicated geometry is essential to modeling the reprocessed emissions from the dense clouds. We develop a new calculation framework of X-ray reflection from molecular clouds based on Monte Carlo simulations for accurate interpretation of high-quality observational data. By utilizing this simulation framework, we present the first calculations of morphologies and spectra of the reflected X-ray emission for several realistic models of Sgr B2, which is the most massive molecular cloud in our Galaxy. The morphology of scattered hard X-rays above 20 keV is significantly different from that of iron fluorescence due to their large penetrating power into dense regions of the cloud, probing the structure of the cloud. High-resolution spectra provide quantitative evaluation of the iron line including its Compton shoulder to constrain the mass and the chemical composition of the cloud as well as the luminosity of the illuminating source. These predictions can be checked in the near future with future X-ray missions such as NuStar (hard X-rays) and ASTRO-H (both iron lines and hard X-rays).

The Hard X-ray Imager (HXI) for the ASTRO-H mission

The 6th Japanese X-ray satellite, ASTRO-H, is scheduled for launch in 2015. The hard X-ray focusing imaging system will observe astronomical objects with the sensitivity for detecting point sources with a brightness of 1/100,000 times fainter than the Crab nebula at > 10 keV. The Hard X-ray Imager (HXI) is a focal plane detector 12 m below the hard X-ray telescope (HXT) covering the energy range from 5 to 80 keV. The HXI is composed of a stacked Si/CdTe semiconductor detector module and surrounding BGO scintillators. The latter work as active shields for efficient reduction of background events caused by cosmic-ray particles, cosmic X-ray background, and in-orbit radiation activation. In this paper, we describe the detector system, and present current status of flight model development, and performance of HXI using an engineering model of HXI.

A new Si/CdTe semiconductor Compton camera developed for high-angular resolution

A semiconductor Compton camera for a balloon borne experiment aiming at observation in high energy astrophysics is developed. The camera is based on the concept of the Si/CdTe semiconductor Compton Camera, which features high-energy and high-angular resolution in the energy range from several tens of keV to a few MeV. It consists of tightly packed double-sided silicon strip detectors (DSSDs) stacked in four layers, and a total of 32 CdTe pixel detectors surrounding them. The Compton reconstruction was successfully performed and gamma-ray images were obtained from 511 keV down to 59.5 keV. The Angular Resolution Measure (ARM) at 511 keV is ~ 2.5 degrees, thanks to the high energy resolution in both the DSSD and CdTe parts.

Fermi-LAT and Suzaku observations of the radio galaxy Centaurus B

Centaurus B is a nearby radio galaxy positioned in the southern hemisphere close to the Galactic plane. Here we present a detailed analysis of about 43 months of accumulated Fermi-LAT data of the γ-ray counterpart of the source initially reported in the 2nd Fermi-LAT catalog, and of newly acquired Suzaku X-ray data. We confirm its detection at GeV photon energies and analyze the extension and variability of the γ-ray source in the LAT dataset, in which it appears as a steady γ-ray emitter. The X-ray core of Centaurus B is detected as a bright source of a continuum radiation. We do not detect, however, any diffuse X-ray emission from the known radio lobes, with the provided upper limit only marginally consistent with the previously claimed ASCA flux. Two scenarios that connect the X-ray and γ-ray properties are considered. In the first one, we assume that the diffuse non-thermal X-ray emission component is not significantly below the derived Suzaku upper limit. In this case, modeling the inverse-Compton emission shows that the observed γ-ray flux of the source may in principle be produced within the lobes. This association would imply that efficient in-situ acceleration of the radiating electrons is occurring and that the lobes are dominated by the pressure from the relativistic particles. In the second scenario, with the diffuse X-ray emission well below the Suzaku upper limits, the lobes in the system are instead dominated by the magnetic pressure. In this case, the observed γ-ray flux is not likely to be produced within the lobes, but instead within the nuclear parts of the jet. By means of synchrotron self-Compton modeling, we show that this possibility could be consistent with the broad-band data collected for the unresolved core of Centaurus B, including the newly derived Suzaku spectrum.

Fine-Pitch Semiconductor Detector for the FOXSI Mission

The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket mission which will study particle acceleration and coronal heating on the Sun through high sensitivity observations in the hard X-ray energy band (5-15 keV). Combining high-resolution focusing X-ray optics and fine-pitch imaging sensors, FOXSI will achieve superior sensitivity; two orders of magnitude better than that of the RHESSI satellite. As the focal plane detector, a Double-sided Si Strip Detector (DSSD) with a front-end ASIC (Application Specific Integrated Circuit) will fulfill the scientific requirements of spatial and energy resolution, low energy threshold and time resolution. We have designed and fabricated a DSSD with a thickness of 500 μm and a dimension of 9.6 mm × 9.6 mm, containing 128 strips with a pitch of 75 μm, which corresponds to 8 arcsec at the focal length of 2 m. We also developed a low-noise ASIC specified to FOXSI. The detector was successfully operated in the laboratory at a temperature of -20°C and with an applied bias voltage of 300 V. Extremely good energy resolutions of 430 eV for the p-side and 1.6 keV for the n-side at a 14 keV line were achieved for the detector. We also demonstrated fine-pitch imaging successfully by obtaining a shadow image. Hence the implementation of scientific requirements was confirmed.

Development of Double-Sided CdTe Strip Detectors for γ-Ray Imaging and Spectroscopy

By reading out both anode and cathode strips, double-sided CdTe strip detectors can achieve a large area and a high position resolution with few readout channels, which makes them very attractive for X-ray and γ-ray imaging and spectroscopy. We have developed double-sided CdTe strip detectors, 1.28 ×1.28 cm2 in size and 0.5 and 2.0 mm in thickness. Both electrodes are divided into 32 orthogonal strips with a pitch of 400 µm. For a detector of 0.5 mm thickness, the energy resolution was measured to be 1.5 keV (FWHM) at 60 keV. For the 2.0-mm-thick detector, an energy resolution of 8.0 keV (FWHM) at 662 keV was obtained using only the anode signal. By combining both the anode and cathode signals, we successfully improved the spectral performance and measured an energy resolution of 5.9 keV (FWHM) at 662 keV.

Soft gamma-ray detector (SGD) onboard the ASTRO-H mission

The Soft Gamma-ray Detector (SGD) is one of observational instruments onboard the ASTRO-H, and will provide 10 times better sensitivity in 60{600 keV than the past and current observatories. The SGD utilizes similar technologies to the Hard X-ray Imager (HXI) onboard the ASTRO-H. The SGD achieves low background by constraining gamma-ray events within a narrow field-of-view by Compton kinematics, in addition to the BGO active shield. In this paper, we will present the results of various tests using engineering models and also report the flight model production and evaluations.