Katsuhiro Hayashi

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.

The soft gamma-ray detector (SGD) onboard ASTRO-H

The Soft Gamma-ray Detector (SGD) is one of science instruments onboard ASTRO-H (Hitomi) and features a wide energy band of 60{600 keV with low backgrounds. SGD is an instrument with a novel concept of Narrow field-of-view Compton camera where Compton kinematics is utilized to reject backgrounds which are inconsistent with the field-of-view defined by the active shield. After several years of developments, the flight hardware was fabricated and subjected to subsystem tests and satellite system tests. After a successful ASTRO-H (Hitomi) launch on February 17, 2016 and a critical phase operation of satellite and SGD in-orbit commissioning, the SGD operation was moved to the nominal observation mode on March 24, 2016. The Compton cameras and BGO-APD shields of SGD worked properly as designed. On March 25, 2016, the Crab nebula observation was performed, and, the observation data was successfully obtained.

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.

The first demonstration of the concept of “narrow-FOV Si/CdTe semiconductor Compton camera”

Abstract The Soft Gamma-ray Detector (SGD), to be deployed on board the ASTRO-H satellite, has been developed to provide the highest sensitivity observations of celestial sources in the energy band of 60–600xa0keV by employing a detector concept which uses a Compton camera whose field-of-view is restricted by a BGO shield to a few degree (narrow-FOV Compton camera). In this concept, the background from outside the FOV can be heavily suppressed by constraining the incident direction of the gamma ray reconstructed by the Compton camera to be consistent with the narrow FOV. We, for the first time, demonstrate the validity of the concept using background data taken during the thermal vacuum test and the low-temperature environment test of the flight model of SGD on ground. We show that the measured background level is suppressed to less than 10% by combining the event rejection using the anti-coincidence trigger of the active BGO shield and by using Compton event reconstruction techniques. More than 75% of the signals from the field-of-view are retained against the background rejection, which clearly demonstrates the improvement of signal-to-noise ratio. The estimated effective area of 22.8xa0cm 2 meets the mission requirement even though not all of the operational parameters of the instrument have been fully optimized yet.

SUZAKU OBSERVATION OF THE FERMI CYGNUS COCOON: THE SEARCH FOR A SIGNATURE OF YOUNG COSMIC-RAY ELECTRONS

The origin of Galactic cosmic rays remains unconfirmed, but promising candidates for their sources are found in star-forming regions. We report a series of X-ray observations, with Suzaku, toward the nearby star-forming region of Cygnus X. They aim at comparing diffuse X-ray emissions on and off the γ-ray cocoon of hard cosmic rays revealed by the Fermi Large Area Telescope. After excluding point sources and small-scale structures and subtracting the non-X-ray and cosmic X-ray backgrounds, the 2–10 keV X-ray intensity distribution is found to monotonically decrease with increasing Galactic latitude. This indicates that most of the extended emission detected by Suzaku originates from the Galactic ridge. In two observations, we derive upper limits of and to X-ray emission in the 2–10 keV range from the γ-ray cocoon. These limits exclude the presence of cosmic-ray electrons with energies above about 50 TeV at a flux level capable of explaining the γ-ray spectrum. They are consistent with the emission cut-off observed near a TeV in γ-rays. The properties of the Galactic-ridge and local diffuse X-rays are also discussed.

Design and performance of Soft Gamma-ray Detector onboard the Hitomi (ASTRO-H) satellite

Abstract. Hitomi (ASTRO-H) was the sixth Japanese x-ray satellite that carried instruments with exquisite energy resolution of <7u2009u2009eV and broad energy coverage of 0.3 to 600 keV. The Soft Gamma-ray Detector (SGD) was the Hitomi instrument that observed the highest energy band (60 to 600 keV). The SGD design achieves a low background level by combining active shields and Compton cameras where Compton kinematics is utilized to reject backgrounds coming from outside of the field of view. A compact and highly efficient Compton camera is realized using a combination of silicon and cadmium telluride semiconductor sensors with a good energy resolution. Compton kinematics also carries information for gamma-ray polarization, making the SGD an excellent polarimeter. Following several years of development, the satellite was successfully launched on February 17, 2016. After proper functionality of the SGD components were verified, the nominal observation mode was initiated on March 24, 2016. The SGD observed the Crab Nebula for approximately two hours before the spacecraft ceased to function on March 26, 2016. We present concepts of the SGD design followed by detailed description of the instrument and its performance measured on ground and in orbit.

Study of the polarimetric performance of a Si/CdTe semiconductor Compton camera for the Hitomi satellite

Abstract Gamma-ray polarization offers a unique probe into the geometry of the γ -ray emission process in celestial objects. The Soft Gamma-ray Detector (SGD) onboard the X-ray observatory Hitomi is a Si/CdTe Compton camera and is expected to be an excellent polarimeter, as well as a highly sensitive spectrometer due to its good angular coverage and resolution for Compton scattering. A beam test of the final-prototype for the SGD Compton camera was conducted to demonstrate its polarimetric capability and to verify and calibrate the Monte Carlo simulation of the instrument. The modulation factor of the SGD prototype camera, evaluated for the inner and outer parts of the CdTe sensors as absorbers, was measured to be 0.649–0.701 (inner part) and 0.637–0.653 (outer part) at 122.2xa0keV and 0.610–0.651 (inner part) and 0.564–0.592 (outer part) at 194.5xa0keV at varying polarization angles with respect to the detector. This indicates that the relative systematic uncertainty of the modulation factor is as small as ∼ 3 % .

Modeling of proton-induced radioactivation background in hard X-ray telescopes: Geant4-based simulation and its demonstration by Hitomi ’s measurement in a low Earth orbit

Abstract Hard X-ray astronomical observatories in orbit suffer from a significant amount of background due to radioactivation induced by cosmic-ray protons and/or geomagnetically trapped protons. Within the framework of a full Monte Carlo simulation, we present modeling of in-orbit instrumental background which is dominated by radioactivation. To reduce the computation time required by straightforward simulations of delayed emissions from activated isotopes, we insert a semi-analytical calculation that converts production probabilities of radioactive isotopes by interaction of the primary protons into decay rates at measurement time of all secondary isotopes. Therefore, our simulation method is separated into three steps: (1) simulation of isotope production, (2) semi-analytical conversion to decay rates, and (3) simulation of decays of the isotopes at measurement time. This method is verified by a simple setup that has a CdTe semiconductor detector, and shows a 100-fold improvement in efficiency over the straightforward simulation. To demonstrate its experimental performance, the simulation framework was tested against data measured with a CdTe sensor in the Hard X-ray Imager onboard the Hitomi X-ray Astronomy Satellite, which was put into a low Earth orbit with an altitude of 570 km and an inclination of 31 ° , and thus experienced a large amount of irradiation from geomagnetically trapped protons during its passages through the South Atlantic Anomaly. The simulation is able to treat full histories of the proton irradiation and multiple measurement windows. The simulation results agree very well with the measured data, showing that the measured background is well described by the combination of proton-induced radioactivation of the CdTe detector itself and thick Bi 4 Ge 3 O 12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.

Fermi LAT study of cosmic-rays and the interstellar medium in nearby molecular clouds

We report an analysis of the interstellar gamma-ray emission from nearby molecular clouds Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope (LAT). The obtained gamma-ray emissivities from 250 MeV to 10 GeV are (5.9 ± 0.1stat−1.0sys+0.9)×10−27 photons s−1 sr−1 H-atom−1, (10.2 ± 0.4stat−1.7sys + 1.2)×10−27 photons s−1 sr−1 H-atom−1, and (9.1±0.3stat−0.6sys+1.5)×10−27 photons s−1 sr−1 H-atom−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, indicating a variation of the CR density by ∼20% among the three regions, even if we consider the systematic uncertainties. The molecular mass calibration ratio, Xcou2009 = N(H2)/Wco, is found to be (0.96 ± 0.06stat−0.12sys+0.15)×1020H2-molecule cm−2 (K km s−1)−1, (0.99 ± 0.08stat−0.10sys+0.18)×1020H2-molecule cm−2 (K km s−1)−1, and (0.63 ± 0.02stat−0.07sys + 0.09)×1020H2-molecule cm−2 (K km s−1)−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, sug...