Takaya Ohashi

Properties of gas scintillation proportional counters for soft X-rays

Abstract Properties of gas scintillation proportional counter for soft X-rays in the energy range 0.1-1.5 keV are studied. This type of counter possesses twice as good an energy resolution as that of ordinary proportional counters. In a xenon-filled counter a significant distortion of the pulse-height distribution due to a low-energy tail is observed. The tail is more enhanced for the energy of X-rays having shorter mean free paths of photoelectric absorption in xenon. This phenomenon was interpreted in terms of a loss of primary electrons captured by the entrance window, and semi-quantitatively analysed by of a simple diffusion model. A significant improvement of the pulse-height distribution is achieved with a mixture of 80% argon and 20% xenon.

Imaging gas scintillation proportional counters for ASTRO-D

The design and performance of imaging gas scintillation proportional counters (IGSPCs) developed for ASTRO-D are described. The 10 micrometers beryllium window has enabled the x- ray sensitivity down to 0.7 keV for a sealed-off gas counter, and a position-sensitive phototube equipped with multiwire anode gives good imaging capability. The laboratory model of the IGSPC shows an energy resolution 8% FWHM at 6 keV and a position resolution 0.5 mm FWHM with an effective area of 50 mm diameter. The onboard data system incorporating a 16-bit microprocessor efficiently discriminates the room background down to the order of magnitude of 3 X 10-4 cm-2 s-1 keV-1, similar to that achieved by the Ginga LAC instrument. These performances make the GIS experiment a very powerful focal plane instrument for ASTRO-D.

X-ray study of hot interstellar medium in early-type galaxies

Abstract We have analyzed ASCA data of about 30 early type galaxies, and studied their X-ray emitting ISM (InterStellar Medium) properties. By carefully examining the abundance ratios and uncertainties in the Fe-L complex, we have concluded that the ISM abundances in X-ray luminous galaxies are in fact about 1 solar. Therefore, the severe discrepancy between the ISM and stellar abundance has been relaxed. The ISM metallicity of X-ray fainter galaxies are uncertain, but at least SNe Ia contribution to the ISM abundance is smaller than in the X-ray luminous ones. We have also discovered that X-ray emissions from X-ray luminous galaxies are very extended, and expressed with two beta models of different angular scales. This means that the X-ray luminous ellipticals are central galaxies of some larger-scale potential structures. We show that presence/absence of such a larger-scale potential can consistently account for several unsolved problems with the ISM.

X-Ray Study of Normal Galaxies with Ginga

Results from X-ray observations of nearby normal galaxies with the X-ray astronomy satellite Ginga are presented. X-ray spectra in the energy range 2-20 keV were obtained for the first time from four spiral (M31, M33, M51 and M81), three starburst (M82, M83 and NGC 253), and three elliptical (NGC 4472, NGC 4636 and NGC 3998) galaxies. M51 and NGC 3998 showed a power-law spectrum suggesting the presence of an active nucleus, while the others exhibited a thermal spectrum with kT ≤ 2 keV for elliptical galaxies and kT ≥ 4 keV for others. Iron emission line was weak or not significant for all spiral and starburst galaxies. Presence of a large extended X-ray emission (~ 2° in full width) around M82 was strongly suggested, which could be a large-scale halo formed by the mass ejection from M82.

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.

Small satellites with MEMS x-ray telescopes for x-ray astronomy and solar system exploration

Toward a new era of X-ray astronomy, next generation X-ray optics are indispensable. To meet a demand for telescopes lighter than the foil optics but with a better angular resolution less than 1 arcmin, we are developing micropore X-ray optics based on micromaching technologies. Using sidewalls of micropores through a thin silicon wafer, this type can be the lightest X-ray telescope ever achieved. Two new Japanese missions ORBIS and GEOX will carry this optics. ORBIS is a small X-ray astronomy mission to monitor supermassive blackholes, while GEO-X is a small exploration mission of the Earth’s magnetosphere. Both missions need a ultra light-weight (<1 kg) telescope with moderately good angular resolution (<10 arcmin) at an extremely short focal length (<30 cm). We plan to demonstrate this optics in these two missions around 2020, aiming at future other astronomy and exploration missions.