Yoshitaka Ishisaki

Structure of the X-Ray-emitting Gas in the Hydra A Cluster of Galaxies

The temperature and abundance structure in the intracluster medium (ICM) of the Hydra A Cluster of galaxies is studied with ASCA and ROSAT. The effect of the large extended outskirts in the point-spread function of the X-ray telescope on ASCA is included in this analysis. In the X-ray brightness profile, the strong central excess above a single β model, identified in the Einstein and ROSAT data, is also found in the harder energy band (>4 keV). A simultaneous fit of five annular spectra taken with the GIS instrument shows a radial distribution of the temperature and metal abundance. A significant central enhancement in the abundance distribution is found, while the temperature profile suggests that the ICM is approximately isothermal, with a temperature of ~3.5 keV. The ROSAT position-sensitive proportional counter (PSPC) spectrum in the central 15 region indicates a significantly lower temperature than the GIS result. A joint analysis of the GIS and PSPC data reveals that the spectra can be described by a two-temperature model as well as by a cooling flow model. In both cases, the hot-phase gas with a temperature of ~3.5 keV occupies more than 90% of the total emission measure within 15 from the cluster center. The estimated mass of the cooler (0.5-0.7 keV) component is ~(2-6) × 109 M☉, which is comparable to the mass of hot halos seen in non-cD ellipticals. The cooling flow model gives the mass deposition rate of 60 ± 30 M☉ yr-1, an order of magnitude lower than the previous estimation.

Detection of Inverse-Compton X-Rays from Lobes of the Radio Galaxy Fornax A

Extended X-ray emission was detected with ASCA from the synchrotron double lobes of the radio galaxy Fornax A. This is thought to be the same emission as was detected with ROSAT. The excess X-rays exhibit very hard spectra, described by a power law of energy index 1.2 ± 0.5. Since this index is consistent with the synchrotron radio index of 0.9 ± 0.2, the X-rays are thought to arise when the radio-emitting relativistic electrons make inverse-Compton scattering off the cosmic microwave photons. Comparison of the X-ray and radio fluxes yields the lobe magnetic field intensity of 2-4 μG. The relativistic electrons in the lobes are inferred to have an energy density close to that of the magnetic fields.

The X-Ray Halo of the Local Group and Its Implications for Microwave and Soft X-Ray Backgrounds

Since recent X-ray observations have revealed that most clusters of galaxies are surrounded by an X-ray-emitting gaseous halo, it is reasonable to expect that the Local Group of galaxies has its own X-ray halo. We show that such a halo, with temperature ~1 keV and column density ~O(1021) cm-2, is a possible source for the excess low-energy component in the X-ray background. The halo should also generate temperature anisotropies in the microwave background via the Sunyaev-Zeldovich effect. Assuming an isothermal spherical halo with the above temperature and density, the amplitude of the induced quadrupole turns out to be comparable to the COBE data without violating the upper limit on the y-parameter. The induced dipole is negligible compared to the peculiar velocity of the Local Group, and multipoles higher than quadrupole are generally much smaller than the observed ones. However, nonsphericity and/or clumpiness of the halo will produce a stronger effect. Therefore the gaseous halo of the Local Group, if it exists, will affect the estimate of the primordial spectral index n and the amplitude of the density fluctuations deduced from the COBE data.

X-Ray Spectrum of Supernova 1993J Observed with ASCA and Its Evolution 8-572 Days after the Explosion

ASCA observations of the supernova in M81, SN 1993J, 8-572 days after the explosion are presented. Utilizing a one-dimensional image-fitting to the Solid-State Imaging Spectrometer (SIS) data, energy spectra of the supernova in 0.5-8 keV are obtained. The spectrum showed drastic softening with a power-law photon index of 0.3-4, while the X-ray intensity decreased from 0.03 to 0.008 counts s-1 SIS-1. The early phase of spectra requires two thermal emission components of different absorption columns if they are fitted with thermal models. The temperatures of two emission components cannot be well constrained with ASCA continuum spectra. However, the detection of an iron K emission line with ASCA and the hard X-ray spectra observed by OSSE on the Compton Gamma-Ray Observatory suggest that the two components have different temperatures: a few keV and about 100 keV. The properties of the two emission components are consistent with emissions from the front and reverse shocks of the supernova explosion. The drastic softening of the X-ray spectra is explained by the decrease of the absorption column density of the reverse shock component and the change of the major contributor to the soft X-ray band from the front to the reverse shocks.

Calibrations of imaging gas scintillation proportional counters on ASTRO-D

The fourth Japanese X-ray astronomy satellite, ASTRO-D, was launched successfully by the Institute of Space and Astronautical Science on February 20, 1993 and was named ASCA. Two of the focal plane detectors are imaging gas scintillation proportional counters (Gas Imaging Spectrometer:GIS). The GIS sensors performed the energy resolution of 8% FWHM at 6 keV, and position resolution of 0.5 mm FWHM on-board, which confirmed their ultimate capability as gas counters. The non-Xray background counting rate was approximately 6 X 10-4 c/s/cm2/keV in the energy range of 2 - 10 keV, which was as low as that achieved by the Ginga instrument. The scientific results obtained by the GIS sensors are also presented.

In-orbit performance of the GIS instrument on board ASCA (ASTRO-D)

The fourth Japanese x-ray astronomy satellite, ASCA, carries two imaging gas scintillation proportional counters (GIS) on its focal plane. Extensive ground calibration has established its position resolution to be 0.5 mm and FWHM energy resolution to be 8.0% both at 6 keV. When combined with the x-ray telescope, a sensitivity range becomes 0.7 - 10 keV. These properties have been confirmed through in-orbit calibrations. The in-orbit background of the GIS has been confirmed to be as low as (5 - 7) X 10-4 c s-1cm-2keV-1 over the 1 - 10 keV range. The long-term detector gain is stable within a few % for two years. Gain dependence on the position and temperature has been calibrated down to 1%. The overall energy response is calibrated very accurately. Thus the GIS is working as an all-round cosmic x-ray detector.

Stray light of Suzaku XRT from Crab offset observations

The stray light is one of the systematic uncertainties in the analysis of faint diffuse objects, such as outskirts of clusters of galaxies. The stray light had been modeled so that researchers can estimate it by xissim. The model prediction sometimes differs from the observation by factors. We summarize characteristics of stray lights obtained from Crab offset observations.

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.

Gamma-ray burst monitoring with the hard X-ray detector onboard the ASTRO-E mission

ASTRO-E is the fifth Japanese X-ray astronomy satellite scheduled to be launched in year 2000. The satellite will carry three scientific instruments, one of which is the Hard X-ray Detector (HXD). HXD will cover the 10–600 keV band with low background of several×10−6c/s/cm2/keV. This will be achieved by the well-type phoswich counter technique together with large active shield counters of BGO (about 1200u2009cm2/side), which are also capable to detect GRBs in the 100–2000 keV band. About 100 GRBs are expected to be detected per year with HXD.