Ryo Shibata

Temperature Map of the Virgo Cluster of Galaxies Observed withASCA

The temperature distribution of the intracluster medium (ICM) in the Virgo Cluster of galaxies has been derived from extensive mapping observations with ASCA covering an area of 19 deg2. In the spectral analysis, the inner region within a radius of ~60 from M87 is characterized by an ICM temperature of kT ~ 2.5 keV with little variation. On the other hand, the outer regions indicate significant variation of the temperature with an amplitude of about 1 keV. The temperature map was produced from the hardness ratio (HR) values with a resolution of about 5. Besides the previously reported hot region with kT > 4 keV between M87 and M49, several hot regions with kT = 3-4 keV are detected in the cluster outskirts. The autocorrelation function for the HR variation shows that the temperature variation is correlated within a size of about 300 kpc, suggesting that gas blobs falling in the Virgo Cluster have a typical size of groups of galaxies. A correlation with the velocity dispersion of member galaxies shows that only the northwest region indicates an unusually large βspec value of 2-4. The upper limit for extended nonthermal emission in the Virgo Cluster is obtained to be LX ~ 9 × 1041 ergs s-1 in the 2-10 keV band. We discuss that these features consistently indicate that the Virgo Cluster is in a relatively early stage of the cluster evolution.

A Possible Emission Feature in an X-Ray Afterglow of GRB 970828 as a Radiative Recombination Edge

A gamma-ray burst of 28 August 1997 was localized by the All-Sky Monitor on the Rossi XTE satellite and its coordinates were promptly disseminated. An ASCA followup started 1.17 days after the burst as a Target of Opportunity Observation and detected an X-ray afterglow. The spectral data displayed a hump around ~5 keV and an absorption column of 7.1 x 10^21 cm^{-2}. This hump structure is likely a recombination edge of iron in the vicinity of the source, taking account of the redshift z = 0.9578 found for the likely host galaxy of the associated radio flare. Radiative Recombination edge and Continuum model can interpret the spectrum from highly ionized plasma in a non equilibrium ionization state. The absorption could be also due to the medium presumably in the vicinity of the GRB.A gamma-ray burst (GRB) of 1997 August 28 was localized by the All-Sky Monitor on the Rossi X-Ray Timing Explorer satellite, and its coordinates were promptly disseminated. An ASCA follow-up started 1.17 days after the burst as a Target-of-Opportunity Observation and detected an X-ray afterglow. The spectral data displayed a hump around ~5 keV and an absorption column of 7.1 × 1021 cm-2. Taking into account the redshift z = 0.9578 found for the likely host galaxy of the associated radio flare, this hump structure is likely a recombination edge of iron in the vicinity of the source. The radiative recombination edge and continuum model can interpret the spectrum from highly ionized plasma in a nonequilibrium ionization state. The absorption could also be due to the medium presumably residing in the vicinity of the GRB.

Characterization of the supermirror hard-x-ray telescope for the InFOCμS balloon experiment

A hard-x-ray telescope is successfully produced for balloon observations by making use of depth-graded multilayers, or so-called supermirrors, with platinum-carbon (Pt/C) layer pairs. It consists of four quadrant units assembled in an optical configuration with a diameter of 40 cm and a focal length of 8 m. Each quadrant is made of 510 pieces of coaxially and confocally aligned supermirrors that significantly enhance the sensitivity in an energy range of 20-40 keV. The configuration of the telescope is similar to the x-ray telescope onboard Astro-E, but with a longer focal length. The reflectivity of supermirrors is of the order of 40% in the energy range concerned at a grazing angle of 0.2 deg. The effective area of a fully assembled telescope is 50 cm2 at 30 keV. The angular resolution is 2.37 arc min at half-power diameter 8.0 keV. The field of view is 12.6 arc min in the hard-x-ray region, depending somewhat on x-ray energies. We discuss these characteristics, taking into account the figure errors of reflectors and their optical alignment in the telescope assembly. This hard-x-ray telescope is unanimously afforded in the International Focusing Optics Collaboration for muCrab Sensitivity balloon experiment.

ASCA Temperature Maps of Three Clusters of Galaxies: Abell 1060, AWM 7, and the Centaurus Cluster

We present two-dimensional temperature maps of three bright clusters of galaxies Abell 1060, AWM7, and the Centaurus cluster, based on multi-pointing observations with the ASCA GIS. The temperatures are derived from hardness ratios by taking into account the XRT response. For the Centaurus cluster, we subtracted the central cool component using the previous ASCA and ROSAT results, and the metallicity gradients observed in AWM7 and the Centaurus cluster were included in deriving the temperatures. The intracluster medium in Abell 1060 and AWM7 is almost isothermal from the center to outer regions with a temperature of 3.3 and 3.9 keV, respectively. The Centaurus cluster exhibits remarkable hot regions within about 30 from the cluster center showing a temperature increase of +0.8 keV from the surrounding level of 3.5 keV, and outer cool regions with lower temperatures by -1.3 keV. These results imply that a strong merger has occurred in the Centaurus in the recent 2-3 Gyr, and the central cool component has survived it. In contrast, the gas in Abell 1060 was well-mixed in an early period, which probably has prevented the development of the central cool component. In AWM7, mixing of the gas should have occurred in a period earlier than the epoch of metal enrichment.

Temperature Map of the Perseus Cluster of Galaxies Observed with ASCA

We present a two-dimensional temperature map of the Perseus Cluster based on multipointing observations with the ASCA Gas Imaging Spectrometer, covering a region with a diameter of ~2°. By correcting for the effect of the X-ray telescope response, the temperatures were estimated from hardness ratios, and the complete temperature structure of the cluster with a spatial resolution of about 100 kpc was obtained for the first time. There is an extended cool region with a diameter of ~20 and kT ~ 5 keV about 20 east of the cluster center. This region also shows higher surface brightness and is surrounded by a large ringlike hot region with kT 7 keV and is likely to be a remnant of a merger with a poor cluster. Another extended cool region is extending outward from the IC 310 subcluster. These features and the presence of several other hot and cool blobs suggest that this rich cluster has been formed as the result of a repetition of many subcluster mergers.

ASCA Observations of the Temperature Structure and Metal Distribution in the Perseus Cluster of Galaxies

Large-scale distributions of hot-gas temperature and Fe abundance in the Perseus cluster have been studied with multi-pointing observations by the GIS instrument onboard ASCA. Within a radius of 20 � from the cluster center, the energy spectra requires two temperature components, in which the cool component indicates kT ∼2keV and the hot-component temperature shows a significant decline from about 8 keV to 6 keV toward the center. In the outer region of the cluster, the temperature shows a fluctuation with an amplitude of about 2 keV, which suggest that a western region at ∼ 16 � from the cluster center is relatively hotter. As for the Fe abundance, a significant decline with radius is detected from 0.44 solar at the center to ∼ 0.1 solar at a 50 � offset region. If the observed Fe-K line intensity within 4 � from the center is suppressed by a factor of 2 due to the resonance scattering effect, the corrected

Characterization of a Hard X-ray Telescope at Synchrotron Facility SPring-8

Space-borne astronomical instruments require extensive characterization on the ground before launch. In the hard X-ray region however, it is difficult for a laboratory-based beamline using a conventional X-ray source to provide a capability sufficient for pre-flight high-precision calibration. In this paper, we describe an experiment to characterize a hard X-ray telescope at a synchrotron facility, mainly on the basis of experimental setup and examples of measured results. We have developed hard X-ray telescopes consisting of Wolter-I grazing incidence optics and platinum-carbon multilayer supermirror coatings. The telescopes have been characterized at the synchrotron facility SPring-8 beamline BL20B2. The measurements at BL20B2 have great advantages such as extremely high flux, large-sized and less-divergent beam, and monochromatic beam covering the entire hard X-ray region from 8 to over 100 keV. The telescope was illuminated by monochromatic hard X-rays, and the focused image was measured by high resolution hard X-ray imagers. The entire telescope aperture was mapped by a small beam, and the effective area and the point spread function were obtained as well as local optical properties for further diagnostics of the characteristics of the telescope.

Temperature Variation in the Cluster of Galaxies Abell 115 Studied with ASCA

Abell 115 exhibits two distinct peaks in the surface brightness distribution. Observation with ASCA shows a significant temperature variation in this cluster, confirmed by a hardness ratio analysis and spectral fits. A linking region between the main cluster and subcluster shows a high temperature compared with other regions. Two possibilities are examined as the cause of the temperature variation: cooling flows in the main cluster and a shock heating due to the collision of the subcluster into the main system. Spectral fits with cooling flow models to the main-cluster data show a mass-deposition rate less than 419 M☉ yr-1. Temperatures in the main cluster, the linking region, and the subcluster are estimated by correcting for the effects of X-ray telescope response as 4.9, 11, and 5.2 keV, respectively. The high temperature in the linking region implies that Abell 115 is indeed a merger system, with possible contribution from cooling flows on the temperature structure.

X-ray telescope onboard Astro-E. III. Guidelines to performance improvements and optimization of the ray-tracing simulator

We present a detailed study of the performance of the Astro-E x-ray telescope (XRT) onboard the Astro-E satellite. As described in preceding papers the ground-based calibrations of the Astro-E XRT revealed that its image quality and effective area are somewhat worse than that expected from the original design. Conceivable causes for such performance degradation are examined by x-ray and optical microscopic measurements at various levels, such as individual reflectors, sectors, and quadrants of the XRT and their alignments. We can attribute, based on detailed measurements, the degradation of the image quality to a slope error in the individual reflectors and the positioning error of reflectors. As for the deficit of the effective area, the shadowing of x rays within the XRT body is the dominant factor. Error budgets for the performance degradation of the Astro-E XRT are summarized. The ray-tracing simulator, which is needed to construct the response function for arbitrary off-axis angles and spatial distributions of any celestial x-ray sources, has been developed and tuned based on the results of detailed measurements. The ray-tracing simulation provides results that are consistent within 3% with the real measurement except for large off-axis angles and higher energies. We propose, based on knowledge obtained from all the measurements and simulations, several plans for future developments to improve the performance of the nested thin-foil mirrors.

First light of a hard-x-ray imaging experiment: the InFOCuS balloon flight

As technological and scientific path-finder towards future observatory missions, a balloon-born hard X-ray imaging observation experiment InFOCμS has been developed. The payload has flown four times since 2000. In its 2004 Fall flight campaign InFOCμS successfully achieved first scientific observations of multiple astronomical objects from galactic compacts to cluster of galaxies. Significant signal has been detected from bright galactic objects while analysis of extragalactic objects is underway. InFOCμS plans additional and upgraded telescope-detector system as early as 2006. High energy telescope for nuclear gamma-ray line observations is under planning.