K. Kikuchi

ASCA Measurements of Silicon and Iron Abundances in the Intracluster Medium

We analyzed the ASCA X-ray data of 40 nearby Clusters of galaxies, whose intracluster-medium temperature distributes in the range of 0.9-10 keV. We measured the Si and Fe abundances of the intracluster medium, spatially averaging over each Cluster, but excluding the central ~ 0.15/I^Q1 Mpc region in order to avoid any possible abundance gradients and complex temperature structures. The Fe abundances of these Clusters are 0.2-0.3 solar, with only weak dependence on the temperature of the intracluster medium, hence on the Cluster richness. In contrast, the Si abundance is observed to increase from 0.3 to 0.6-0.7 solar from the poorer to richer Clusters. These results suggest that the Supernovae of both type-Ia and type-II significantly contribute to the metal enrichment of the intracluster medium, with the relative contribution of type-II Supernovae increasing towards richer Clusters. We suggest a possibility that a considerable fraction of type-II Supernova products escaped from poorer Systems.

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.

Development of Cryogenic System for Smiles

Superconducting Submillimeter‐Wave Limb‐Emission Sounder (SMILES) is to be operated aboard the Japanese Experiment Module (JEM) of the International Space Station (ISS) in 2007. SMILES uses two Superconductor‐insulator‐superconductor (SIS) mixers for submillimeter‐wave atmospheric observation, and they are cooled to 4 K levels by a cryogenic system with a two‐stage Stirling cooler, a Joule‐Thomson (JT) cycle cooler and a cryostat composed of three stages. The cooling capacity is designed as about 20 mW at 4.5 K, 200 mW at 20 K and 1 W at 100 K with the total input power of approximately 140 W. The proto‐flight model (PFM) of the cryogenic system has achieved such cooling capacity with significantly less input power, as well as mechanical capability required for launching conditions.

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.

Submillimeter Limb-emission Sounder JEM/SMILES aboard the Space Station

A submillimeter limb-emission sounder, that is to be aboard the Japanese Experiment Module (JEM, dubbed as KIBO) at the International Space Station, has been designed. This payload, Superconducting Submillimeter-wave Limb-emission Sounder (SMILES), is aimed at global mappings of stratospheric trace gases by means of the most sensitive submillimeter receiver ever operated in space. Such sensitivity is ascribed to a Superconductor-Insulator- Superconductor (SIS) mixer, which is operated at 4.5 K in a dedicated cryostat combined with a mechanical cooler. SMILES will observe ozone-depletion-related molecules such as ClO, Hcl, HO2, HNO3, BrO and O3 in the frequency bands at 624.32-626.32 GHz and 649.12-650.32 GHz. A scanning antenna will cover tangent altitudes from 10 to 60 km in every 53 seconds, while tracing the latitudes form 38 S to 65 N along its orbit. This global coverage makes SMILES a useful tool of observing the low- and mid- latitudinal areas as well as the Arctic peripheral region. The molecular emissions will be detected by two units of acousto-optic spectrometers (AOS), each of which has coverage of 1.2 GHz with a resolution of 1.8 MHz. This high-resolution spectroscopy will allow us to detect weak emission lines attributing to less-abundant species.

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

Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA

Based on mapping observations with ASCA, an unusual hot region with a spatial extent of 1 deg2 was discovered between M87 and M49 at a center coordinate of R.A.=12h27m36s and decl.=9&j0;18 (J2000). The X-ray emission from the region has a 2-10 keV flux of 1x10-11 ergs s-1 cm-2 and a temperature of kT greater, similar4 keV, which is significantly higher than that in the surrounding medium of approximately 2 keV. The internal thermal energy in the hot region is estimated to be VnkT approximately 1060 ergs with a gas density of approximately 10-4 cm-3. A power-law spectrum with a photon index of 1.7-2.3 is also allowed by the data. The hot region suggests there is an energy input due to a shock that is probably caused by the motion of the gas associated with M49, infalling toward the M87 cluster with a velocity greater, similar1000 km s-1.

Possible site of heating and acceleration in clusters of galaxies

The result of the Virgo cluster mapping project utilizing ASCA is reported. We discoverd a hot region in the emission bridge between M87 and M49. The physical parameters of the hot region and the heating or acceleration process are examined.