U. Morita

Study of the X-Ray Background Spectrum and Its Large-Scale Fluctuation with ASCA

We studied the energy spectrum and the large-scale fluctuation of the X-ray background with the ASCA GIS based on the ASCA MSS and LSS observations. A total of 91 fields with Galactic latitude |b|>10 deg were selected with a sky coverage of 50 deg^2 and 4.2 Ms of exposure. For each field, sources brighter than \sim 2\times 10^{-13}\ergs (2-10 keV) were eliminated. Spectral fits with a single power-law model for the individual 0.7-10 keV spectra showed a significant excess below \sim 2 keV, which could be expressed by an additional thermal model with kT\simeq 0.4 keV or a steep power-law model with photon index GS\simeq 6. The 0.5-2 keV intensities of the soft thermal component varied significantly by 1\sigma=52^{+4}_{-5}%, and showed a maximum toward the Galactic Center. This component is considered to be entirely Galactic. As for the hard power-law component, an average photon index of 91 fields was obtained to be GH = 1.412\pm 0.007\pm 0.025 and the average 2-10 keV intensity was calculated as FH = (6.38\pm 0.04\pm 0.64)\times 10^{-8} \ergss. The Galactic component is marginally detected in the hard band. The 2-10 keV intensities shows a 1\sigma deviation of 6.49^{+0.56}_{-0.61}%, while deviation due to the NXB is 3.2%. The observed deviation can be explained by the Poisson noise of source count in the f.o.v. (\sim 0.5 deg^2), even assuming a single \logn relation on the whole sky. Based on the observed fluctuation and the absolute intensity, an acceptable region of the \logn relation was derived, showing a consistent feature with the recent Chandra and XMM-Newton results. Fluctuation of the spectral index was also examined, and it implied a large amount of hard sources and a substantial variation in the intrinsic source spectra (GS\simeq 1.1\pm 1.0).

Detection of a Fully Resolved Compton Shoulder of the Iron Kα Line in the Chandra X-Ray Spectrum of GX 301–2

We report the detection of a fully resolved, Compton-scattered emission line in the X-ray spectrum of the massive binary GX 301-2 obtained with the High Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The iron Kα fluorescence line complex observed in this system consists of an intense narrow component centered at an energy of E = 6.40 keV and a redward shoulder that extends down to ~6.24 keV, which corresponds to an energy shift of a Compton backscattered iron Kα photon. From detailed Monte Carlo simulations and comparisons with the observed spectra, we are able to directly constrain the physical properties of the scattering medium, including the electron temperature and column density, as well as an estimate for the metal abundance.

Chandra and XMM-Newton Observations of a Group of Galaxies, HCG 62

We present results from Chandra and XMM-Newton observations of the bright group of galaxies HCG 62. The two cavities in the central region show no significant change of temperature compared with that in the surrounding region. We studied radial distributions of temperature and metal abundance. Two temperatures are required in the inner γ <2’ (35 kpc) region, and a sharp drop of temperature is seen at γ ∼ 5’ where the gas may not be in hydrostatic equilibrium. The metal distribution suggests that iron and silicon are produced by type Ia supernova in the central galaxy, while galactic winds by type II supernova have caused a wider distribution of oxygen. The pressure due to electrons and magnetic fields is too low to displace the group hot gas, and other pressure contributions from high energy protons or by galaxy-scale dynamical motions are nearly 700 times higher. Detailed accounts are given in [5].

Present performance of a single pixel Ti/Au bilayer TES calorimeter

We are developing a superconducting transition-edge sensor (TES) calorimeter for future Japanese X-ray astronomy missions (e.g. NeXT mission). The performance of our single pixel TES calorimeter is presented. We fabricated a Ti/Au (40 nm/110 nm) bilayer TES on a thin silicon-nitride membrane, which is adjusted to have a transition temperature of about 100 mK. The size of the TES is 500μm × 500μm, and 300μm × 300μm gold with a thickness of 300 nm is deposited with sputtering as an X-ray absorber. The TES calorimeter was installed in a dilution refrigerator operated at about 40 mK, with a combination of 400-series SQUID array as an ammeter. Collimated 5.9 keV X-rays (200 um in diameter) from 55Fe isotope were irradiated and X-ray pulses were obtained. Simultaneously with a fast falling time constant of 74.2 us, the energy resolution of 6.6+-0.4 eV was attained, while the baseline noise was 6.4 eV. The contents of the energy resolution are 5.1 eV of the excess noise, 3.3 eV of the readout noise, 1.6 eV of the pulse by pulse variation, and 1.9 eV of the intrinsic noise. The baseline noise are dominated by an unknown excess noise, which increases roughly in proportion to the inverse of the TES resistance. The pulse height is sensitive to the operating conditions, and the superconducting shield appears to have improved it by a factor of about 2. The calorimeter works fine over six months surviving five thermal cycles, even though it is kept in air.

Development of a microcalorimeter array for the Diffuse-Intergalactic Oxygen-Surveyor (DIOS) mission

We are developing a superconducting transition-edge sensor (TES) microcalorimeter array for the Diffuse Intergalactic Oxygen Surveyor (DIOS) mission. DIOS is a relatively small Japanese X-ray mission which will study large-scale distribution of the warm-hot intergalactic medium (WHIM) using OVII and OVIII emission lines. The satellite weighs about 400 kg equipped with a four-reflection X-ray telescope (FXT) and a TES microcalorimeter array (XSA). The design goal of the observing system is an effective area larger than 100 cm2 at the oxygen line energy, a field of view about 50 arcmin square, and an energy resolution about 2 eV in the energy range of 0.3-1 keV. The TES microcalorimeter array provides the large field of view and good energy resolution at the same time. We plan to install an array comprising 16 x 16 pixels with an overall size of 1 cm square, which is cooled with a cryogen-free cooler. Pixels are readout by multiplexing signals using a multi-input SQUID amplifier, with each input connected to a TES microcalorimeter which is AC biased with a different frequency. We report the design and present status of the XSA system development.

High sensitive X-ray microcalorimeter using Bi-Au microabsorber for imaging applications

An X-ray microcalorimeter is a cryogenic energy-dispersive spectrometer, which has an energy resolution almost comparable to that of conventional wavelength-dispersive spectrometers. Using a transition edge sensor (TES) as a temperature sensor, the energy resolution can be further improved. We have developed a new method of achieving an array of TES microcalorimeters for the purpose of X-ray imaging. To achieve this, mushroom-shaped X-ray microabsorbers formed using electrodeposition were applied. The temperature of the TES, which is easily degraded by thermal diffusion, was kept sufficiently low throughout the process to achieve practical use. On the bases of this new method, a 2×2 (× 4) array of TES microcalorimeters was fabricated and tested. A high energy resolution of 13.0 eV at 6 keV was achieved and the filling factor was improved to 83%. Although several issues still need to be investigated, we verified that our method is useful for fabricating a Ti–Au TES microcalorimeter array.

Soft x-ray measurement of the toroidal pinch experiment RX reversed field pinch plasma using transition edge sensor calorimeter

A superconductive transition edge sensor (TES) calorimeter is for the first time applied for the diagnostics of the reversed field pinch plasma produced in the toroidal pinch experiment RX (TPE-RX), and the instrumental system is fully described. The first result from the soft x-ray spectroscopy in 0.2–3keV with an energy resolution ∼50eV are also presented. The TES calorimeter is made of a thin bilayer film of titanium and gold with a transition temperature of 151mK and its best energy resolution at our laboratory is 6.4eV, while it was significantly degraded by about a factor of eight during the plasma operation. The TES microcalorimeter was installed in a portable adiabatic demagnetization refrigerator (ADR), which is originally designed for a rocket experiment. The detector box is carefully designed to shield the strong magnetic field produced by the ADR and TPE-RX. The ADR was directly connected to TPE-RX with a vacuum duct in the sideway configuration, and cooled down to 125mK stabilized with an acc...

Status of X-ray microcalorimeter development at ISAS

A superconducting phase-transition microcalorimeter is a promising detector for high resolution X-ray spectroscopy. We are developing such a detector for future Japanese X-ray astronomy missions. In our design, a tin absorber is electrodeposited as a mushroom structure to achieve a high covering fraction. We have succeeded in detecting X-rays with the first model of our calorimeter with an electrodeposited absorber, though the energy resolution was limited due to residual resist. We are also developing a phase-transition microcalorimeter with a low transition temperature. We describe the results of these experiments, and discuss the limiting factors of their performance.

The XRS Microcalorimeter on Astro‐E2

The XRS microcalorimeter will be launched in 2005 as part of the Astro‐E2 mission. It will cover the energy band from 0.3 to 10 keV with a nearly constant energy resolution of 6.0 eV and a peak effective area of 200 cm2 at 1.5 keV. The XRS will provide unprecedented throughput and resolving power, particularly at high energies. Detailed spectral features in the Fe K region will be resolved for the first time, providing access to spectroscopic diagnostics for a wide range of astrophysical objects. In this presentation we will describe the XRS instrument, details of its spectral performance, and how it compares to the current high‐resolution instruments on the Chandra and XMM‐Newton observatories.

Chandra Observation of a Group of Galaxies HCG 80: Does the Spiral-Only Group Have Hot Intragroup Gas?

We present an analysis of Chandra X-ray observations of a compact group of galaxies, HCG 80 (z=0.03). The system is a spiral-only group composed of four late-type galaxies, and has a high-velocity dispersion of 309 km/s. With high-sensitivity Chandra observations, we searched for diffuse X-ray emission from the intragroup medium (IGM); however, no significant emission was detected. We place a severe upper limit on the luminosity of the diffuse gas as LX < 6e40 erg/s. On the other hand, significant emission from three of the four members were detected. In particular, we discovered huge halo emission from HCG 80a that extends on a scale of ~30 kpc perpendicular to the galactic disk, whose X-ray temperature and luminosity were measured to be ~0.6 keV and ~4e40 erg/s in the 0.5-2 keV band, respectively. It is most likely to be an outflow powered by intense starburst activity. Based on the results, we discuss possible reasons for the absence of diffuse X-ray emission in the HCG 80 group, suggesting that the system is subject to galaxy interactions, and is possibly at an early stage of IGM evolution.