M. Hirayama

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.

Hard X-Ray Emission from the Galactic Ridge

Hard X-ray and γ-ray emissions from the Galactic ridge were studied with the large area proportional counter (LAC) on the Ginga satellite and a balloon-borne detector Welcome-1. In the scanning observations with the LAC, diffuse hard X-rays were detected along the Galactic plane between l = -20° and l = 40°. The measured spectrum shows that a hard component exists in the Galactic ridge emission above 10 keV, in addition to the hot plasma component. The estimated luminosity of the Galactic ridge emission is around 2 × 1038 ergs s-1 in the 3-16 keV band. Welcome-1 observed the γ-ray ridge emission at around l ~ 345° between 50 and 600 keV. These two results and a recent COMPTEL study suggest that the spectrum of the diffuse Galactic ridge emission extends over the keV-MeV range. From the observed spectral slope, bremsstrahlung by electrons is shown to be the dominant emission mechanism. This implies that low-energy electrons must be supplied continuously to sustain emission in the hard X-ray band. We propose a scenario in which the thermal electrons in the hot plasma responsible for the strong Fe K X-ray emission are shock-accelerated continuously in supernova remnants (SNRs), producing the observed hard X-ray and γ-ray emissions from the Galactic ridge.

X-Ray Detection of PSR B1259-63 at Periastron

We report on results of X-ray observations of the unique radio pulsar/Be star binary PSR B1259?63, obtained using the ASCA satellite two weeks before, during , and two weeks after the pulsars most recent periastron passage on January 9, 1994. The source was detected at all three epochs, with an X-ray luminosity in the 1{10 keV band of 110 34 (d=2 kpc) 2 erg s ?1 at the pre-and post-periastron epochs, and a factor of 2 smaller at periastron. The X-ray emission can be characterized by power-law spectra with a photon index in the range 1.5{1.9, with evidence for spectral softening at periastron. The photoelectric absorption, N H 510 21 cm ?2 , was constant for the three observations within measurement uncertainties, and is consistent with the galactic contribution. We detect no pulsations , and derive an upper limit on periodicities close to the PSR B1259?63 spin period of 7% of the total observed ux, conservatively assuming a sine-wave proole. We argue that accretion of gaseous material onto the surface of the neutron star is an unlikely origin for the observed X-rays. The characteristics of the X-ray emission from the PSR B1259?63 system are in good qualitative agreement with models of non-thermal acceleration of relativistic particles from the pulsar wind in a shock at the location where the pulsar and Be star wind pressures balance. Assuming reasonable Be star equatorial outtow and pulsar wind pressures, we nd that the mass-loss rate _ M and surface velocity v are constrained by the relation 100 < (v=10 km s ?1)(_

Astro-E hard x-ray detector

Astro-E is the x-ray satellite to be launched in the year 2000 by Inst. of Space & Astronautical Science. This report deals with the design and expected performance of the hard x-ray detector (HXD), one of the 3 experiments aboard Astro- E. The HXD is a combination of GSO/BGO well-type phoswich counters and silicon PIN diodes: the two combined will cover a wide energy band of 10 - 700 keV. The detector is characterized by its low background of approximately 10-5/s/cm2/keV and its sensitivity higher than any past missions between a few 10 keV and several 100 keV. Combined with the other 2 experiments, a micro-calorimeter array (XRS) and 4 CCD arrays (XIS), both with x-ray mirrors, the mission will cover the soft and hard x-ray range at a highest sensitivity.

Well-type phoswich counter for low-flux X-ray/ gamma -ray detection

Novel phoswich counters have been developed that are capable of detecting low flux hard X-rays gamma -rays from localized sources. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator, hard X-rays/ gamma -rays that have deposited energy only in the detection part can be selected. The first model counter was built by using a new scintillator, GSO, in the detection part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area approximately 740 cm/sup 2/) was flown on board a balloon, setting a limit to the /sup 57/Co line flux from SN 1987A at around 10/sup -4//cm/sup 2/-s. The sensitivity for continuum flux was around a few*10/sup -6//cm/sup 2/-s-keV between 100 and 200 keV. >

Pulsed X-Ray Emission from the Fastest Millisecond Pulsar: PSR B1937+21 with ASCA

We have detected pulsed X-ray emission from the fastest millisecond pulsar known, PSR B1937+21 (P = 1.558 ms), with ASCA. The pulsar is detected as a point source above ~1.7 keV, with no indication of nebulosity. The source flux in the 2-10 keV band is found to be f = (3.7 ± 0.6) × 10-13 ergs s-1 cm-2, which implies an isotropic luminosity of LX = 4πD2f ~ (5.7 ± 1.0) × 1032(D/3.6 kpc)2 ergs s-1, where D is the distance, and an X-ray efficiency of ~5 × 10-4, relative to the spin-down power of the pulsar. The pulsation is found at the period predicted by the radio ephemeris with a very narrow primary peak, the width of which is about 1/16 phase (~100 μs), near the time resolution limit (61 μs) of the observation. The instantaneous flux in the primary peak (1/16 phase interval) is found to be (4.0 ± 0.8) × 10-12 ergs s-1 cm-2. Although there is an indication for the secondary peak, we consider its statistical significance too low to claim a definite detection. The narrow pulse profile and the detection in the 2-10 keV band imply that the X-ray emission is caused by the magnetospheric particle acceleration. Comparison of X-ray and radio arrival times of pulses indicates, within the timing errors, that the X-ray pulse is coincident with the radio interpulse.

The silicon tracker readout electronics of the Gamma-ray Large Area Space Telescope

A unique electronics system has been built and tested for reading signals from the silicon-strip detectors of the Gamma-ray Large Area Space Telescope mission. The system amplifies and processes signals from 884 736 36-cm strips using only 160 W of power, and it achieves close to 100% detection efficiency with noise occupancy sufficiently low to allow it to self trigger. The design of the readout system is described, and results are presented from ground-based testing of the completed detector system.

Newly developed low background hard X-ray/gamma-ray telescope with the well-type phoswich counters

A low background hard X-ray/gamma-ray telescope (Welcome-1) for balloon-borne experiments in the energy range from 40 keV to 800-1000 keV is discussed. The detector is based on newly developed well-type phoswich counters. In the first well-type phoswich counter, GSO(Ce) (Gd/sub 2/SiO/sub 5/ doped with Ce) is used as the detection part and CsI(Tl) as the shielding part. Welcome-1 consists of 64 GSO/CsI well-type phoswich counters assembled in the compound-eye configuration, and the effective area is 740 cm/sup 2/ at 122 keV and 222 cm/sup 2/ at 511 keV line. The well-type phoswich counter and the compound eye configuration reduce background significantly both externally and internally, and allow a high signal-to-noise ratio in balloon-borne experiments. The background levels at an attitude of 4.5 g/cm/sup 2/ are 1*10/sup -4//cm/sup 2//s/keV at 122 keV. Crab nebula is observed with the signal to background ratio better than unity between 100 and 200 keV. The design of the telescope and the flight performance are presented. >

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

Well-type phoswich counters for low-flux x-ray/gamma-ray detection

Novel phoswich counters have been developed that are capable of detecting low flux hard X-rays gamma -rays from localized sources. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator, hard X-rays/ gamma -rays that have deposited energy only in the detection part can be selected. The first model counter was built by using a new scintillator, GSO, in the detection part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area approximately 740 cm/sup 2/) was flown on board a balloon, setting a limit to the /sup 57/Co line flux from SN 1987A at around 10/sup -4//cm/sup 2/-s. The sensitivity for continuum flux was around a few*10/sup -6//cm/sup 2/-s-keV between 100 and 200 keV.<<ETX>>