Hidehiro Kaneda

COMPLEX SPECTRA OF THE GALACTIC RIDGE X-RAYS OBSERVED WITH ASCA

X-ray spectra of the Galactic ridge emission in the Scutum arm region have been obtained with ASCA GIS and SIS in the energy range 0.7-10 keV. The observed spectra are basically of thermal emission from thin hot plasmas, and individual K emission lines from helium-like Mg, Si, S, and Fe ions are confirmed in both the GIS and SIS spectra. This means that the Galactic ridge X-ray emission cannot be explained by a single-temperature ionization-equilibrium plasma model. It cannot, however, be reproduced even if a nonequilibrium ionization model is introduced; thus multiple plasma components are required. The GIS spectra are fairly well fitted by a double-temperature nonequilibrium ionization plasma model with temperatures of kT ~ 0.8 keV and kT ~ 7 keV. The softer component is found to be in an extremely low ionization state, with net ~ 109 cm-3 s, while the harder component is in a relatively high ionization state, though not yet in a full equilibrium. The GRXE properties obtained with the GIS are carefully reexamined by the highly resolved spectral-line features with the SIS. The soft and hard components are absorbed by equivalent hydrogen columns of 0.7 × 1022 cm-2 and 4.6 × 1022 cm-2, respectively. The surface brightness of the soft and hard components at b ~ 0° are estimated to be 1.9 × 10-6 and 5.3 × 10-7 ergs cm-2 s-1 sr-1 respectively, both in the 0.5-10 keV band. The surface brightness of the softer component extends toward significantly higher (~2°) Galactic latitudes than the harder component, although their actual scale heights may be similar at ~100 pc if the differences in their observable depths are taken into account. Spectral properties of the two components are seen to depend on the latitude; the most noticeable effect is a rapid decrease in the Fe K line equivalent width seen in the hard component. Attempts are made to interpret the two components in terms of diffuse hot plasmas filling the interstellar space.

Evidence of Energy Nonequipartition between Particles and Fields in Lobes of the Radio Galaxy PKS 1343?601 (Centaurus B)

We observed the radio galaxy PKS 1343-601 (Centaurus B) in X-rays with ASCA and discovered diffuse hard X-ray emission from its radio lobe region. The obtained flux and spectrum strongly suggest that the X-rays are produced via an inverse-Compton (IC) process, in which the cosmic microwave background photons are boosted up to X-rays by synchrotron electrons. This is the second case, following the case of Fornax A, of the detection of the IC X-rays from radio lobes. The observed X-ray emission is brighter than expected in the case of minimum energy density, indicating particle-dominated lobes. Comparison of the X-ray and radio images reveals an outward increase in the magnetic energy density. This infers a gradual compression of magnetic fields toward the edge of the lobes.

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.

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.

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.

Transient response of stressed Ge:Ga detector for ASTRO-F

Abstract Two types of gallium-doped germanium detector arrays, stressed and unstressed ones, are onboard ASTRO-F, the next Japanese infrared astronomical satellite. Prior to measurements of such detector arrays, we investigated the transient properties of a single-element stressed gallium-doped germanium detector. In order to obtain calibration data for ASTRO-F, our measurements cover wide ranges of signal and background photon flux levels. The current status of our experiment is described in this paper.

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.

Fourier synthesis image reconstruction by use of one-dimensional position-sensitive detectors

An improvement of Fourier synthesis optics for hard x-ray imaging is described, and the basic performance of the new optics is confirmed through numerical simulations. The original concept of the Fourier synthesis imager utilizes nonposition-sensitive hard x-ray detectors coupled to individual bigrid modulation collimators. The improved concept employs a one-dimensional position-sensitive detector (such as a CdTe strip detector) instead of the second grid layer of each bigrid modulation collimator. This improves the imaging performance in several respects over the original design. One performance improvement is a two-fold increase in the average transmission, from 1/4 to 1/2. The second merit is that both the sine and cosine components can be derived from a single grid-detector module, and hence the number of imaging modules can be halved. Furthermore, it provides information along the depth direction simultaneously. This in turn enables a three-dimensional imaging hard x-ray microscope for medical diagnostics, incorporating radioactive tracers. A conceptual design of such a microscope is presented, designed to provide a field of view of 4 mm and a spatial resolution of 400 microm.

Development of the large-area silicon PIN diode with 2 millimeter-thick depletion layer for hard x-ray detector (HXD) on board ASTRO-E

ASTRO-E is the next Japanese x-ray satellite to be launched in the year 2000. It carries three high-energy astrophysical experiments, including the hard x-ray detector (HXD) which is unique in covering the wide energy band from 10 keV to 700 keV with an extremely low background. The HXD is a compound-eye detector, employing 16 GSO/BGO well-type phoswich scintillation counters together with 64 silicon PIN detectors. The scintillation counters cover an energy range of 40 - 700 keV, while the PIN diodes fill the intermediate energy range from 10 keV to 70 keV with an energy resolution about 3 keV. In this paper, we report on the developments of the large area, thick silicon PIN diodes. In order to achieve a high quantum efficiency up to 70 keV with a high energy resolution, we utilize a double stack of silicon PIN diodes, each 20 by 20 mm2 in size and 2 mm thick. Signals from the two diodes are summed into a single output. Four of these stacks (or eight diodes) are placed inside the deep BGO active-shield well of a phoswich counter, to achieve an extremely low background environment. Thus, the HXD utilizes 64 stacked silicon PIN detectors, achieving a total geometrical collecting area of 256 cm2. We have developed the 2 mm thick silicon PIN diodes which have low leakage current, a low capacitance, and a high breakdown voltage to meet the requirements of our goal. Through various trials in fabricating PIN diodes with different structures, we have found optimal design parameters, such as mask design of the surface p+ layer and the implantation process.

ASCA measurements of field-particle energy distribution in radio lobes

Abstract The ASCA detections of inverse-Compton (IC) X-rays from lobes of radio galaxies are reported. Significant IC X-rays are observed from radio lobes of Fornax A (Kaneda et al. 1995; Feigelson et al. 1995), Centaurus B (PKS 1343-601) and NGC 612. The energy density and magnetic field intensity in the lobes are sorted out by comparing the IC X-ray flux with the synchrotron radio flux. The results from Centaurus B strongly suggest particle domination in the lobes; meanwhile, X-ray and radio image analysis shows an outward increase in the magnetic energy density. The X-ray image and spectrum observed from NGC 612 reveal a heavily absorbed low luminosity core (nucleus) emission, together with an extended emission associated with the lobes. The extended source luminosity is rather close to the expected value assuming the equipartition state, although the possibility of particle dominant lobes cannot be excluded.