Naoki Isobe

A Chandra detection of diffuse hard X-ray emission associated with the lobes of the radio galaxy 3C 452

An 80 ks Chandra ACIS observation of the radio galaxy 3C 452 is reported. A diffuse X-ray emission associated with the lobes has been detected with high statistical significance, together with the X-ray nucleus of the host galaxy. The 0.5-5 keV ACIS spectrum of the diffuse emission is described by a two-component model, consisting of a soft thermal plasma emission from the host galaxy halo and a hard nonthermal power-law component. The hard component is ascribed to the inverse Comptonization of cosmic microwave background photons by the synchrotron-emitting electrons in the lobes, because its spectral energy index, 0.68 ± 0.28, is consistent with the radio synchrotron index, 0.78. These results reveal a significant electron dominance in the lobes. The electrons are inferred to have a relatively uniform distribution, while the magnetic field is compressed toward the lobe periphery.

Detection of excess hard X-ray emission from the group of galaxies HCG 62

We detected an excess of hard X-ray emission at energies above ~4 keV from the group of galaxies HCG 62 using data from the ASCA satellite. The excess emission is spatially extended up to ~10 from the group center and somewhat enhanced toward the north. Its spectrum can be represented by either a power law of photon index 0.8-2.7 or a bremsstrahlung of temperature greater than 6.3 keV. In the 2-10 keV range, the observed hard X-ray flux, (1.0 ± 0.3) × 10-12 ergs cm-2 s-1, implies a luminosity of (8.0 ± 2.0) × 1041 ergs s-1 for a Hubble constant of 50 km s-1 Mpc-1. The emission is thus too luminous to be attributed to X-ray binaries in the member galaxies. We discuss possible origins of the hard X-ray emission.

X-Ray Measurements of the Field and Particle Energy Distributions in the West Lobe of the Radio Galaxy NGC 1316 (Fornax A)

A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax A (NGC 1316) that was based on new ASCA observations made in 1997 for 98 ks and that incorporated the previous observation in 1994 for 39 ks. The 0.7-10 keV spectrum of the emission can be described by a power law with an energy index of 0.74 ± 0.10, which agrees with the synchrotron radio index of 0.9 ± 0.2. Therefore, the X-rays are reconfirmed to arise via the inverse Compton scattering of the cosmic microwave photons, as Kaneda et al. and Feigelson et al. concluded. The surface brightness of the inverse Compton X-rays exhibits a relatively flat distribution over the west lobe, indicative of an approximately spherical emissivity distribution with a radius of ~11 (75 kpc). In contrast, the 1.4 GHz radio image by Ekers et al. exhibits a rim-brightened surface brightness, consistent with a shell-like emissivity distribution whose inner and outer boundaries are 4 and 11, respectively. These morphological differences between radio and X-rays suggest that the relativistic electrons are distributed homogeneously over the lobe volume, whereas the magnetic field is amplified toward the lobe rim region.

Improvement on the light yield of a high-Z inorganic scintillator GSO(Ce)

Abstract Cerium-doped gadolinium silicic dioxide crystal, GSO(Ce), is a high-Z non-hydroscopic scintillator that gives higher light yield than BGO, and can potentially replace NaI(Tl), CsI(Tl) and BGO in many applications. Its production cost, however, has been substantially higher than any of them, while its energy resolution has been worse than that of NaI(Tl) or CsI(Tl). The merit did not overcome these deficiencies except in limited applications. We developed a low background phoswich counter (the well-type phoswich counter) for the Hard X-ray Detector of the Astro-E project based on GSO scintillator. In the developmental work, we have succeeded in improving the light yield of GSO(Ce) by 40–50%. For energies above 500 keV , a large GSO(Ce) crystal (4.5 cm ×4.5φ cm ) now gives energy resolution comparable to or better than the best NaI(Tl) when read out with a phototube. With a small GSO(Ce) crystal (5×5×5 mm 3 ) and a photodiode, an energy resolution comparable to or better than the best CsI(Tl) has been obtained. With this improved performance, we find that the much higher photopeak efficiency and the shorter scintillation decay time of GSO(Ce) offsets its higher cost for many applications. We summarize our past developmental work to decrease radioactive contamination and to increase light yield of GSO(Ce) for astronomical hard X-ray detection. Included also are measurements done after the unsuccessful launch of the Astro-E mission. The work is still continuing for the remake version of Astro-E Hard X-ray Detector.

Preflight performance of the Astro-E hard X-ray detector

The hard x-ray detector (HXD) is one of the three experiments of the Astro-E mission, the fifth Japanese X-ray Satellite devoted to studies of high energy phenomena in the universe in the x-ray to soft gamma-ray region. Prepared for launch at the beginning of 200 via the newly developed M-V launch vehicle of the Institute of Space and Astronomical Science, the Astro-E is to be thrown in to a near-circular orbit of 550 km altitude, with an inclination of 31 degrees. The flight model has been finished assembled this year, and we carried out various tests to verify the performance. We acquired the background spectrum at sea level, and confirmed that our system is operating effectively in reducing the background level. The HXD will observe photons in the energy range of 10-600 keV, and the calculations based on the preflight calibration suggest that the HXD will have the highest sensitivity ever achieved in this energy range. We also verified that our electronic system will maintain its performance against charged particle events expected in orbit.

Fabrication of the ASTRO-E hard-x-ray detector

The Hard X-ray Detector (HXD) is one of the three instruments on the fifth Japanese cosmic X-ray satellite ASTRO-E, scheduled for launch in January 2000. The HXD covers a wide energy range of 10-600 keV, using 16 identical GSO/BGO phoswich-counter modules, of which the low-energy efficiency is greatly improved by adding 2 m-thick silicon PIN diodes. Production of the HXD has been completed and pre-flight calibration is now in progress. The design concept of the HXD sensor, detail of the production process, and a brief summary of the measured performance is reported.

Spatially dependent response of thick and large area p-i-n diode for ASTRO-E hard X-ray detector

The ASTRO-E hard X-ray detector utilizes GSO(Gd/sub 2/SiO/sub 5/:Ce 0.5% mol)-BGO(Bi/sub 4/Ge/sub 3/O/sub 12/) well-type phoswich counters in compound-eye configuration to achieve an extremely low background level of about a few times 10/sup -5/ counts s/sup -1/ cm/sup -2/ keV/sup -1/. The GSO scintillators placed at the bottom of the BGO well observe photons in the energy range 30-600 keV. To cover the lower energy range of 10-60 keV, Si p-i-n diodes of 2 mm in thickness and 21.5/spl times/21.5 mm/sup 2/ in size were newly developed and placed in front of the GSO scintillators. The p-i-n diode exhibits complex spectral responses, including subpeak and low energy tail components. To examine the origin of these components, we measured the spatially resolved response of the p-i-n diode and confirmed that the subpeak and the low energy tail are related to the electrode structures and electric fields in the p-i-n diode, respectively.

Thick and large area PIN diodes for hard X-ray astronomy

Abstract Thick and large area PIN diodes for the hard X-ray astronomy in the 10–60 keV range are developed. To cover this energy range in a room temperature and in a low background environment, Si PIN junction diodes of 2 mm in thickness with 2.5 cm2 in effective area were developed, and will be used in the bottom of the Phoswich Hard X-ray Detector (HXD), on-board the ASTRO-E satellite. Problems related to a high purity Si and a thick depletion layer during our development and performance of the PIN diodes are presented in detail.

Observational response of MAXI onboard ISS

The current status is reported of the development of Monitor of All-sky X-ray Image and the measurement of its observational response. MAXI is a scanning X-ray camera to be attached to the Japanese Experiment Module of the International Space Station in 2008. MAXI is mainly composed of two kinds of instruments, GSC which is sensitive to the 2 - 30 keV photons, and SSC to the 0.5 - 10 keV ones. As an X-ray all-sky monitor, MAXI has an unprecedented sensitivity of 7 mCrab in one orbit scan, and 1 mCrab in one week. Using the engineering mode of the proportional counter and of the collimator for GSC, the observational response of GSC is extensively measured. The acceptable performances are obtained as a whole for both the collimator and the counter. The engineering models of the other part of MAXI are also constructed and the measurement of their performance is ongoing.

ASCA Observations of the BL Lacertae Object OJ 287 in 1997 April and November

The X-ray properties of the BL Lacertae object OJ 287, observed with ASCA on 1997 April26 and November17, are reported. The 0.5–10 keV flux was lower than that obtained in previous X-ray observations, and no evidence of intensity variations was found during each observation. The obtained flux densities at 1 keV, 0.22–0.26 µJy, exceed the extrapolations from lower frequency synchrotron continua, which were measured in nearly the same period as the present ASCA observations. The X-ray spectra acquired with the GIS and SIS were consistently described with a single power-law model modified by the Galactic absorption, and the derived photon indices, 1.5–1.6, are flatter than those observed so far. These results strongly suggest that the X-ray spectra observed in 1997 arise via an inverse Compton process alone. The X-ray spectra obtained in 1994 (Idesawa et al. 1997, AAA 68.159.334), exhibiting a steeper slope than those in 1997, is thought to be contaminated by a “synchrotron soft tail.”