Kazuo Makishima

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

In-orbit performance of the hard X-ray detector on board Suzaku

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10–600keV, energy resolutions of ∼ 4keV (FWHM) at 40keV and ∼ 11% at 511keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5–20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15–70 and 150–500keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

High-Resolution Observations of the Elliptical Galaxy NGC 4636 with the Reflection Grating Spectrometer on Board XMM-Newton

We present the first high spectral resolution X-ray observation of the giant elliptical galaxy NGC 4636, obtained with the reflection grating spectrometer on board the XMM-Newton Observatory. The resulting spectrum contains a wealth of emission lines from various charge states of oxygen, neon, magnesium, and iron. Examination of the cross-dispersion profiles of several of these lines provides clear, unambiguous evidence of resonance scattering by the highest oscillator strength lines, as well as a weak temperature gradient in the inner regions of the interstellar medium. We invoke a sophisticated new Monte Carlo technique that allows us to properly account for these effects in performing quantitative fits to the spectrum. Our spectral fits are not subject to many of the systematics that have plagued earlier investigations. The derived metal abundances are higher than have usually been inferred from prior, lower spectral resolution observations of this source, but are still incompatible with conventional chemical-enrichment models of elliptical galaxies. In addition, our data are incompatible with standard cooling-flow models for this system; our derived upper limit to the mass deposition rate is below the predicted value by a factor of 3-5.

Detection of high-energy gamma rays from winter thunderclouds

A report is made on a comprehensive observation of a burstlike gamma-ray emission from thunderclouds on the Sea of Japan, during strong thunderstorms on 6 January 2007. The detected emission, lasting for approximately 40 sec, preceded cloud-to-ground lightning discharges. The burst spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung photons originating from relativistic electrons. This ground-based observation provides the first clear evidence that strong electric fields in thunderclouds can continuously accelerate electrons beyond 10 MeV prior to lightning discharges.

Development of the HXD-II wide-band all-sky monitor onboard Astro-E2

The hard X-ray detector (HXD-II) is one of the three scientific instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled to be launched in 2005. This mission is very unique in a point of having a lower background than any other past missions in the 10-600 keV range. In the HXD-II, the large and thick BGO crystals are used as active shields for particle and gamma-ray background to the main detector. They have a wide field of view of ~2pi and a large effective area of 400 cm2 even at 1 MeV. Hence, the BGO shields have been developed as a wide-band all-sky monitor (WAM) with a broadband coverage of 50-5000 keV. In this paper, overall design and performance of the HXD-II/WAM based on the results of preflight calibration tests carried out in June 2004 are described. By irradiating various radio isotopes with the WAM flight model, we verified that it had comparable capabilities with other gamma-ray burst detectors

Suzaku Results on Cygnus X-1 in the Low/Hard State

AbstractThe black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005October, while it was in a low/hard state with a 0.7–300 keV luminosity of 4.6×10 37 erg s −1 . The XIS andHXD spectra, spanning 0.7–400 keV, were reproduced successfully incorporating a cool accretion disk anda hot Comptonizing corona. The corona is characterized by an electron temperature of ∼100 keV, and twooptical depths of ∼0.4 and ∼1.5 which account for the harder and softer continua, respectively. The disk hasthe innermost temperature of ∼0.2 keV, and is though to protrude half way into the corona. The disk notonly provides seed photons to the Compton cloud, but also produces a soft spectral excess, a mild reflectionhump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655−40 revealsseveral interesting spectral differences, which can mostly be attributed to inclination effects assuming thatthe disk has a flat geometry while the corona is grossly spherical. An intensity-sorted spectroscopy indicatesthat the continuum becomes less Comptonized when the source flares up on times scales of 1–200 s, whilethe underlying disk remains unchanged.Key words: accretion disks — black hole physics — stars: individual (Cygnus X-1)— X-ray: binaries1. IntroductionLuminous soft X-ray radiation of accreting stellar-massblack holes (BHs) has generally been explained as thermalemission from optically-thick (in particular “standard”)accretion disks (Shakura & Sunyaev 1973; Makishimaet al. 1986; Dotani et al. 1997; Remillard & McClintock2006), which are expected to form around them underrather high accretion rates. In contrast, their hard X-rayproduction process is much less understood, even thoughintense hard X-ray emission characterizes black-hoe bina-ries (BHBs) among varioustypes of compact X-raysourcesin the Milky Way and Magellanic clouds.Indeed, BHBs often emit a major fraction of their ra-diative luminosity in the hard X-ray band, in the formof spectral hard-tail component if they are in so-calledhigh/soft state, or as the entire power-law (hereafter PL)like continua if they are in so-called low/hard state (here-after LHS) which appears under relatively low accretionrates. Furthermore, the hard X-ray emission (partic-ularly in the LHS) involves another interesting aspect,namely the long-known aperiodic variation over a widefrequency range (e.g., Oda et al. 1971; Oda 1977; Nolanet al. 1981; Miyamoto et al. 1991; Pottschmidt et al.2003; Remillard & McClintock 2006). These spectral andtiming studies are not limited to stellar-mass BHs, since a

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.

Improvements of the astro-E2 hard X-ray detector (HXD-II)

We summarize significant improvements which have been achieved in the development of Astro-E2 Hard X-ray Detector (HXD-II). An expanded energy range and better energy resolution have been achieved from progresses in device materials and redesigning of the front-end electronics. An improved estimation for the detector background in orbit has also been conducted based upon results from our proton irradiation experiment. The sensitivity of HXD-II can be expected to reach an order of 10/sup -6/ [cs/sup -1/ keV/sup -1/ cm/sup -2/].

BROADBAND STUDY WITH SUZAKU OF THE MAGNETAR CLASS

Broad-band (0.8-70 keV) spectra of the persistent X-ray emission from 9 magnetars were obtained with Suzaku, including 3 objects in apparent outburst. The soft X-ray component was detected from all of them, with a typical blackbody temperature of kT ~ 0.5 keV, while the hard-tail component, dominating above ~10 keV, was detected at ~1 mCrab intensity from 7 of them. Therefore, the spectrum composed of a soft emission and a hard-tail component may be considered to be a common property of magnetars, both in their active and quiescent states. Wide-band spectral analyses revealed that the hard-tail component has a 1-60 keV flux, Fh, comparable to or even higher than that carried by the 1-60 keV soft component, Fs. The hardness ratio of these objects, defined as xi=Fh/Fs, was found to be tightly anti-correlated with their characteristic age tau as xi=(3.3+/-0.3)x(tau/1 kyr)^(-0.67+/-0.04) with a correlation coefficient of -0.989, over the range from xi~10 to xi~0.1. Magnetars in outburst states were found to lie on the same correlation as relatively quiescent ones. This hardness ratio is also positively correlated with their surface magnetic fields with a correlation coefficient of 0.873. In addition, the hard-tail component becomes harder towards sources with older characteristic ages, with the photon index changing from ~1.7 to ~0.4.

The ASCA Medium Sensitivity Survey (The GIS Catalog Project): Source Catalog II.

In succession to the first ASCA Medium Sensitivity Survey catalog (AMSS-I), we present the second X-ray source catalog (AMSS-II), constructed from the Gas Imaging Spectrometer data covering Galactic latitudes |b| > 10° observed between 1997 January and 2000 May. The AMSS-II catalog uses 306 fields and contains a total of 1190 sources detected with significance of >5 σ in either the 0.7-7, 2-10, or 0.7-2 keV band. The AMSS-I and AMSS-II catalogs list a total of 2533 sources from an area of 278 deg2 and provides a unique database of X-ray sources in the flux range of 10-13-10-11 ergs cm-2 s-1 (0.7-10 keV). We summarize statistical properties of a complete X-ray sample consisting of 1969 serendipitous selected from AMSS-I and AMSS-II.