Kazuhisa Mitsuda

The nature of ultraluminous compact X-ray sources in nearby spiral galaxies

Studies were made of ASCA spectra of seven ultraluminous compact X-ray sources in nearby spiral galaxies: M33 X-8, M81 X-6, IC 342 source 1, Dwingeloo 1 X-1, NGC 1313 source B, and two sources in NGC 4565. With the 0.5-10 keV luminosities in the range 1039-1040 ergs s-1, they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all seven sources have been described successfully with so-called multicolor disk blackbody emission arising from optically thick standard accretion disks around black holes. Except for the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ~100 M☉. However, the observed innermost disk temperatures of these objects, Tin = 1.1-1.8 keV, are too high to be compatible with the required high black hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole and hence hotter.

Searches for millisecond pulsations on low-mass x-ray binaries

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Spectral variations of LMC X-3 observed with Ginga

The prime black hole candidate LMC X-3 was observed over three years with the Ginga satellite, and a characteristic spectral variation was found accompanying the periodic intensity variation of about 198 (or possibly about 99) days (Cowley et al., 1991). The energy spectrum of LMC X-3 consists of the soft, thermal component and the hard, power-law component, which are respectively dominant below and above about 9 keV. The soft component, which carries most of the X-ray intensity, shows a clear correlation between the intensity and the hardness, while the hard component varies independently of the soft component. It was found that the spectral variation of the soft component is well described by an optically thick accretion disk model with a remarkably constant innermost radius and variable mass accretion rate. The constancy of the innermost radius suggests it is related to the mass of the central object.

IRON K EMISSION LINES IN THE ENERGY SPECTRA OF LOW-MASS X-RAY BINARIES OBSERVED WITH ASCA

ASCA archive data of 20 low-mass X-ray binaries (LMXBs) were analyzed to study the iron K emission lines. The LMXBs analyzed here include Z sources, atoll sources, dippers, bursters, and ADC (accretion disk corona) sources. We paid particular attention to reducing systematic errors for the estimation of line parameters, and the combination of SIS and GIS, which have different energy resolutions, was found to be very useful for this purpose. We detected significant iron lines from about one-half of the sources. The line center was 6.56 keV on average, and a finite width of ~0.5 keV (FWHM) was obtained from six sources; these properties may be common to LMXBs. On the other hand, equivalent width (EW) of the lines showed large scatter among the sources, from less than 10 to 170 eV. These line parameters do not show any correlation with the source categories or luminosities. The iron K lines from LMXBs are likely produced through the radiative recombination of photoionized plasma. We discuss the origin of the line width, which may result from the combination of the line blending, Doppler broadening, and Compton scattering.

A uniform X-ray analysis of 79 distant galaxy clusters with ROSAT and ASCA

We present a uniform analysis of the ROSAT HRI and the ASCA GIS/SIS data for 79 distant clusters of galaxies in the redshift range 0.1 < z < 0.82 to study the global structures of the intracluster medium. We have constructed an X-ray catalog consisting of the largest sample of clusters in the redshift range for which pointed X-ray observations were carried out with both the observatories. We determined the emission-weighted X-ray temperatures of the clusters with ASCA, while we studied surface brightness distribution with the ROSAT HRI utilizing the isothermal β model. We investigated the statistical properties and trends for redshift evolution of the X-ray parameters including the temperature, the density profile of the intracluster gas and the gas-mass fraction within r500. We also present correlations of the cluster parameters with the X-ray temperature and with the core radius and compare them with the predictions of the self-similar model, from which we discuss the possible origin of the double structure discovered in the core radius distribution.

Application of a general relativistic accretion disk model to LMC X-1, LMC X-3, X1608−522, and X1636−536

The persistent X-ray spectra of black hole candidates and X-ray bursters are compared with a model spectrum of an accretion disk around either a black hole or a neutron star. The model takes account of general relativity and assumes that the accretion disk is geometrically thin and in a steady state. This general relativistic model is compared with other conventional model spectra that have been used to fit observations so far. It is shown that the general relativistic model, in shape, is quite similar to the multicolor disk model and the Sunyaev-Titarchuk Comptonization model. These conventional models are used to perform spectral fitting analysis on the black hole candidates LMC X-1 and LMC X-3 and on the X-ray bursters X1608 - 522 and X1636 - 536 using the general relativistic model. As a result, the lower limit of the masses of LMC X-1 and LMC X-3 and the probable masses of X1608 - 522 and X1636 - 536 are obtained. The former masses are larger than the theoretical upper limit of the neutron star mass, 2.5 solar masses, and the latter masses are consistent with the canonical neutron star mass, 1.4 solar mass. 38 refs.

Radiation damage to charge coupled devices in the space environment

We have investigated the characteristics of radiation damage to charge coupled devices (CCDs) in the space environment. The X-ray astronomy satellite ASCA launched on February 20, 1993 in low Earth orbit carries CCDs specially developed for soft X-ray detection. We have traced the performance of the CCDs for 3 years. We have observed both the gradual decrease of charge transfer efficiency (CTE) and the increase of dark current. These are phenomenologically explained by the increase of charge traps due to irradiation by high energy charged particles. However, some of the effects of the radiation damage in the CCD are quite non-uniform over the chip on various scales. We discuss characteristics of the charge traps and possible origins of the non-uniformity.

Suzaku Observations of the Local and Distant Hot ISM

Suzaku observed the molecular cloud MBM 12 and a blank field less than 3 ◦ away to separate the local and distant components of the diffuse soft X-ray background. Towards MBM 12, a local (D 275pc) O VII emission line was clearly detected with an intensity of 3.5 photons cm −2 s −1 sr −1 (or line units, LU), and the O VIII flux was < 0.34 LU. The origin of this O VII emission could be hot gas in the Local Hot Bubble (LHB), charge exchange within the heliosphere with oxygen ions from the solar wind (SWCX), or both. If entirely from the LHB, the emission could be explained by a region with emission measure of 0.0075cm −6 pc and a temperature of 1.2 ×10 6 K. However, this temperature and emission measure implies 1/4 keV emission in excess of observations. There is no evidence in the X-ray light curve or solar wind data for a significant contribution from geocoronal SWCX, although interplanetary SWCX is still possible. In any case, the observed O VII flux represents an upper limit to both the LHB emission and interplanetary SWCX in this direction. The blank field was observed immediately afterwards. The net off-cloud O VII and O VIII intensities were (respectively) 2.34 ±0.33 and 0.77 ±0.16 LU, after subtracting the on-cloud foreground emission. If this more distant O VII and O VIII emission is from a thermal plasma in collisional equilibrium beyond the Galactic disk, we infer it has a temperature of (2.1 ±0.1) ×10 6 K with an emission measure of (4 ±0.6) ×10 −3 cm −6 pc.

O and Ne K Absorption Edge Structures and Interstellar Abundance toward Cygnus X-2

We have studied the O and Ne absorption features in the X-ray spectrum of Cyg X-2, observed with the Chandra LETG. The O absorption edge is represented by the sum of three absorption edge components within the limit of the energy resolution and the photon counting statistics. Two of them are due to the atomic O; their energies correspond to two distinct spin states of photoionized O atoms. The remaining edge component is considered to represent compound forms of oxide dust grains. Since Cyg X-2 is about 1.4 kpc above the Galactic disk, the H column densities can be determined by radio (21 cm and CO emission line) and Hα observations, with relatively small uncertainties. Thus, the O abundance relative to H can be determined from the absorption edges. We found that the dust scattering can affect the apparent depth of the edge of the compound forms. We determined the amplitude of the effect, which we consider to be the largest possible correction factor. The ratio of column densities of O in atomic to compound forms and the total abundance of O were determined to be in the ranges 1.7 to 2.8 (ratio) and 0.63 ± 0.12 to 0.74 ± 0.14 solar (abundance), taking into account the uncertainties in the dust-scattering correction and in the ionized H column density. We also determined the Ne abundance from the absorption edge to be 0.75 ± 0.20 solar. These abundance values are smaller than the widely used solar values, but consistent with the latest estimates of solar abundance.

ASCA Observation of Cygnus X-1 in the Soft State: Mass of the Compact Object

We describe the ASCA observation of the black hole candidate Cyg X-1 in the soft state of 1996 May and report the mass estimation of the central object from X-ray spectral analysis. The energy spectrum in 0.7-10 keV can be described by the sum of a power law (Γ = 2.3 ± 0.1) and an ultrasoft component; the latter is most probably the emission from an optically thick accretion disk. Through the spectral analysis of the ultrasoft component, we estimated the size of the accretion disk, which is closely related to the Schwarzschild radius and, hence, to the mass of the central object. Taking the general relativistic effects into account, the most probable mass of the compact object is estimated as 12+ 3−1M☉. This value is independent of, yet consistent with, the mass estimate based on the orbital kinematics information.