海老沢 研

The Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and the Galactic superluminal jet sources share the common spectral characteristic that they have extremely high disk temperatures which cannot be explained in the framework of the standard accretion disk model in the Schwarzschild metric. We examine several possibilities to solve this "too-hot" disk problem. In particular, we have calculated an extreme Kerr disk model to fit the observed spectra. We found that the Kerr disk will become significantly harder compared to the Schwarzschild disk only when the disk is highly inclined. For super-luminal jet sources, which are known to be inclined systems, the Kerr disk model may work if we choose proper values for the black hole angular momentum. For the ULXs, however, the Kerr disk interpretation will be problematic, as it is highly unlikely that their accretion disks are preferentially inclined.

We have carried out a Chandra ACIS-I observation on a typical Galactic plane where no previous X-ray sources have been known. We have discovered more than 50 unidentified sources in the 3-8 keV band above the flux level similar to 3 x 10(-15) ergs/s/cm(2) at the 3 sigma confidence level. The total point source flux accounts for only 10 % of the total X-ray flux in the field of view. The observed source number density shows little excess over the extragalactic log N - log S curve determined from high Galactic latitude regions. The present result indicates that significant parts of the detected sources have extragalactic origin. The Galactic Ridge hard X-ray Emission has truly diffuse origin, and there are few Galactic X-ray sources as dim as similar to 3 x 10(-15) ergs/s/cm(2) (2 - 10 keV).

The current understanding of Galactic Ridge X-ray Emission (GRXE) is summarized on the basis of observational results from ASCA and the other new missions. GRXE possibly originates from a truly diffuse interstellar plasma, not a collection of discrete sources.

Analysis was made of the multiple RXTEIPCA data on the black hole candidate with superluminal jet, GRO J1655 - 40, acquired during its 1996-1997 outburst. The source spectra can be adequately described by the sum of a multi-color disk spectrum (MCD) and a power-law. When the power-law luminosity is high, the inner disk radius and the maximum color temperature derived from the MCD model vary significantly with time. In this strong power-law state, the MCD luminosity decreases with temperature, in contradiction to the prediction of the standard Shakura-Sunyaev model. In the same state, the intensity of the power-law component correlates negatively with that of the MCD component, and positively with the power-law photon index, suggesting that the strong power-law is simply the missing MCD emission. One possible explanation for this behavior is inverse-Compton scattering in the disk. By re-fitting the same data incorporating a disk Comptonization, the inner radius and temperature of the underlying disk are found to become more constant.

The nearly final in-orbit calibration of the GIS, installed on the focal plane of the X-ray astronomy satellite ASCA, are summarized. With the successive in-orbit calibration with celestial sources and comparison with the results from the SIS, the GIS team and ASCA-GOF performed fine recalibration resulting released four and a newly presented final versions energy response matrices in collaboration with the XRT, SIS and ASCA operation teams.

We summarize the ASCA observations of the two Galactic superluminal sources GRO J1655-40 and GRS 1915+105. focusing on the absorption line features. The high spectroscopic capability of ASCA enabled us to detect iron-K absorption line features from both objects. This is direct evidence for the presence of highly ionized plasma in a non-spherical configuration around a black hole, and is considered to be a unique signature of superluminal jet systems. The candidate of the line-absorber would be a part of an accretion disk at a distance of 10(9-11) cm from the central X-ray source.

The bulge X-ray emission has been systematically studied with the ASCA and RXTE satellites. After removing the contribution from point sources, the presence of diffuse bulge X-ray emission consisting three distinct emission components, two thermal (cooler and hotter) and one non-thermal, has been established. The temperatures of the thermal components are essentially constant (0.6 keV and 3 keV) all over the bulge region, and surface brightness distributions of the hotter and non-thermal components are strongly correlated with each other. From the estimated scale of the hard X-ray bulge, total X-ray luminosity of the bulge emission is estimated to be similar to 10(38) erg s(-1), which is almost comparable to that of the well-known GRXE.

Energy spectra of four luminous supersoft X-ray sources (SSS) are investigated combining the ASCA SIS and the ROSAT PSPC data. We found that the energy spectra of the SSS are commonly reproduced by a blackbody continuum model superposed with sharp edge-like structures. Despite of several years intervals between the ASCA, and ROSAT observations, differences of the best-fit parameters are found to be small, This indicates that X-ray emissions from these foils SSS are rather stable. The blackbody temperature and the edge energies show large differences among the sources, which may reflect differences in the white dwarf mass. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

K-line absorption features of highly ionized iron were discovered in X-ray spectra of super-luminal transient jets GRS 1915+105 and GRO J1655-40 with ASCA. The absorption lines were identified as Fe XXV K alpha, Fe XXVI K alpha and Ni XXVII K alpha resonance lines with equivalent width of 20-50 eV. They are evidence of highly ionized plasma in anisotropic configuration or in anisotropic illumination in the vicinity of the source. We adopted models of photoionized plasma with a cylindrical geometry, and estimated physical parameters such as density, dimension, solid angle from the source, temperature and xi-parameter. It is suggested that the kinetic temperature of resonant scattering atoms is very high, at least 100 keV. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

K-line absorption features of highly ionized iron were discovered in X-ray spectra of super-luminal transient jets GRS 1915 + 105 (Ebisawa 1997; Kotani et al. 1997) and GRO J1655-40 (Ueda et al. 1998) with ASCA. The absorption features were identified as Fe XXV K alpha, Fe XXVI K alpha, and Ni XXVII K alpha resonance lines with equivalent widths of 20-50 eV. Such absorption lines have never been found in any other X-ray binary, and considered to be a peculiar characteristic of transient jets. They are evidence of highly ionized plasma in anisotropic configuration or in anisotropic illumination in the vicinity of the source. We adopted models of torus and disk-like plasma, and estimated physical parameters such as density, dimension, solid angle from the source, temperature and xi-parameter. The nature of the plasma and its implication is discussed.

We have investigated the energy spectrum and the orbital modulation of the supersoft X-ray source CAL 87 using the ASCA satellite. We have detected a deep absorption edge at 0.85(-0.01)(+0.02) keV superposed on a continuum that can be approximated with an absorbed blackbody (kT = 0.05(-0.01)(+0.06) keV). The edge structure may be produced by the highly ionized species of O VIII and O VII. Such deep edge structures from highly ionized heavy elements are expected for X-ray emission from a white dwarf with an envelope of stable hydrogen burning and strongly supports the model that a white dwarf with a large mass accretion rate is involved in the supersoft X-ray sources. Epoch-folding analysis reveals the presence of a partial X-ray eclipse with broad wings at orbital phase 0; the eclipse covers a quarter of the orbital phase and the X-ray flux decreases down to similar to 40% of the out-of-eclipse level. We discuss the possible cause of the partial eclipse, including the extended emission region due to the accretion disk corona, and the X-ray scattering site formed by the gas escaping from the binary system, where the gas is highly ionized by the large X-ray luminosity from the white dwarf.

We report results from simultaneous ASCA and RXTE observations of Cygnus X-1 when the source made a rare transition from the hard (=low) state to the soft (=high) state in 1996. These observations together cover a broad energy range similar to 0.7-50 keV with a moderate energy resolution at the iron K band and thus make it possible to disentangle various spectral components. The low-energy spectrum is dominated by an ultrasoft component, which is likely to be the emission from the hottest inner portion of the accretion disk around the black hole. At high energies, the X-ray spectrum can be described by a Comptonized spectrum with a reflection component. The Compton corona, which upscatters soft "seed photons" to produce the hard X-ray emission, is found to have a y-parameter similar to 0.28. The hard X-ray emission illuminates the accretion disk, and the reemitted photons produce the observed "reflection bump." We show that the reflecting medium subtends only a small solid angle (similar to 0.15 x 2 pi but has a large ionization parameter such that iron is ionized up to Fe XXIV-Fe XXVI. The presence of a broad iran line at 6.58 +/- 0.04 keV is also consistent with a highly ionized disk, if we take into account the gravitational and Doppler shift of the line energy. These results imply a geometry of a central corona surrounding the black hole and the reflection occurring in the innermost region of the disk where matter is highly ionized.

We report the detection of iron K-absorption lines in the energy spectra of the superluminal jet source GRO J1655-40. We observed the source with ASCA on four occasions from 1994 through 1996. Significant absorption-line features were found at 6.63 +/- 0.07 keV when the X-ray intensity was low (0.27-0.57 crab) and at 6.95 +/- 0.10 keV when the intensity was high (2.2 crab). The former are interpreted as resonance absorption lines from helium-like iron and the latter are those from hydrogen-like iron, respectively. This gives direct evidence for the presence of highly ionized plasma in a nonspherical configuration in the vicinity of the black hole. The change of the ionization stale of iron suggests that the plasma is photoionized by X-ray irradiation. The observed equivalent width (about 60 eV for the helium-like K alpha line) and the ionization parameter xi (similar to 10(3)) constrain the size of the plasma to similar to 10(10) cm. The similarity of these properties to those of the other superluminal source GRS 1915+105 may imply some connection between the presence of such highly ionized plasma and the formation of relativistic jets.

The X-ray/radio transient superluminal jet source GRO J1655-40 was recently suggested to contain a black hole from optical observations. Because it is a relatively close-by system (d similar to 3.2 kpc), it can likely provide us with rich information about the physics operating in both Galactic and extragalactic jet sources. We present the first simultaneous broadband high-energy observations of GRO J1655-40 during the 1995 July-August outburst by three instruments: ASCA, WATCH/Granat, and BATSE/CGRO, in the energy band from 1 keV to 2 MeV. Our observations strengthen the interpretation that GRO J1655-40 contains a black hole. We detected a two-component energy spectrum, commonly seen from other Galactic black hole binaries, but never detected from a neutron star system. Combining our results with the mass limits derived from optical radial velocity and orbital period measurements, we further constrain the mass of the central object to be between 3.3 and 5.8 M., above the well-established mass upper limit of 3.2 M. for a neutron star (the optical mass function for GRO J1655-40 is 3.16 +/- 0.2 M.). This system is therefore the first Galactic superluminal jet source for which there is strong evidence that the system contains a stellar mass black hole. The inclination angle of the binary system is constrained to be between 76 degrees and 87 degrees, consistent with estimates obtained from optical light curves and radio jet kinematics.

Recent studies with ASCA have shown very complicated, strange iron K features in the spectra or galactic jet systems. SS 433, the "classic" jet, was found to have pairs of Doppler-shifted lines, contrary to the previous belief that the receding X-ray jet is short and hidden behind the accretion disk, The transient jets, GRS 1915+105 and GRO J1655-40, show spectral dips, which have never been observed in ally other source and are interpreted as absorption lines or Doppler-shifted absorption edges. K they are resonant absorption lines of helium-like iron, they would be the evidence of highly ionized, anisotropically distributed plasma near the jet engine. These features peculiar to galactic jet systems are expected to be explained in terms of the nature of the sources and the jet-formation mechanisms. Since ASCA was proved to be an excellent tool for diagnostics of jets, observation campaigns of the jet systems were planned and performed. SS 433 was observed about thirty times in the three years of Che campaign, covering the phase space of the 162.5-day precession and the 13-day orbital motion. The extracted physics of the system, such as X-ray-jet length ten times longer than previous estimations, jet kinetic luminosity exceeding 10(40) erg s(-1), etc., draw a highly energetic and stormy, new picture of SS 433. The transient jets, GRS 1915+105 and GRO J1655-40, were also observed several times. GRS I915+105 was found to be active: in ASCA band even months after onsets of outburst. Violent variations were not seen. GRO J1655-40 was observed to be transit between high and low states, and the low state is consistent to occultation of a component, We review ASCA Observations of galactic jet systems and present some topics from recent progresses.

We present spectral properties of an accretion disk model, in which a Keplerian accretion disk is flanked by a sub-Keplerian halo component terminating at a standing shock. The post-shock region (which may be considered to be the boundary layer of a black hole) reprocesses the soft photons emitted from the Keplerian accretion disk. We show that switching of states (from hard to soft and vice versa) could be accomplished by a change in the accretion rate of the Keplerian disk component. Our consideration, for the first time, resolves a long-standing problem of identifying the illusive 'Compton cloud' responsible for the switching of states.

The black hole candidate Cyg X-l was observed with the ASCA satellite on multiple occasions in 1993 and 1994 for a total exposure of about 7 days. The energy resolution of the ASCA SIS (Delta E/E approximate to 2% at 6 keV) allows the spectral structure in the iron K band to be studied in greater detail than before. We detected an iron emission line at 6.4 keV in almost all the data with an intrinsic line width of less than or similar to 0.2 keV (1 sigma) and an equivalent width of 10-30 eV. The line parameters show no clear orbital phase dependence. A broad edge feature was observed at E greater than or similar to 7 keV in all the spectra. Both the narrow iron emission-line and broad edge features are consistent with disk reflection from the outer parts of an ionized accretion disk where the most populous iron ion species is Fe XI.

We investigate the properties of the 8.7 s X-ray pulsar 4U 0142+61 using new data obtained with the RSCA observatory and archival data from the Einstein and ROSAT observatories. New measurements of the pulse period from 1979 and 1994 confirm that 4U 0142+61 is spinning down on a timescale of 127,000 yr. The RSCA spectrum is featureless and requires two components consisting of a similar to 0.4 keV blackbody plus a power law with a photon index of similar to 3.7. The blackbody flux is similar to 40% the total and for a distance greater than 0.5 kpc covers more than 12% of the neutron star surface. This covering fraction is 2 orders of magnitude larger than expected for thin disk accretion onto a magnetized neutron star. These results suggest 4U 0142+61 is probably not a low-mass X-ray binary system, but rather is an isolated pulsar undergoing a combination of spherical and disk accretion. The observed properties seem consistent with the suggestion by van Paradijs, Taam, & van den Heuvel that this pulsar is powered by accretion from the remnant of a Thorne-Zytkow object (TZO). The ROSAT PSPC image shows a dust-scattering halo that is a factor of 2 less than predicted by the measured equivalent hydrogen column density of 8 x 10(21) cm(-2), suggesting half of the absorbing material is located in the vicinity of the pulsar and possibly the remains of the TZO envelope.

The X-ray binary pulsar Cen X-3 was observed with the ASCA satellite over an entire binary eclipse. With unprecedented energy resolution at the iron K-energy band (FWHM ∼ 2%), three iron emission lines at 6.4 keV, 6.7 keV, and 6.97 keV were clearly resolved. At lower energies, lines from hydrogenic ions of Ne, Mg, Si, and S were also observed. Spectral variations over the eclipse were studied, in particular variations of the iron emission lines. Although the intensity of the 6.4 keV line, which is due to fluorescence from low ionized iron, decreased about one order of magnitude during the eclipse, the intensity drop of the 6.7 and 6.97 keV lines, which are respectively from FeXXV and FeXXVI, was at most a factor of three. This, confirming the conjecture by Nagase et al. (1992, AAA56.117.175), indicates that the 6.4 keV fluorescent line is emitted from the region close to the neutron star while the highly photoionized plasma which emits 6.7 and 6.97 keV lines is more extended than the size of the companion star (∼ 12 R(Fish eye sign)). The 6.4 keV line equivalent-width did not show a large variation over the eclipse, and was 75-145 eV. The dense Alfvén shell or an optically thick accretion disk is a likely candidate for reprocessing the fluorescent iron line. From the intensity ratio of the 6.7 and 6.97 keV lines during the eclipse, the ionization parameter of the photoionized plasma is estimated as ξ ∼ 103-4. The size and the density of the plasma are estimated as ∼ 16 R• and ∼ 1.3 x 1011 cm-3, respectively. It is suggested that a lower ionization state is dominated in the pre-eclipse phase, since the 6.7 and 6.97 keV lines were not clearly seen and other K-lines from lighter elements were strong.

The energy spectra of the black-hole candidate GX 339-4 in the low-intensity state were observe on four occasions through 1989 to 1991 with the Large Area Counter on board the Ginga satellite. The spectra showed significant deviations from a power-law, with an iron K(alpha) emission line at approximately 6.4 keV and a broad iron K-edge structure above approximately 7 keV. The energy spectra above approximately 4 keV were successfully explained with a reflection model, in which part of the incident X-rays with a power-law spectrum is Compton reflected by optically thick matter, resulting in a harder continuum component with iron K-edge absorption and an iron fluorescent line. The line equivalent width with respect to the reflection component decreases as the source flux increases. This is consistent with an increase in the ionization state of the material, so that resonant absorption followed by Auger ionization depletes the line. The photon-index of the power-law component was clearly variable, and it correlated with the relative amount of the reflection component. Such a correlation may be explained in the context of the anisotropic Comptonization models of Haardt et al. (1993 ApJL 411 L95), or by a variation of the relative geometry of the source and disk.

An ASCA observation of one complete 2-day orbit of the eclipsing RS CVn binary AR Lac is reported. The time averaged X-ray spectrum reveals an emission line from Fe K at 6.7 keV, lines at lower energies from the Fe L complex, and K lines of Si, S, and other medium-Z elements. The previously established two-temperature thermal equilibrium emission model was fit to the spectra, but gives a poor fit. An improved fit is obtained if the abundance of O, Mg, Si, S, Ar, Ca, and Fe are 2-4 times less than the solar photospheric value. The underabundance of these elements lends support to the idea that weaker than expected absorption lines in the optical spectra of the secondary stars in RS CVn binaries are caused by these stars being metal poor. The overall orbital lightcurve is similar to that seen previously with a 50% reduction in flux centered on the primary eclipse and energy independent across the 0.4-7 keV band. At energies >2 keV the lightcurve shows continuous low level flaring activity on a timescale of 20-60 min.

A bright X-ray nova, GS 1124-68 (Nova Muscae 1991, GRS 1124-68), which later turned out to be a promising black-hole binary with evidence of large mass, was observed with the Large Area Counters onboard Ginga from its discovery in 1991 January to its quiescent state in 1991 September. The X-ray intensity reached a maximum (similar to 8 Crab) eight days after its discovery, then decreased exponentially with an e-folding time of about similar to 30 d. Temporary increases in the luminosity were found similar to 80 d and similar to 200 d after the outburst. The source was last detected at similar to 2 mCrab on the 239th day after the outburst; although subsequent observations were carried out on days 260 and 282, the source was below the detection limit (similar to 0.3 mCrab). GS 1124-68 exhibited such a drastic state transition that was very reminiscent of the high-low transition of black-hole candidates Cyg X-l and GX 339-4, between the 131st day (May 18) and 157th day (June 13) after the outburst. Before the transition, the energy spectrum was characterized by a thermal soft component which was dominant below similar to 8 keV, and a power-law like hard-tail component, which was significant above that energy. After the transition, the energy spectrum became much harder. The spectrum was approximately represented by a single power-law function with a photon index of similar to 1.7. Intense short time variations, which were quite similar to those in the low state of Cyg X-l, GX 339-4 and GS 2023+338, became prominent down to a timescale of milliseconds. The outburst mechanism, the origin of the soft component and the state transition can be favorably explained in terms of an optically thick accretion-disk model around a similar to 6M. black-hole at a distance of similar to 2.5 kpc.

The X-ray source X1957 + 11 has an unusually soft X-ray spectrum, very similar to the X-ray spectra of Galactic black hole candidates. We report on a recent Ginga observation of X1957 + 11, the first X-ray observation since HEAO-1, and find significant spectral variability, on time-scales of hours, in the sense that the spectrum becomes harder as the luminosity increases. We model the spectrum with a two-component model: a soft component associated with the disc emission and a hard component which is best described by a power law. The spectral variability is predominantly due to changes in the hard component while the soft component remains relatively stable. Recent results suggest that Galactic black hole candidates might be distinguished from X-ray binary systems in which the compact object is a neutron star by their X-ray spectral signatures alone. We discuss the extent to which this may be true and compare the results of our spectral analysis for X1957 + 11 with those for other low-mass X-ray binary systems (LMXBs) and black hole candidates, and argue that the compact object in X1957 + 11 is likely to be a neutron star.

The low-mass X-ray binary X1608-522 was observed with Ginga on three occasions from 1989 to 1991. The source was in the low state throughout the observations. The energy spectrum in the energy range of 2-60 keV can be approximated by a power-law function with a broad absorption-like structure. The structure can be represented either by partial absorption or by a reflection of the power-law spectrum. The ionization degree of the absorbing/reflecting medium increases when the source luminosity increases by a factor of about three, while the percentage of the absorbed/reflected component relative to the total intensity decreases. The power spectrum is characterized by a flat top below a certain frequency, while it decreases approximately as 1/f above that frequency. The turn-over frequency varies according to the source luminosity, and is in a range 0.2 to 1 Hz. Additional structures, which can be represented by a broad Lorentzian peak, also exist in the power spectrum. The physical state of the accretion disk is discussed in conjunction with the low state of black-hole candidates.

We present the results of X-ray scan observations of X-ray novae during quiescence, performed with the y Large-Area Proportional Counters (LAC) onboard Ginga. The scans of GS 2000+25 were made 155d before the peak of the 1988 outburst and 382d after the peak. The derived upper limits to the X-ray luminosity in the energy range between 1.2 and 37.1 keV are (3-9)x10(33) erg s-1 for a distance of D almost-equal-to 2 kpc. For such X-ray luminosities, the mass-overflow instability of the companion star is difficult to trigger. GS 2023+338 was X-ray scanned 320d before the peak of the 1989 outburst and 442-443d after the peak. LAC found an X-ray source at a distance of approximately 0.1 arc degree from the position of GS 2023+338 after the eruption. The possibility that this source is GS 2023+338, itself cannot be discarded if its luminosity is highly variable. If this is the case, the quiescent X-ray luminosity of GS 2023+338 is of order 10(34) erg s-1 for a distance of D almost-equal-to 3 kpc, and may be sufficient to trigger a mass-overflow instability. The disk instability model is, on the other hand, viable for both stars.

We detected an X-ray outburst in the Vela-Puppis region with the All Sky Monitor (ASM) aboard Ginga on 1990 November 22. Following observations by the Large Area proportional Counter (LAC) revealed that this outburst consisted of two transient sources, GS0834-430 and GS0836-429, accidentally in the same field of view of the LAC by a separation of only 0.4 degree. These two transient X-ray sources were subsequentially monitored with the LAC until 1991 May. GS 0834-430 was identified by an X-ray pulsar with a period of 12.s3. This pulsar shows a power-law energy spectrum with a photon index of approximately 0.9, and with a high-energy cutoff that is typical of X-ray pulsars. GS 0836-429 was identified by an X-ray burster, which shows typical type-I bursts approximately every 2 h.

GX 339-4 was observed with the large area counters (LAC) onboard Ginga in its very high state, where the X-ray intensity was about a factor of 2-3 larger than its high state and it showed very rapid variations on time scales of less than several minutes, which had not been observed earlier in the high state of this source. The X-ray energy spectrum was very soft; it consisted of a low-energy component and a high-energy tail. The low-energy component could be interpreted as being due to disk blackbody radiation (the disk blackbody component) and the high-energy tail as being due to Compton-scattering radiation (the Comptonized blackbody component). The X-ray energy spectrum also showed K-edge absorption of highly ionized iron atoms of about 10(19) cm-2 and an iron emission line with an equivalent width of about 60-100 eV. On short time scales of less than several minutes, the X-rays showed rapid time variations. For instance, in power spectral density functions, 6 Hz quasi-periodic oscillations (QPOs), very low frequency noise (VLF noise), and low-frequency noise (LF noise) were recognized. There are three types of power spectral density functions. Time variations such as dips and flip-flops were also observed. These rapid time variations are due to the Comptonized blackbody component. On long time scales larger than hours, the disk blackbody component and the Comptonized blackbody component changed independently. However, these changes took place within some restricted regions in an X-ray hardness ratio (color)-intensity or a color-color diagrams. These are a hardness ratio increasing (with the X-ray flux) branch, a hardness ratio decreasing branch, and their crossing region. These two energy spectral branches and their crossing region have three different types of power density spectra: these different branches and regions have different time variations on time scales of less than several minutes. In the hardness ratio increasing branch, the hard Compton-scattering component is the main cause of the long-term time variation of the X-ray flux, and in the hardness ratio decreasing branch, the disk blackbody component is the main cause of the long-term time variation of the X-ray flux. On a time lag versus Fourier period diagram, the time lag between time variations of different energy X-rays showed shoulder structures in addition to the large time lags at large Fourier periods similar to those observed in Cygnus X-1. The time variations of the X-rays of 2.3-4.6 keV were most advanced: time variations of X-rays with both lower and higher energy than 2.3-4.6 keV showed time lags relative to those of 2.3-4.6 keV X-rays. These facts together with the X-ray energy spectrum can be explained in terms of a high-energy component being due to a Compton-scattering cloud of size of about 10(9) cm, temperature kT(e) of about 30 keV, and Thomson scattering depth of about 0.5-1.0; the variable blackbody radiation with kT = approximately 1 keV is incident to the large Compton-scattering cloud.

The highly variable X-ray luminosity of LMC X-3 is found to be strongly modulated with a period of ∼198 (or possibly ∼99) days. Observations from both Ginga and HEAO 1 satellites show this periodic variation. For energies &lt 13 keV, the X-ray intensity and hardness are positively correlated for higher energies, there appears to be no correlation. Available optical photometry indicates the mean V brightness also varies by &gt 1 mag with this same long-term period. The regularity of this "clock" in LMC X-3 suggests that it may be related to an accretion disk precession, similar to that seen in LMC X-4, Her X-1, SS 433, or possibly periodic variations in the mass transfer rate.

We compare the persistent X-ray spectra of black hole candidates and X-ray bursters 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. First we compare this general relativistic model 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. Through the medium of these conventional models, we perform spectral fitting analysis on the black hole candidates LMC X-1 nad LMC X-3 and on the X-ray bursters X1608-522 and X1636-536 using the general relativistic model. As a result, we obtain the lower limit of the masses of LMC X-1 and LMC X-3 and the probable masses of X1608-522 and X1636-533. The former masses are larger than the theoretical upper limit of the neutron star mass, 2.5 M., and the latter masses are consistent with the canonical neutron star mass, 1.4 M..

The black hole candidate LMC X-3 was observed simultaneously at X-ray (Ginga), UV (I U E), and optical frequencies in 1988 January. The X-ray energy distribution is the superposition of a thermal component and a hard tail. The former component can be described equally well by a Comptonization spectrum or by a disk blackbody. In the latter picture, the high-energy tail may derive from the inner part of the accretion disk, which is hot and transparent. Optical and UV data indicate a very low state of the source. Comparing with 1984 observations, one finds that the optical - to - X-ray flux ratio is not maintained, possibly indicating a secular variation of the disk structure, or a change of the anisotropy of the X-ray emission.