海老沢 研

We report on the current status of our astronomical data sonification project. This project aims to sonify astronomical data, that is, to convert a visual medium into an audio medium for both scientists and the visually impaired. We hope that sonification can lead to a new way of conceptualizing scientific data. The primary sourcesof the data used for this project come from Japanese satellites dedicated to X-ray astronomy and geophysics. This project is performed in collaboration with Nihon Fukushi University and the Center for Planning and Information Systems (PLAIN center) of the Japan Aerospace Exploration Agency (JAXA) Institute of Space andAstronautical Science (ISAS). Our project began in March 2006. Since this time we have sonified astronomical data sets, including data from X-ray pulsars, and have published these results. In this paper we also discuss future plans for the project, as well as its implications for visually impaired scientists and public.

X-ray spectra of black hole binaries in the standard high/soft state were studied comprehensively using ASCA GIS data and, partially, RXTE PCA data. A mathematical disk model was applied to several black hole binaries to see whether the observed accretion disk temperature profile was consistent with that expected from the standard accretion disk model. This model is called a p-free disk and assumes that the spectrum is composed of multi-temperature blackbody emission, where the local temperature T(r) at radius r is given by T(r) ∝ r-p, with p being a positive free parameter. The standard disk, where the gravitational energy of the accreted matter is effectively released as blackbody radiation, roughly requires p ≈ 3/4, while a small deviation is expected depending on the inner boundary conditions, general relativistic effects, and disk vertical structures. Our sample objects included LMC X-1, LMC X-3, XTE J2012+381, and GRO J1655-40. During the ASCA observations, these black hole binaries showed characteristics of the standard high/soft state. Under the standard modeling of high-state black hole binaries, the sources show disk inner temperatures of 0.76-1.17 keV, a disk fraction of the total 0.7-10 keV flux of 54%-98%, and absorption columns of (0.7-12) × 1021 cm -2. The best-fit values of p were found in the range 0.6-0.8, and the standard value of p = 3/4 was accepted for all the sources. The obtained values of p are also compared with those expected for the standard accretion disk in the Schwarzschild metric using the so-called GRAD model. The observed p-values were indeed found to be consistent with those expected from the standard accretion disk in the Schwarzschild metric. © 2005. The American Astronomical Society. All rights reserved.

Cal 87 was observed with XMM-Newton in April of 2003. The source shows a rich<br /> emission line spectrum, where lines can be identified if they are red-shifted<br /> by 700-1200 km/s. These lines seem to have been emitted in a wind from the<br /> system. The eclipse is observed to be shifted in phase by 0.03 phi(orb), where<br /> phi(orb) is the phase of the optical light curve.

The imaging instruments oil board INTEGRAL have wide fields of view and high time resolution. Therefore, they are ideal instruments to search for pulsating sources and/or transient events. We are systematically searching for pulsations and transient events from known and serendipitous sources in the Galactic Plane Scan (GPS) and Galactic Center Deep Exposure (GCDE) core program data. We analyze the standard pipe-line data using ISDC Off-line Science Analysis (OSA) system, so that our results are reproducible by general guest users. In this paper, we describe our system and report preliminary results for the. first year of operation.

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a, face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (greater than or similar to 300 M-circle dot). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super-Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and that their X-ray emission is from the slim disk shining at super-Eddington luminosities.

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a, face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (greater than or similar to 300 M-circle dot). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super-Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and that their X-ray emission is from the slim disk shining at super-Eddington luminosities.

We describe the INTEGRAL reference catalog which classifies previously known bright X-ray and gamma-ray sources before the launch of INTEGRAL. These sources are, or have been at least once, brighter than similar to1 mCrab above 3 keV, and are expected to be detected by INTEGRAL. This catalog is being used in the INTEGRAL Quick Look Analysis to discover new sources or significantly variable sources. We compiled several published X-ray and gamma-ray catalogs, and surveyed recent publications for new sources. Consequently, there are 1122 sources in our INTEGRAL reference catalog. In addition to the source positions, we show an approximate spectral model and expected flux for each source, based on which we derive expected INTEGRAL counting rates. Assuming the default instrument performances and at least similar to10(5) s exposure time for any part of the sky, we expect that INTEGRAL will detect at least similar to700 sources below 10 keV and similar to400 sources above 20 keV over the mission life.

We present the Chandra results of AX J1843.8-0352, a supernova remnant (SNR) recently identified with ASCA. Chandra spatially resolved two components from this SNR: one nonthermal and one thermal. The morphology of the nonthermal component is clumpy and elliptical, elongated from the north to the south with a mean diameter of about 9′. The spectrum is fitted with a power-law model of photon index 2.1, and the east rim is associated with the nonthermal radio sources C and F. Therefore, the nonthermal component is probably synchrotron X-rays produced by energetic electrons accelerated at the shell of the SNR. The thermal component is the brightest clump located within the nonthermal component and shows the spectrum of a thin plasma of about 0.7 keV temperature. A notable discovery is its peculiar morphology a head of 50″ 30″ size near the southeast rim of the SNR and a 30″ long "jet" pointing to the southwest. Although this emission is associated with the western part of the radio source F, the absorption is 2 times larger than that of the nonthermal X-rays or the bulk of the SNR emission. Therefore, it is unclear whether this peculiar plasma is a thermal component associated with AX J1843.8-0352, a Galactic source located in the far side of our Galaxy, or an extragalactic source.

We have carried out a deep X-ray observation on a typical Galactic plane<br /> region with the Chandra ACIS-I instrument with unprecedented sensitivity and<br /> spatial resolution, and detected 274 unidentified X-ray point sources in the<br /> \~500 arcmin2 region. In order to identify these new X-ray sources, we have<br /> carried out a near infrared follow-up observation using ESO/NTT infrared camera<br /> on 2002/7/28 and 2002/7/29. Initial results are briefly reported.

We are systematically analyzing ASCA GIS data taken during the satellite<br /> attitude maneuver operation. Our motivation is to search for serendipitous hard<br /> X-ray sources and make the ASCA Slew Survey catalog.<br /> During its operational life from 1993 February to 2000 July, ASCA carried out<br /> more than 2,500 maneuver operations, and total exposure time during the<br /> maneuver was ~415 ksec after data screening. Preliminary results are briefly<br /> reported.

We present the X-ray spectrum of MR Vel/RXJ 0925.7-4758, obtained with the Medium Energy Grating spectrometer of the Chandra X-ray Telescope. The simplest models used by earlier authors, stellar atmospheres in combination with a thermal plasma in collisional ionization equilibrium, cannot explain the spectrum. Neither does a photo-ionized plasma. We identify PCygni profiles of Fe XVII and OVIII, from which we conclude that these lines arise in a wind. We conclude that major uncertainty exists about the bolometric luminosity of MR Vel, and perhaps of supersoft sources in general, so that the theoretical prediction that this luminosity derives from steady nuclear burning cannot be verified.

AX J1843.8-0352 is a new X-ray SNR identified to the radio complex G28.6-0.1<br /> with the ASCA satellite. Chandra discovered two distinct components from this<br /> SNR: non-thermal and thin thermal X-ray emissions. The non-thermal component is<br /> fitted with a power-law spectrum of photon index 2.0. The morphology is<br /> complicated, but roughly an elliptical shape with a mean diameter of about<br /> 7&#039;-10&#039;. The east to south rims of the ellipse are associated with the<br /> non-thermal radio sources C, F and G (Helfand et al. 1989). The power-law slope<br /> of the radio spectrum can be smoothly connected to that of X-rays with a break<br /> at around the optical-IR band, hence would be due to synchrotron X-rays<br /> accelerated probably to &gt; 1 TeV at the shell of the SNR. The thermal component<br /> near the southeast rim is a thin plasma of about 0.8 keV temperature. It has<br /> the appearance of a &quot;Tadpole&quot; figure with a head of 30&quot;x 40&quot;-size and a tail of<br /> 30&quot;-long. Although this emission is associated with the west part of the radio<br /> source F, the absorption is about two times larger than that of the non-thermal<br /> X-rays, the bulk of the SNR emission. Therefore, together with the peculiar<br /> morphology, whether the thermal plasma is a part of the SNR or a background<br /> object is unclear.

An analysis was made of the multiple Rossi X-Ray Timing Explorer/Proportional Counter Array data on the candidate black hole binary system with superluminal jet, GRO J1655-40, acquired during its 1996-1997 outburst. The X-ray spectra can be adequately described by the sum of an optically thick disk spectrum and a power law. When the estimated 1-100 keV power-law luminosity exceeds 1 × 1037 ergs s-1 (assuming a distance of 3.2 kpc), the inner disk radius and the maximum color temperature derived from a simple accretion disk model (a multicolor disk model) vary significantly with time. These results reconfirm the previous report by Sobczak and coworkers. In this strong power-law state (once called the "very high state"), the disk luminosity decreases with temperature, in contradiction to the prediction of the standard Shakura-Sunyaev model. In the same state, the power-law component becomes stronger and steeper (softer), as the disk component decreases in intensity, suggesting that some of the strong power-law emission is simply the missing optically thick disk emission. One possible explanation for this behavior is inverse Compton scattering around the disk. By refitting the same data incorporating a disk Comptonization model, the inner radius and temperature of the underlying disk are found to become more constant. These results provide one of the first observational confirmations of the scenario of disk Comptonization in the strong power-law state. This strong power-law state seems to appear when the color temperature of the disk exceeds a certain threshold, ∼1.2-1.3 keV.

The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17' by 17' field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space.

We present a general relativistic accretion disc model and its application to<br /> the soft-state X-ray spectra of black hole binaries. The model assumes a flat,<br /> optically thick disc around a rotating Kerr black hole. The disc locally<br /> radiates away the dissipated energy as a blackbody. Special and general<br /> relativistic effects influencing photons emitted by the disc are taken into<br /> account. The emerging spectrum, as seen by a distant observer, is parametrized<br /> by the black hole mass and spin, the accretion rate, the disc inclination angle<br /> and the inner disc radius. We fit the ASCA soft state X-ray spectra of LMC X-1<br /> and GRO J1655-40 by this model. We find that having additional limits on the<br /> black hole mass and inclination angle from optical/UV observations, we can<br /> constrain the black hole spin from X-ray data. In LMC X-1 the constrain is<br /> weak, we can only rule out the maximally rotating black hole. In GRO J1655-40<br /> we can limit the spin much better, and we find 0.68 &lt; a &lt; 0.88. Accretion discs<br /> in both sources are radiation pressure dominated. We don&#039;t find Compton<br /> reflection features in the spectra of any of these objects.

The diffuse X-ray background (DXB) emission near the Galactic plane ($l,b<br /> \sim 25.6^{\circ},0.78^{\circ}$) has been observed with $ASCA$. The observed<br /> region is toward a Galactic molecular cloud which was recently reported to cast<br /> a deep X-ray shadow in the 0.5 $-$ 2.0 keV band DXB. The selection of this<br /> particular region is intended to provide a constraint on the spatial<br /> distribution of the DXB emission along the line of sight: i.e., the molecular<br /> cloud is optically thick at $&lt;$2 keV and so the bulk of the observed soft<br /> X-rays {\it must} originate in the foreground of the cloud, which is at $\sim$3<br /> kpc from the Sun. In the 0.8 $-$ 9.0 keV band, the observed spectrum is<br /> primarily from multiple components of thermal plasmas. We here report a<br /> detection of soft X-ray (0.5 $-$ 2 keV) emission from an $\sim10^{7}$ K thermal<br /> plasma. Comparisons with the {\it ROSAT} data suggest that this soft X-ray<br /> emission is absorbed by $N_H$ = 1 $-$ 3 $\times$ 10$^{21}$ cm$^{-2}$, which<br /> implies a path-length through the soft X-ray emitting regions of $\la$1 kpc<br /> from the Sun.

We propose a mechanism for the origin of the Galactic ridge X-ray background<br /> that naturally explains the properties of the Fe K line, specifically the<br /> detection of the centroid line energy below 6.7 keV and the apparent broadness<br /> of the line. Motivated by recent evidence of nonthermal components in the<br /> spectrum of the Galactic X-ray/gamma-ray background, we consider a model that<br /> is a mixture of thermal plasma components of perhaps supernova origin and<br /> nonthermal emission from the interaction of low energy Cosmic ray electrons<br /> (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova<br /> explosions or by ambient interstellar plasma turbulence. Atomic collisions of<br /> fast electrons produce characteristic nonthermal, narrow X-ray emission lines<br /> that can explain the complex Galactic background spectrum. Using the ASCA GIS<br /> archival data from the Scutum arm region, we show that a two-temperature<br /> thermal plasma model with kT~0.6 and ~2.8 keV, plus a LECRe component models<br /> the data satisfactorily. Our analysis rules out a purely nonthermal origin for<br /> the emission. It also rules out a significant contribution from low energy<br /> Cosmic ray ions, because their nonthermal X-ray production would be accompanied<br /> by a nuclear gamma-ray line diffuse emission exceeding the upper limits<br /> obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate.<br /> The proposed model naturally explains the observed complex line features and<br /> removes the difficulties associated with previous interpretations of the data<br /> which evoked a very hot thermal component (kT~7 keV).

We have discovered errors in the calculation of some of the Einstein<br /> coefficients in Table 5 and some plots in Figure 3. Due to the errors, the<br /> square-root section of the curves of growth of Fe xxvi Ka and Kb, and Ni xxviii<br /> Ka in Figure 3 were a few times underestimated. We correct Table 5 and re-plot<br /> Figure 3 together with the unaffected curves. The program to calculate the<br /> corrected curves of growth is available at<br /> http://www.hp.phys.titech.ac.jp/kotani/cog/index.html

Studies were made of ASCA spectra of seven ultra-luminous compact X-ray<br /> sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6<br /> (Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al.<br /> 1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano &amp;<br /> Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al.<br /> 1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they<br /> are thought to represent a class of enigmatic X-ray sources often found in<br /> spiral galaxies. For some of them, the ASCA data are newly processed, or the<br /> published spectra are reanalyzed. For others, the published results are quoted.<br /> The ASCA spectra of all these seven sources have been described successfully<br /> with so called multi-color disk blackbody (MCD) emission arising from<br /> optically-thick standard accretion disks around black holes. Except the case of<br /> M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not<br /> to exceed the Eddington limits, the black holes are inferred to have rather<br /> high masses, up to ~100 solar masses. However, the observed innermost disk<br /> temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be<br /> compatible with the required high black-hole masses, as long as the standard<br /> accretion disks around Schwarzschild black holes are assumed. Similarly high<br /> disk temperatures are also observed from two Galactic transients with<br /> superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high<br /> disk temperature may be explained by the black hole rotation, which makes the<br /> disk get closer to the black hole, and hence hotter.

We present X-ray/gamma-ray spectra of the binary GX 339-4 observed in the<br /> hard state simultaneously by Ginga and CGRO OSSE during an outburst in 1991<br /> September. The Ginga spectra are well represented by a power law with a photon<br /> spectral index of 1.75 and a moderately-strong Compton reflection component<br /> with a fluorescent Fe K alpha line. The OSSE data require a sharp high-energy<br /> cutoff in the power-law spectrum. The broad-band spectra are very well modelled<br /> by repeated Compton scattering in a thermal plasma with tau=1 and kT=50 keV. We<br /> also find the distance to the system to be &gt; 3 kpc, ruling out earlier<br /> determinations of 1.3 kpc. Using this limit, the observed reddening and the<br /> orbital period, we find the allowed range of the mass of the primary is<br /> consistent with it being a black hole. The data are inconsistent with models of<br /> either homogenous or patchy coronae above the surface of an accretion disc.<br /> Rather, they are consistent with the presence of a hot inner hot disc accreting<br /> at a rate close to the maximum set by advection and surrounded by a cold outer<br /> disc. The seed photons for Comptonization are supplied by the outer cold disc<br /> and/or cold clouds within the hot disc. Pair production is negligible if<br /> electrons are thermal. The hot disc model, which scaled parameters are<br /> independent of the black-hole mass, is supported by the similarity of the<br /> spectrum of GX 339-4 to those of other black-hole binaries and Seyfert 1s. On<br /> the other hand, their spectra in the soft gamma-ray regime are significantly<br /> harder than those of weakly-magnetized neutron stars. Based on this difference,<br /> we propose that the presence of broad-band spectra corresponding to thermal<br /> Comptonization with kT of 50 keV or more represents a black-hole signature.

We analyze X-ray/gamma-ray observations of Cyg X-1 in the soft state. We find<br /> the hybrid thermal/non-thermal model fits the data very well. The significant<br /> contribution from the non-thermal electrons is required to account for the<br /> observed power law extending without a break to at least 800 keV. The presence<br /> of electron-positron pairs in the hot source is unlikely. We propose the<br /> geometry in which a cold accretion disk, surrounded by a hot corona, extends<br /> down to the last stable orbit. At the observed accretion rate the corona<br /> ensures the stability of the cold disk.

We present X-ray/gamma-ray observations of Cyg X-1 in the soft state during 1996 May-June. We analyze ASCA, RXTE and OSSE data. The spectrum consists of soft S-rag blackbody emission of an optically thick accretion disk in the vicinity of a black hole and a power law with an energy index alpha similar to 1.2-1.5 extending to at least several hundred keV. In the spectra, we find the presence of strong Compton reflection, which probably comes from the disk.

We present four X-ray/gamma-ray spectra of Cyg X-1 observed in the hard<br /> (&#039;low&#039;) state simultaneously by Ginga and GRO/OSSE on 1991 July 6. The 3-30 keV<br /> Ginga spectra are well represented by power laws with an energy spectral index<br /> of alpha~0.6 and a Compton reflection component including a fluorescent Fe<br /> K-alpha corresponding to the solid angle of the reflector of ~0.3 times 2 Pi.<br /> The overall Ginga/OSSE spectra can be modelled by repeated Compton scattering<br /> in a mildly-relativistic, tau ~1, plasma. However, the high-energy cutoff is<br /> steeper than that due to single-temperature thermal Comptonisation. It can be<br /> described by a superposition of dominant optically-thin, thermal emission at<br /> kT~140 keV and a Wien-like component from an optically-thick plasma at kT~50<br /> keV. The X-ray spectra do not show the presence of an anisotropy break required<br /> if thermal Compton scattering takes place in a corona above a cold disc. Also,<br /> the flat spectral index shows that the plasma is soft-photon starved, i.e., the<br /> luminosity in incident soft X-ray seed photons is very much less than that in<br /> the hard X-rays. Furthermore, the observed solid angle of the reflector is<br /> significantly less than 2 pi. These facts taken together strongly rule out a<br /> disc-corona geometry. Rather, the observed spectra are consistent with a<br /> geometry in which the cold accretion disc (which both supplies the seed soft<br /> X-rays and reflects hard X-rays) only exists at large radii, while the<br /> Comptonising hot plasma is located in an inner region with no cold disc. This<br /> hot plasma consists of either pure electron-positron pairs if the source size<br /> is ~5 Schwarzschild radii or it contains also protons if the size is larger.

We describe the ASCA observation of the black hole candidate Cyg X-l 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 (F = 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 12-1 +3M⊙. This value is independent of, yet consistent with, the mass estimate based on the orbital kinematics information. © 1997. The American Astronomical Society. All rights reserved.

Correlated variations in timing and spectral properties were observed near the peak of the outburst of Nova Muscae 1991 (GS 1124-683). Two regions with different spectral and timing characteristics can be distinguished in color-color and hardness-intensity diagrams. Two types of quasi-periodic oscillations (QPOs), each associated with regions in these diagrams, have been observed. Rapid flip-flop transitions sometimes occur between the regions. In one region the QPOs&#039; centroid frequency is a strong, increasing function of X-ray intensity. Regions with similar characteristics and similar forms of QPOs were earlier found in GX 339-4 in its very high state. These two regions may turn out to be common and unique to black hole candidates in their very high state, even though they bear some similarities to the spectral branches in neutron star Z sources. © 1997. The American Astronomical Society. All rights reserved.

In the high state of black hole candidates, in which 2-10 keV luminosity is relatively high, the energy index of the power-law like hard X-ray emission above similar to 10 keV is typically similar to 1.5. On the other hand, in the low state, the hard X-ray energy index is 0.3-0.9. In this paper, we suggest that this difference of the hard X-ray spectral slopes may be due to two different Comptonization mechanisms. We propose that, in the high state, the hard component is governed by the Comptonization due to the built motion of the almost freely falling (convergent accretion) flow close to the black hole, rather than thermal Comptonization. The spectral slope of the hard component is insensitive to the disk accretion rate governing the soft component, hence is nearly invariant in spite of the soft component variations. The power-law component due to the bulk motion Comptonization has a sharp cut-off at around the electron rest mass energy, which is consistent with high energy observations of the high state. In the low state, the spectrum is formed due to thermal Comptonization of the low-frequency disk radiation by a sub-Keplerian component (possibly undergoing a centrifugally-supported shock) which is originated from the Keplerian disk. In the limit of low disk accretion rate, the power law index is uniquely determined by the mass accretion rate of the sub-Keplerian component.