The complex radio source Sgr A is embedded in a region near our Galactic Center. The dynamical center of our Galaxy is considered to be Sgr A*, the compact non-thermal radio source. Dynamical mass within ~0.1 pc from Sgr A* has been estimated to be ~ 3×106 M⊙. This places Sgr A* to be a candidate of a massive blackhole (Eckart and Genzel, 1997 and reference therein).

Absorption line features were detected at 4.1 keV from X 1636-536 with the Tenma satellite in the spectra of X-ray bursts (Waki et al., 1984). Similar features were also detected from X 1608-52 and EXO 1747-214 during bursts (Nakamura et al., 1988; Magnier et al., 1989). These features at 4.1 keV may be interpreted as the redshifted Kα absorption line of helium-like iron atoms. However, such interpretation requires extremely soft equation of state for the nuclear matter, and confirmation with high resolution detectors is urged (Lewin et al., 1993). To investigate the line features, we observed X 1636-536 with ASCA for ~ 240 ksec.

X-ray source in Cyg X-3 is embedded in the photo-ionized circumstellar gas. Recombination edges and emission lines were detected by ASCA observation (Kawashima, Kitamoto 1996; Liedahl, Paerels 1996). The strength of the Kα line and the recombination edge are simply determined by the atomic process. Therefore, the comparison of observed these intensities is a good tool to study the Cyg X-3 system.

Here we present a very short review of the ASCA observation campaign of the enigmatic galactic jet system SS433. The campaign started in 1994 just after the launch, and ended in 1996. Various phases of the 162.5-day precession and 13-day orbital motion were sampled. With ASCA, the Doppler-shifted pairs from various ion species from Si to Ni were resolved for the first time (Kotani et al. 1994). The Doppler-shift parameters were determined with an accuracy comparable to optical spectroscopy (Kawai 1995). No velocity gradient was found between the X-ray emission region of the jet and the optical. The distance between them was constrained to be less than 1015 cm. Line intensity ratios of Fe XXVI/Fe XXV give the base temperature of the jet to be 20 keV (Kotani et al. 1996). The variation of the apparent base temperature of the jet can be explained in terms of the partial occultation of the jet by a precessing accretion disk (Kotani et al. 1997a). From the variation, the disk radius and the disk height in unit of the X-ray jet length were estimated to be 0.23 ± 0.10 and 0.0232 ± 0.0049, respectively. (These are an improved version of the values in Kotani et al. (1997a).) SS433 is also known as an eclipsing binary. Because the emission from each jet with ASCA, it is possible to know how much of which jet is occulted by the companion star during an eclipse. Relative size of the companion star gives Roche lobe size and thus mass ratio MX/MC = 0.22+0.09-0.16 (Kotani 1997b). With the help of Doppler modulation, compact star mass is constrained. However, the values of Doppler modulation reported from optical observations largely scatters. D'Odorico et al. (1991) reported 112 km s−1 and this gives MX = 0.68+0.43-0.53 M⊙, i.e., a white dwarf, while Fabrika and Bychkova (1990) reported 175 km s−1, which gives 2.6+1.6-2.0 M⊙. (This error includes systematic errors of the X-ray data, and will be reduced in future analysis.) On the other hand, the absolute size of the system were determined with a satisfactory precision. For example, the X-ray jet length was determined to be 2×1013 cm, ten times larger than previous estimations (Kotani et al. 1997c). Other physical parameters of the jet can be derived from the X-ray jet length. Mass outflow rate and the kinetic luminosity of both jet were determined to be 8 × 10−6 M⊙ yr−1 and 1.6 × 1040 erg s−1 (Kotani et al. 1997d), implicating a highly super critical accretion. Most of these values are first precise measurements and/or “radical” revisions of previous estimations. The new picture of SS433 drawn here is far stormy and highly energetic.

We present the ASCA/GIS results of the transient source GRO J1744-28, the bursting X-ray pulsar, for the two times observations about one year apart. (the first and the second observations are on Feb.27th 1996 and Mar.16th 1997 respectively.) Since the discovery of GRO J1744-28 on Dec. 2 1995 with the BATSE observatory (Fishman et al. 1995; Kouveliotou et al.), thousands of Type II X-ray bursts, and sinusoidal pulsations have been observed in the X-ray band. No other source has ever been found to exhibit such unusual characteristics. Following each burst the flux decreases below, and recovers to the pre-burst persistent level (here after the dip) within a few seconds to a few minutes, depending on the total flux of the burst. We examined the spectrum of GRO J1744-28 during each persistent, dip, burst phases at the two observations. This is the first detail study of the spectra (1-10keV band) of GRO J1744-28.

We consider the structure and time dependent behavior of the outflow in disks of Be stars in Be/X-ray binaries, based on the viscous decretion disk scenario (Lee et al. 1991). In this scenario, the matter ejected from the star with the Keplerian velocity at the equatorial surface of the star drifts outward because of the effects of viscosity, and forms the disk.

In the light curves of SS 433, in addition to a binary modulation with a period of 13.1 d, there is a precessional one with a period of 162.5 d (Kemp et al. 1986). In the current studies, however, the attention was focused mainly on the light curves during eclipse (see, however, e.g., Antokhina, Cherepashchuk 1987). Thus, in this paper we examine light curves during precession and compare them with observational ones (Fukue et al. 1997b).

This paper report the ASCA observations of the three brightest persistent X-ray stars near the Galactic Center: an X-ray burster A1742-294, black hole candidate 1E 1740.7-2942, and unclassified source 1E 1743.1-2843. Emission mechanism is briefly discussed based on the new ASCA results.

X Persei is a Be X-ray binary pulsar with ~835-sec pulsation period. Different from most other Be X-ray binaries, X Persei does not exhibit X-ray outbursts. A binary period of ~580 days has been suggested, but is not confirmed the by follow-up observations.

In numerous low-mass X-ray binaries quasi-periodic oscillations (QPOs) between 300 and 1200 Hz have been discovered (the kHz QPOs; see van der Klis 1997 for a recent review on kHz QPOs). Here we present the search for kHz QPOs in the atoll sources GX 9+1 and GX 9+9.

XBT 0748-676 is a dipping LMXB source, with dips in X-ray intensity occurring at the orbital period of ~ 3.8 hrs. It is a member of the sub-group of dipping sources also including XB 1916-053 and XB 1254-690 in which the spectral evolution in dipping has previously been modelled by the “absorbed plus unabsorbed” approach, in which the dip spectra are modelled by two terms, each having the same form as used for non-dip emission, one of which is strongly absorbed but one which is not absorbed, which has a normalisation decreasing strongly in dipping. This energy-independent decrease has sometimes been taken to imply electron scattering in the absorber.

We investigate the accretion disks around compact objects with high mass accretion rates near the Eddington's critical value ME, where radiation pressure and electron scattering are dominant. This raises next problems: (a) whether stable disks could exist in relation to the theory of thermal instabilities of the disk and (b) what characteristic features the disks have if the stable disks exist. A non-rotating neutron star with the mass M = 1.4M⊙, radius R* = 107cm and the accretion rate Mac = 2.0 and 0.5Mac (models 1 and 2) is considered as the compact object. We assume the α-model for the viscosity and solve the set of two-dimensional time-dependent hydrodynamic equations coupled with radiation transport. The numerical method used is basically the same as one described by Kley and Hensler (1987) and Kley (1989) but we include some improvements in solving the difference equations (Okuda et al. 1997). The initial configuration consists of a cold, dense, and optically thick disk which is given by the standard α-model (Shakura and Sunyaev 1973) and a rarefied optically thin atmosphere around the disk.

A model for the stationary radial distribution of the magnetic energy-stress tensor <BB> in a standard thin disc is presented, with allowance for magnetic torques and dissipation, see Fig. 1. The model is an extension of earlier work by Schramkowski et al. (1996). For LMC X-3, <B2>1/2 reaches ~ 4 × 105 G near the inner edge of the disc, and the magnetic pressure is much smaller than the sum of gas and radiation pressure.

Quasi-periodic oscillations (QPOs) of a few Hz are observed in the very high state of some black hole candidates (GX 339-4 and GS 1124-68). This is the Kepler frequency at the radius of a few hundred Schwarzschild radii. As a possible mechanism of the QPOs in these objects, the trapped oscillations in the accretion disks are considered. The trapped oscillations of the disks were investigated by several authors. They studied the trapped oscillations in the standard radiative cooling-dominated disks. Recently, the advection-dominated accretion flow is considered, as a possible model to explain the hard X-ray spectra of the black hole candidates or the active galactic nuclei. In particular, in the very high state of some black hole candidates, the spectrum can be explained by the disk-corona model which comprises the cold standard accretion disk and the advection-dominated corona above the cold disk. We thus investigated the trapped axi-symmetric oscillations in the advection-dominated corona by the global linear analysis.

We have made observations of the black hole binary Cyg X-1 with the Indian X-ray Astronomy Experiment (IXAE). Observations made with time resolution ranging from 0.4 ms to 1 s showed variations and flaring activity on sub-sec and longer time scales. Results on time variability on different time scales and flaring characteristics in the two states of Cyg X-1 are presented.

Photoelectric observations of Cyg X-1 in 1996-97 fulfilled in Tien-Shan, Kazakhstan (WBVR), Mt. Maidanak, Uzbekistan (UBVR) and in Crimea, Ukraine (UBV) have detected unusual optical flare activity. During May-August 1996 Cyg X-1 was in high soft X-ray state and after August 9, 1996 it returned to its ordinary state but with some evidences of X-ray activity.

On 1996 May 10, the All-Sky Monitor aboard RXTE revealed that Cyg X-1 started a transition from its hard state to soft state (Cui 1996). Throughout this interesting episode, snapshots were taken with more sensitive detectors on ASCA, RXTE, and CGRO to monitor the temporal and spectral variability of the source over a broad energy range.

About 10 X-ray binaries in our Galaxy and LMC/SMC are considered to contain black hole candidates (BHCs). Among these objects, Cyg X-1 was identified as the first BHC, and it has led BHCs for more than 25 years(Oda 1977, Liang and Nolan 1984). It is a binary system composed of normal blue supergiant star and the X-ray emitting compact object. The orbital kinematics derived from optical observations indicates that the compact object is heavier than ~ 4.8 M⊙ (Herrero 1995), which well exceeds the upper limit mass for a neutron star(Kalogora 1996), where we assume the system consists of only two bodies. This has been the basis for BHC of Cyg X-1.

Cygnus X-1 was in the Soft State between May and September 1996 for the first time in more than 20 years creating an opportunity to study the thermal component, its spectral evolution, time variability and its relation to the hard component during intensity variations. We will show how the luminosity of the thermal component changes in relation to its temperature and attempt to determine whether the emitting area varied during the variations.

GRS1915+105 is an extraordinary X-ray transient which exhibits superluminal radio jets. In this paper, ASCA observations of the GRS1915+105 conducted from 1994 to 1997 are reported. Observations are carried out on the following dates each for ~ 20 ksec exposure; Sep 27 1994, April 20 1995, Oct 23 1996 and Apr 25 1997.