We have made photometric observations of the galactic superluminal jet source GRS 1915+105 in the energy bands of 2-6 and 6-18 keV during 1997 June 12-29 and August 8-10. During our observations, different types of very intense, quasi-regular X-ray bursts have been observed from this source. We present here the light curves and the power density spectra of our observation of this source in its bright state.

The transient X-ray source GRS 1915+105 was discovered in August 1992 with the GRANAT/WATCH all-sky monitor (Castro-Tirado et al. 1994). Subsequent VLA observations from March through April 1994 led to the discovery of apparent superluminal motion in a pair of radio condensations moving away from the compact radio core (Mirabel & Rodriguez 1994). These jet-like features are interpreted as a bipolar outflow with bulk velocity ~ 0.9c. Although no optical counterpart has been observed, due to the heavy extinction in the Galactic plane, and therefore not enabling measurements of the mass of the compact object, the hard X-ray spectrum and high luminosity (~ 1039 erg s−1), extreme variability in the X-ray light curve and the relativistic jets make GRS 1915+105 a strong black hole candidate.

The soft X-ray transient (SXT) GRO J0422+32 (Nova Persei 1992) was detected with the Burst And Transient Source Experiment (BATSE) on board the CGRO on 1992 August 5 (Paciesas et al. 1992) (Truncated Julian Day [TJD] 8839). The source intensity of GRO J0422+32 increased rapidly, reaching a flux of ~ 3 Crab (40-230 keV) within days after its first detection (Harmon et al. 1992). Hereafter, the X-ray intensity of the source decreased exponentially with a decay time of ~ 43 days (Vikhlinin et al. 1995). A secondary maximum of the X-ray intensity was reached at TJD 8978, 139 days after the first detection of the source. The daily averaged flux history of GRO J0422+32 in the 40-150 keV energy band is presented in Figure 1.

Black hole candidates sometimes show a transition between the high (or soft) state and the low (or hard) state. In the low state, low frequency time variations are much larger than the high state. A possible mechanism of the large-amplitude, sporadic time variabilities in the low-state is the magnetic energy release in low-β (β = Pgas/Pmag < 1) disks (Mineshige, Kusunose & Matsumoto 1995). It had been thought that low-β disks cannot exist because buoyant escape of magnetic flux due to the Parker instability may set the lower limit for β inside the disk. Shibata, Tajima & Matsumoto (1990), however, pointed out that in accretion disks, once a low-β disk is formed, it can stay in low-β state partly because the growth rate of the Parker instability decreases when β < 1. They suggested that magnetic accretion disks fall into two types; high-β disks and low-β disks.

The accretion disks are now supposed to be the main driving source of the active astrophysical phenomena. Even the electron-positron pair plasma will be created at the surface of the sufficiently luminous disk. While the effect of radiation drag which causes in the intense radiation fields around the accretion disk is examined recently. Then, we numerically consider the radiative accelerated pair-winds, which blow off from central luminous accretion disk surrounding a black hole, taking into account radiation drag of the order of v/c.

The new model for X-ray fluctuations of Cyg X-1, which is based on the fluid dynamics, is presented. The model is the optically thin and advection-dominated accretion disk model, which has a critical behavior.

We study the spectra arising from Compton reflection in the innermost parts of the accretion disk. We emphasize that the so far neglected relativistic distortion of the Compton reflection continuum may strongly affect the derived Fe Kα line shapes.

The optically thin, advection-dominated accretion flows are thermally stable against global perturbations. In addition, they have high temperatures because of inefficient radiative cooling. They are thus promising candidates of models to explain the high energy emission of X-ray stars and AGNs. So far, models, however, take no account of the advective heat transport in determining the thermal structure of the electron system. The validly of this neglect, however, must be checked by integrating the electron energy equation globally as well as the ion energy one.

A model of magnetohydrodynamic (MHD) turbulence in accretion disks with global magnetic fields is constructed using a second-order closure modeling of turbulence. The transport equations of the Reynolds stress tensor, the Maxwell stress tensor, and the cross-helicity tensor (the correlation of velocity fluctuation and magnetic fluctuation) are closed by second-order quantities using a two-scale direct interaction approximation (TSDIA). The quantities appearing these equations are considered to be those averaged in the vertical direction of the disks. The turbulence is assumed to be stationary. We are interested only in the effects of the global magnetic fields on the turbulence in the disks, i.e., no dynamo processes are considered, and the global magnetic fields are supposed to be embedded in the disk a priori.

Accretion-disk corona (ADC) is required from observational as well as theoretical reasons. In almost all of traditional studies, however, a stationary corona has been assumed; i.e., the corona gas corotates with the underlying (Keplerian) accretion disk, and the radial motion is ignored. Recently, in the theory of accretion disks a radiative interaction between the gas and the external radiation field has attracted the attention of researchers. In particular the radiation drag between the gas and the external radiation field becomes important from the viewpoint of the angular-momentum removal. We thus examine the effect of radiation drag on the accretion-disk corona above/below the accretion disk (Watanabe, Fukue 1996a, b). We suppose that an accretion disk can be described by the standard disk, and that radiation fields are produced by the central luminous source and the accretion disk, itself. In general an accretion-disk corona under the influence of strong radiation fields dynamically infalls (advected) toward the center.

The radio jets ejected from active galactic nuclei (AGNs) sometimes show proper motions with apparent velocity exceeding the speed of light. This phenomenon, called superluminal motion, is explained as relativistic jets propagating in a direction almost toward us, and has been thought to be ejected from the close vicinity of hypothetical supermassive black holes powering AGNs (Rees 1996). The magnetic mechanism has been proposed not only for AGN jets (Lovelace 1976; Blandford & Payne 1983) but also for protostellar jets (Pudritz & Norman 1986; Uchida & Shibata 1985; Shibata & Uchida 1986), although no one has yet performed nonsteady general relativistic magnetohydrodynamic (GRMHD) numerical simulations on the formation of jets from the accretion disk around a black hole.

Advection-dominated accretion flow (hereafter ADAF) is the only self-consistent solution to describe the optically thin accretion flows around compact objects. The main feature of ADAF is that the dynamics of the flow is dominated by accretion process rather than radiation process. As a result of advection domination, the luminosity of ADAFs is very low. Coupled with the existence of the event horizon, ADAF has been successfully applied to the dim accretion black holes such as central core of our Galaxy: Sgr A*. In this issue, we calculate the spectrum radiated from the optically thin ADAFs and show that the observed spectrum of Sgr A* is explained with the accretion massive black hole.

We examined an accretion-disk corona around a black hole immersed in the disk radiation fields (cf. Watanabe, Fukue 1996a, b). The corona is supposed to be initially at rest far from the center. During infall above and below the disk, the corona is suffered from the disk radiation fields. As a disk model, we adopted the standard α-disk, and in order to mimic the general relativisitic effects, we use the pseudo-Newtonian force proposed by Artemova et al. (1996). Moreover, we assume that the corona is geometrically thin and optically thin, and ignored any motion such as wind. We consider the cold case, where the pressure-gradient force is ignored. Under these assumptions, we calculated the motion of the corona gas and found that the infall of corona is supressed due to disk radiation fields.

The paper considers the possibility of AGN mass estimation from observations of spectral lines emitted by jets. Jets are a typical for AGNs.

Similar to SS433 the inner parts of AGN jets may emit spectral lines. 88433 spectra show that jet lines are week in comparison with those of slowly outflowing gas. Consequently the proper time for AGNs jet lines observations is the deep brightness minima (so-called transition from Sy1 type to Sy2 type of spectra). It is possibly the only chance for jet lines observations, because on the Sy1 stage gas surrounding the central object can. be optically thick in the broad lines wings or at least light up the week jet lines. The only well studied case of A(;N transition from Sy1 to Sy2 with evidence of jets is NGC 4151 when this galaxy was in a deep minimum of brightness during the 1984-1987.

We report investigation of the iron Kα line in a long (100 ks) ASCA observation of NGC 4151. This observation offers unprecedented good statistics; however, the situation is complicated by the fact that the absorption in NGC 4151 is complex and therefore it is difficult to deconvolve a broad iron line from the power law strongly curved by the absorption. Preliminary spectral fitting with a dual absorber model, using updated abundances and response matrices, and also allowing for iron overabundance, revealed significant spectral residuals around 5 keV which could be modeled with a broad Gaussian. This profile resembles the line characteristic of emission from a relativistic accretion disk; however, that model fit the spectra poorly. Since the energy of the narrow core is nearly 6.4 keV, the orientation of the accretion disk should be nearly face-on, because if the inclination were higher, the blue horn should be shifted to higher energies. If the orientation is face-on, there should be no emission blueward of 6.4 keV; however, a small blue wing as well as a long red wing are present in the residuals.

One of the primary goals of AGN variability studies has been to determine the size of broad-line region (BLR) through the reverberation mapping technique. In a recent international multiwavelength spectroscopic monitoring campaign, NGC 4151 has been observed intensively by ground-based telescopes for a period of over 2 months, with a typical temporal resolution of 1 day. The main result from this optical campaign is that finding the variation in the emission line flux (Hβ or Hα) lagging the continuum by 0-3 days (1993 campaign: Kaspi et al. 1996). This is in contrast to the past results in which a time lag of 9±2 days was found for the same emission line (1988 campaign: Maoz et al. 1991). Such a BLR “size problem” may be caused by a different variability timescale of the ionizing continuum or a real change in BLR gas distribution in the 5.5 yr interval between the two watch campaigns. In order to clarify which of the two possibilities is most likely the real case, we performed further reverberation analysis on both optical datasets.

The first-ever high energy (~ 1-30 keV) X-ray spectra of three z ~ 2 radio-quiet quasars (RQQs) are presented. If confirmed, the most interesting result is the marginal, but consistent, evidence for FeKα emission lines in at least two of the sources. Further ASCA observations of high-z RQQs are needed for firm conclusions.

Study of nine Narrow Line Seyfert 1 galaxies (NLS1) observed with ASCA is presented. The X-ray spectra are fitted with a two component model. The soft component can be represented by a blackbody model with kT~100eV. To make the observed luminosity below the Eddignton limit, we have to assume the emission region has a size much smaller than 3Rs. The X-ray variabilities of the NLS1s have systematically shorter time scale than the typical (broad line) Seyferts 1 (BLS1), which suggests lower black hole masses in the NLS1s.

We also study the the X-ray spectral structure around 1.1 keV, which has been reported for some NLS1s. Although the structure has been interpreted as an absorption feature, we propose an alternative model employing emission lines. One possible origin of the emission lines is preprocessing from a highly ionized accretion disk.

We discuss a model of the central source in Seyfert 1 galaxy NGC 5548. The model assumes a three phase disk structure consisting of a cold outer disk, a hot central disk constituting a Comptonizing X/γ source, and an intermediate unstable and complex phase emitting a soft excess component. The model qualitatively explains broad-band spectrum and variability behavior assuming that the soft excess contributes significantly to the continuum emission and drives variability by geometrical changes of the intermediate disk zone.

We have observed a dozen bright Seyfert 2's selected by their [OIII]λ5007 emission line flux (Ueno et al 1996, Awaki et al 1996). We chose this to be a flux-limited sample free from X-ray selection effects such as intrinsic absorbing columns (NH) and differences in viewing angle (Mulchaey et al 1994). Is it genuinely free from these selection effects? To investigate this, we compare the [OIII] luminosity of various Seyfert 2 galaxies. For this paper, we assume the Hubble constant to be H0 = 50km s−1 Mpc−1.