arXiv subjects

Shin Mineshige

Publications and source records attributed to Shin Mineshige.

81 records · Page 5Linked to original sources

arXiv2000

Slim Disk Model for Narrow-Line Seyfert 1 Galaxies

We argue that both the extreme soft X-ray excess and the large-amplitude variability of Narrow-Line Seyfert 1 galaxies (NLS1s) can be explained in the framework of the slim disk model. When the disk luminosity approaches the Eddington luminosity, the disk becomes a slim disk, exhibiting a multi-color blackbody spectrum with a maximum temperature, T(bb), of about 0.2 (M/1e5 solar masses)e(-1/4) keV, and size of the X-ray emitting region, r(bb), of about R(S) (the Schwarzschild radius). Furthermore, magnetic energy can be amplified up to a level exceeding radiation energy emitted from the disk, causing substantial variability in X-rays by consecutive magnetic flares.

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arXiv2000

Slim Disk Model for Soft X-Ray Excess and Variability of Narrow-Line Seyfert 1 Galaxies

Narrow-line Seyfert 1 galaxies (NLS1s) exhibit extreme soft X-ray excess and large variability. We argue that both features can be basically accounted for by the slim disk model. We assume that a central black-hole mass in NLS1 is relatively small, $M \sim 10^{5-7}M_\odot$, and that a disk shines nearly at the Eddington luminosity, $L_{\rm E}$. Then, the disk becomes a slim disk and exhibits the following distinctive signatures: (1) The disk luminosity (particularly of X-rays) is insensitive to mass-flow rates, $\dot M$, since the generated energy is partly carried away to the black hole by trapped photons in accretion flow. (2) The spectra are multi-color blackbody. The maximum blackbody temperature is $T_{\rm bb} \simeq 0.2(M/10^5 M_\odot)^{-1/4}$ keV, and the size of the blackbody emitting region is small, $r_{\rm bb} \lsim 3 r_{\rm S}$ (with $r_{\rm S}$ being Schwarzschild radius) even for a Schwarzschild black hole. (3) All the ASCA observation data of NLS1s fall onto the region of $\dot M/(L_{\rm E}/c^2)>10$ (with $L_{\rm E}$ being the Eddington luminosity) on the ($r_{\rm bb},T_{\rm bb}$) plane, supporting our view that a slim disk emits soft X-rays at $\sim L_{\rm E}$ in NLS1s. (4) Magnetic energy can be amplified, at most, up to the equipartition value with the trapped radiation energy which greatly exceeds radiation energy emitted from the disk. Hence, energy release by consecutive magnetic reconnection will give rise to substantial variability in soft X-ray emission.

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arXiv1999

Evolution of Hydromagnetic Disturbances in Low Ionized Cosmic Plasmas

We consider the propagation of hydromagnetic waves generated by a compact turbulent source in low ionized plasmas, applying the Lighthill theory. We assume the plasma to be isothermal, and adopt a uniform, stationary medium thread by ordered magnetic fields as an initial condition. Then, the distinct properties of the hydromagnetic waves originating from a source oscillating with a fixed frequency are studied in the linear regime. As is well known, in low ionized plasmas, the generated waves dissipate due to ion-neutral damping. In this paper, the dependence of the dissipation rate on the frequency of the oscillating source is investigated. The larger the frequency becomes, the more substantial is the wave dissipation. Implications of our results on the energy source in molecular clouds are also discussed. Interestingly, since the outflow lobes associated with young stellar objects act as compact turbulent sources, hydromagnetic waves are generated by them. From our order-estimations, about 70% of the energy of the outflow itself propagates as waves or turbulences, while the remaining 30% dissipates and heats the neutrals via ion-neutral damping. Then, we confirm that the outflows are significant energy sources in molecular clouds in the context of the Lighthill theory.

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arXiv1999

Black-Hole X-Ray Transients: The Effect of Irradiation on Time-Dependent Accretion Disk Structure

Some effects of irradiation on time-dependent accretion-disk models for black hole X-ray novae are presented. Two types of irradiation are considered: direct irradiation from the inner hot disk and indirect irradiation as might be reflected by a corona or chromosphere above the disk. The shadowing effect of the time-dependent evolution of the disk height and consequent blocking of the outer disk by the inner and middle portions of the disk from the direct irradiation is included. The direct irradiation of the disk by inner layers where the soft X-ray flux is generated is found to have only a small effect on the outer disk because of shadowing. Mild indirect irradiation that flattens, but otherwise does not affect the light curve substantially, still has interesting non-linear effects on the structure of the disk as heating and cooling waves propagate. The irradiated disks do not always make simple transitions between the hot and cold states, but can linger at intermediate temperatures or even return temporarily to the hot state, depending on the irradiation and the activity in adjacent portions of the disk.

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arXiv1999

Gravitational Mapping of a Quasar Disk

Since the radiation from different portions in the central region of a quasar can be successively amplified during a microlensing event, microlensing light curves provide us with fruitful information regarding the emissivity distribution of an accretion disk located at the quasar center. We present a basic methodology how to map the emissivity distribution of the disk as a function of the radial distance from the center, $Q(r)$, from `observed' microlens light curves during a caustic crossing event. Our proposed method is based on the standard inversion technique, the so-called regularization method, and the Abel's transformation of the one-dimensional luminosity profile integrated along the line parallel to the caustics. The technique will be used to map the disk structure in Q2237+0305, for which the HST and AXAF observations are scheduled. A reconstruction of the image on length scales of several to ten AUs is quite feasible for this source, as long as measuring errors are within 0.02 mag and the observation time intervals are a week or less.

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arXiv1998

Radiatively Driven Mass Accretion onto Galactic Nuclei by Circumnuclear Starbursts

We examine the radiatively driven mass accretion onto galactic nuclei due to the intensive radiation from circumnuclear starbursts. The radiation from a starburst not only contracts an inner gas disk by the radiation flux force, but also extracts angular momenta due to the relativistic radiation drag, thereby inducing an avalanche of the surface layer of disk. The present radiatively driven mass accretion may provide a physical mechanism which enables mass accretion from 100 pc scales down to pc, and it can eventually link to an advection-dominated viscous accretion onto a massive black hole. The radiation-hydrodynamical and self-gravitational instabilities of the disk are briefly discussed. In particular, the radiative acceleration possibly builds up a dusty wall, which provides another version for the formation of an obscuring torus.

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arXiv1998

An X-Ray Microlensing Test of AU-Scale Accretion Disk Structure in Q2237+0305

The innermost regions of quasars can be resolved by a gravitational-lens {\lq}telescope{\rq} on scales down to a few AU. For the purpose, X-ray observations are most preferable, because X-rays originating from the innermost regions, can be selectively amplified by microlensing due to the so-called `caustic crossing'. If detected, X-ray variations will constrain the size of the X-ray emitting region down to a few AU. The maximum attainable resolution depends mainly on the monitoring intervals of lens events, which should be much shorter than the crossing time. On the basis of this idea, we performe numerical simulations of microlensing of an optically-thick, standard-type disk as well as an optically-thin, advection-dominated accretion flow (ADAF). Calculated spectral variations and light curves show distinct behaviors, depending on the photon energy. X-ray radiation which is produced in optically thin region, exhibits intensity variation over a few tens of days. In contrast, optical-UV fluxes, which are likely to come from optically thick region, exhibit more gradual light changes, which is consistent with the microlensing events so far observed in Q2237+0305. Currently, Q2237+0305 is being monitored in the optical range at Apache Point Observatory. Simultaneous multi-wavelength observations by X-ray sattelites (e.g., ASCA, AXAF, XMM) as well as HST at the moment of a microlens event enable us to reveal an AU scale structure of the central accretion disk around black hole.

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arXiv1997

Evolution of X-Ray Clusters of Galaxies and Shock Heating of Intracluster Medium

Evolutions of X-ray clusters of galaxies are studied by N-body (shell model) + mesh code (TVD) simulations on the assumption of spherical symmetry. We consider a density perturbation of $10^{15} M_{\odot}$ composed of dark matter and gas in cold dark matter dominated universe with the cosmological density parameter, $Ω_0 = 1$ or $0.2$. A shock front appears during its initial collapse, moving outward as ambient gas accretes towards cluster center. The shock front separates the inner X-ray emitting, hot region, where gas is almost in hydrostatic equilibrium but with small radial infall ($\sim 100$km s$^{-1}$) being left, from the outer cool region. Gas inside the shock is strongly compressed and heated by shock so that X-ray luminosity rapidly rises in the early stage. In the late stage, however, the X-ray luminosity rises only gradually due partly to the expansion of the inner high temperature region and partly to the increase of X-ray emissivity of gas as the results of continuous adiabatic compression inside the shock. The shock front structure, which was not clearly resolved in the previous SPH simulations, is clearly captured by the present simulations. In addition, we find a sound wave propagating outward, thereby producing spatial modulations with amplitudes of $\sim$ 10 % in the radial temperature and density profiles and time variations in the strength of the shock. Such modulations, if observed, could be used as a probe to investigate the structure of clusters.

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arXiv1997

Self-Similar Collapse of a Self-Gravitating Viscous Disk

A self-similar solution for time evolution of isothermal, self-gravitating viscous disks is found under the condition that $α' \equiv α(H/r)$ is constant in space (where $α$ is the viscosity parameter and $H/r$ is the ratio of a half-thickness to radius of the disk). This solution describes a homologous collapse of a disk via self-gravity and viscosity. The disk structure and evolution is distinct in the inner and outer parts. There is a constant mass inflow in the outer portions so that the disk has flat rotation velocity, constant accretion velocity, and surface density decreasing outward as $Σ\propto r^{-1}$. In the inner portions, in contrast, mass is accumulated near the center owing to the boundary condition of no radial velocity at the origin, thereby a strong central concentration being produced; surface density varies as $Σ\propto r^{-5/3}$. Moreover, the transition radius separating the inner and outer portions increases linearly with time. The consequence of such a high condensation is briefly discussed in the context of formation of a quasar black hole.

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