arXiv subjects

S. Mineshige

Publications and source records attributed to S. Mineshige.

44 records · Page 3Linked to original sources

arXiv1998

AGN Variability

Number of monitoring observations of continuum emission from Active Galactic Nuclei (AGNs) have been made in optical--X-ray bands. The results obtained so far show (i) random up and down on timescales longer than decades, (ii) no typical timescales of variability on shorter timescales and (iii) decreasing amplitudes as timescales become shorter. The second feature indicates that any successful model must produce a wide variety of shot-amplitudes and -durations over a few orders in their light curves. In this sense, we conclude that the disk instability model is favored over the starburst model, since fluctuations on days are hard to produce by the latter model. Inter-band correlations and time lags also impose great constraints on models. Thus, constructing wavelength and time dependent models remains as a future work.

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arXiv1998

Microlens Diagnostics of Accretion Disks in Active Galactic Nuclei

The optical-ultraviolet continuum from active galactic nuclei (AGN) seem to originate from optically thick and/or thin disks, and occasionally from associated circumnuclear starburst regions. These different possible origins can, in principle, be discriminated by observations of gravitational microlensing event. We performed numerical simulations of microlensing of an AGN disk by a single lensing star passing in front of the AGN. Calculated spectral variations and light curves show distinct behavior, depending on the nature of emitting region; the time variation of a few months with strong wavelength dependence is expected in the case of an optically thick disk ({\it standard disk}), while an optically thin disk ({\it advection-dominated disk}) will produce shorter, nearly wavelength-independent variation. In the case of a starburst region much slower variations (over a year) will be superposed on the shorter variations.

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arXiv1998

Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?

Aperiodic optical variability is a common property of Active Galactic Nuclei (AGNs), though its physical origin is still open to question. To study the origin of the optical -- ultraviolet variability in AGN, we compare light curves of two models to observations of quasar 0957+561 in terms of a structure function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produce aperiodic luminosity variations, while in the disk-instability (DI) model, variability is caused by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, $V(τ)$, by simple Monte-Carlo simulations on the basis of the two models. Each resultant $V(τ)$ possesses a power-law portion, $[V(τ)]^{1/2} \propto τ^β$, at short time lags ($τ$). The two models can be distinguished by the logarithmic slope, $β$; $β\sim$ 0.74--0.90 in the SB model and $β\sim$ 0.41--0.49 in the DI model, while the observed light curves exhibit $β\sim$ 0.35. Therefore, we conclude that the DI model is favored over the SB model to explain the slopes of the observational structure function, in the case of 0957+561, though this object is a radio-loud object and thus not really a fair test for the SB model. In addition, we examine the time-asymmetry of the light curves by calculating $V(τ)$ separately for brightening and decaying phases. The two models exhibit opposite trends of time-asymmetry to some extent, although the present observation is not long enough to test this prediction.

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arXiv1997

Spectrum of Optically Thin Advection Dominated Accretion Flow around a Black Hole: Application to Sgr A*

The global structure of optically thin advection dominated accretion flows which are composed of two-temperature plasma around black holes is calculated. We adopt the full set of basic equations including the advective energy transport in the energy equation for the electrons. The spectra emitted by the optically thin accretion flows are also investigated. The radiation mechanisms which are taken into accout are bremsstrahlung, synchrotron emission, and Comptonization. The calculation of the spectra and that of the structure of the accretion flows are made to be completely consistent by calculating the radiative cooling rate at each radius. As a result of the advection domination for the ions, the heat transport from the ions to the electrons becomes practically zero and the radiative cooling balances with the advective heating in the energy equation of the electrons. Following up on the successful work of Narayan et al. (1995), we applied our model to the spectrum of Sgr A*. We find that the spectrum of Sgr A* is explained by the optically thin advection dominated accretion flow around a black hole of the mass M_bh=10^6 M_sun. The parameter dependence of the spectrum and the structure of the accretion flows is also discussed.

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arXiv1997

The influence of Galactic wind upon the star formation histories of Local Group galaxies

We examine the possibility that ram pressure exerted by the galactic wind from the Galaxy could have stripped gas from the Local Group dwarf galaxies, thereby affecting their star formation histories. Whether gas stripping occurs or not depends on the relative magnitudes of two counteracting forces acting on gas in a dwarf galaxy: ram pressure force by the wind and the gravitational binding force by the dwarf galaxy itself. We suggest that the galactic wind could have stripped gas in a dwarf galaxy located within the distance of $R_{c}\simeq 120(r_{s}/1 {kpc})^{3/2} ({\cal E}_{b}/10^{50} {erg})^{-1/2}$ kpc (where $r_{s}$ is the surface radius and ${\cal E}_{b}$ is the total binding energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of Gyr, thereby preventing star formation there. Our result based on this Galactic wind model explains the recent observation that dwarfs located close to the Galaxy experienced star formation only in the early phase of their lifetimes, whereas distant dwarfs are still undergoing star formation. The present star formation in the Large Magellanic Cloud can also be explained through our Galactic wind model.

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arXiv1996

X-Ray Fluctuations from Locally Unstable Advection-Dominated Disks

The response of advection-dominated accretion disks to local disturbances is examined by one-dimensional numerical simulations. It is generally believed that advection-dominated disks are thermally stable. We, however, find that any disurbance added onto accretion flow at large radii does not decay so rapidly that it can move inward with roughly the free-fall velocity. Although disturbances continue to be present, the global disk structure will not be modified largely. This can account for persistent hard X-ray emission with substantial variations observed in active galactic nuclei and stellar black hole candidates during the hard state. Moreover, when the disturbance reaches the innermost parts, an acoustic wave emerges, propagating outward as a shock wave. The resultant light variation is roughly (time) symmetric and is quite reminiscent of the observed X-ray shots of Cygnus X-1.

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arXiv1996

The Physics of Black Hole X-ray Transients

The most plausible mechanism for triggering the outburst of black hole candidate X-ray transients is the ionization thermal instability. The disk instability models can give the observed mass flow in quiescence, but not the X-ray spectrum. Self-irradiation of the disk in outburst may not lead to X-ray reprocessing as the dominant source of optical light, but may play a role in the "reflare." The hard power-law spectrum and radio bursts may be non-thermal processes driven by the flow of pair-rich plasma from the disk at early times and due to the formation of a pair-rich plasma corona at late times. The repeated outbursts suggest some sort of clock, but it is unlikely that it has anything to do with a simple X-ray heating of the companion star.

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arXiv1995

Subcluster merging in clusters of galaxies and the cosmological density parameter

Recent X-ray observations have established that collisions between subclusters of galaxies are rather common phenomena. Prompted by such observations, we have performed N-body simulations of two equal-mass subclusters of galaxies, which are going to merge. We first have confirmed that only a part of kinetic energy associated with the relative motion of two subclusters is converted to internal energy of each subcluster at the first encounter. We performed simulations for a variety of dark-matter (DM) distributions, and find that the time scale for washing out the double peak structures after the first encounter strongly depends on the distribution of the density and velocity distributions of the DM. It takes longer when the DM is spatially extended. According to our calculation it takes more than $4 imes 10^9$yr after the first encounter until the density contour shows only a single peak. To explain the high fraction of the clusters with substructures among nearby clusters, Richstone,Loeb Turner(1992; hereafter RLT) required recent($$ $10^9 h^{-1}$yr) cluster formation. However, our results show that the timescale for subcluster merging is still uncertain and possibly much longer than the time scale assumed by RLT. Caution should be exercised when concluding that the density of the universe is high by using RLT's method.

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