arXiv subjects

T. Kawaguchi

Publications and source records attributed to T. Kawaguchi.

29 records · Page 2Linked to original sources

arXiv2007

First Detection of 12CO (1--0) Emission from Two Narrow-Line Seyfert 1 Galaxies

In order to investigate how the growth of galactic bulges progresses with the growth of central black holes (BHs), we observed molecular gas (fuel for the coming star formation) in possibly young active galaxies, narrow-line Seyfert 1 galaxies (NLS1s). We present the results of radio observations of 12CO(1--0) using the Nobeyama Millimeter Array (with 2--4 kpc spatial resolution) for two FIR-bright NLS1s, yielding the first detection of their CO emission. Corresponding molecular--gas masses M(H2) of (1-3) X 109 Msun are the 2nd and 4th largest ones among NLS1s. By estimating dynamical masses and bulge masses M(bulge) for these two NLS1s using CO channel map and CO line widths, we found M(H2) amount to 0.13--0.35 of these masses. Taking account the star formation efficiency (~ 0.1), the increase in M(bulge) in those NLS1s in the near future (~< 10^{7.5} yr) is expected not to be a huge fraction (1--5% of the preexisting stars). Bulge growth may have finished before BH growth, or bulge--BH coevolution may proceed with many, occasional discrete events, where one coevolution event produces only a small amount of mass growth of BHs and of bulges. We also discuss the ratios of star-formation rate--to--gas accretion rate onto BHs, finding that two NLS1s have very small ratios (~ 1) compared with the M(bulge)/M(BH) ratios found in active and inactive galaxies (~ 700). This huge difference suggests either the non-overlapped coevolution, long star formation duration or temporarily high accretion rate during NLS1 phase.

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arXiv2005

The application of slim disk models to ULX: the case of M33 X-8

A comparative XMM-Newton archival data spectral study of the ultraluminous X-ray source (ULX) M33 X-8 has been performed by using both the standard disk model and the newly developed slim disk models. The results of this analysis tend to confirm the hypothesis that M33 X-8 is an X-ray binary with a stellar-mass black hole accreting at super-Eddington rate.

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arXiv2004

Growth of massive black holes by super-Eddington accretion

Narrow-Line Seyfert 1 galaxies (NLS1s) and Narrow-Line quasars (NLQs) seem to amount to ~ 10-30 % of active galactic nuclei (AGNs) in the local universe. Together with their average accretion rate, we argue that the black hole (BH) growth by factor of 8-800 happens in these super-Eddington accretion phase of AGNs. Moreover, there is a possible, systematic underestimation of accretion rates (in the Eddington unit) due to an overestimation of BH mass by massive accretion discs for super-Eddington objects. If it is true, the factor of BH growth above may be larger by order(s) of magnitude. In contrast, the growth factor expected in sub-Eddington phase is only ~ 2. Therefore, the cosmic BH growth by accretion is likely dominated by super-Eddington phase, rather than sub-Eddington phase which is the majority among AGNs. This analysis is based on the fraction and the average accretion rate of NLS1s and NLQs obtained for z ~< 0.5. If those numbers are larger at higher redshift (where BHs were probably less grown), super-Eddington accretion would be even more important in the context of cosmic BH growth history.

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arXiv2003

The origin of optical emission from super-Eddington accreting Active Galactic Nuclei: the case of Ton S 180

Self-gravitating accretion discs have only been studied in a few nearby objects using maser spots at the parsec-scale. We find a new spectral window for observing the self-gravitating accretion disc in super-Eddington accreting Active Galactic Nuclei (AGNs). This window is determined by calculating the outermost radius (rsg) of a non self-gravitating disc and the corresponding emission wavelength (lsg) as a function of various disc parameters. We find that lsg reaches ~ 4000 AA for alpha=0.1, when Mdot > 70 (M_BH / 10^7 M_Sun)^-1 L_Edd/c^2 (where alpha, Mdot, M_BH and L_Edd are, respectively, the viscosity parameter, gas accretion rate onto the central black hole (BH), the BH mass and the Eddington luminosity). Moreover, lsg is as small as 1500 AA for alpha = 0.001, which is the smallest alpha case in this study. Therefore, the window for observing the self-gravitating part of an AGN accretion disc is from 2 micron to lsg. Incidentally, rsg can be less than the photon trapping radius for Mdot >~ 10^3.3 L_Edd/c^2. Namely, a self-gravitating, optically-thick, advection-dominated accretion disc is expected to appear in the extremely high accretion rate regime. Next, we demonstrate that the Mid-Infrared to X-ray spectrum of a bright, well-studied Narrow-Line Seyfert 1 galaxy, Ton S 180, is indeed well fitted by the spectrum arising from the following three components: an inner slim disc (with a corona), an outer, self-gravitating non-Keplerian disc and a dusty torus. The total mass, BH mass plus the entire disc mass, is found to be about (1.4 - 8.0) M_BH. If the surface density varies with radius r in proportion to r^-0.6, the total mass is consistent with the central mass estimated by H_beta and [O III] widths.

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arXiv2003

The origin of emission and absorption features in Ton S180 Chandra observations

We present new interpretation of Ton S180 spectrum obtained by {\it Chandra} Spectrometer (Low Energy Transmission Grating). Several narrow absorption lines and a few emission disk lines have been successfully fitted to the data. We have not found any significant edges accompanying line emission. We propose the interpretation of narrow lines consistent with the paper recently written by Krolik (2002), where warm absorber is strongly inhomogeneous. Such situation is possible in so called multi-phase medium, where regions with different ionization states, densities and temperatures may coexist in thermal equilibrium under constant pressure. We illustrate this scenario with theoretical spectra of radiation transfered through a stratified cloud with constant pressure (instead of constant density) computed by code {\sc titan} in plane parallel approximation. Detected spectral features are faint and their presence do not alter the broad band continuum. We model the broad band continuum of Ton S180 assuming an irradiated accretion disk with a dissipative warm skin. The set of parameters appropriate for the data cannot be determined uniquely but models with low values of the black hole mass have too hot and radially extended warm skin to explain the formation of soft X-ray disk lines seen in the data.

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arXiv2002

Two-temperature coronal flow above a thin disk

We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu, & Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate and hence the fraction of accretion energy released in the corona depend strongly on the rate of incoming mass flow from outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation and a weaker corona. We also found a strong dependence on the viscosity, higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate our results become independent on the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies (NLS1s), in agreement with observations. For luminous active galactic nuclei (AGN) strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.

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arXiv2001

Theoretical Models of Multi-waveband QSO Luminosity Functions

Cosmological evolution of the QSO luminosity functions (LFs) at NIR/optical/X-ray bands for 1.3 < z < 3.5 is investigated based on the realistic QSO spectra. The accretion-disk theory predicts that although QSO luminosities only depend on mass-accretion rate, \Mdot, QSO spectra have a dependence on black-hole mass, M_{BH}, as well. The smaller M_{BH} is and/or the larger \Mdot is, the harder becomes the QSO NIR/optical/UV spectrum. We model disk spectra which can reproduce these features and calculated LFs for redshift z ~ 3 with the assumption of new-born QSOs being shining at the Eddington luminosity. The main results are: (i) the observed LFs at optical and X-rays can be simultaneously reproduced. (ii) LFs at optical and X-ray bands are not sensitive to M_{BH}, while LFs at NIR bands are; about one order of magnitude difference is expected in volume number densities at L_{I, J} ~ 10^{46} erg s^{-1} between the case that all QSOs would have the same spectral shape as that of M_{BH} = 10^{9} M_sun and the case with M_{BH} = 10^{11} M_sun. (iii) The resultant LFs at NIR are dominated by 10^{7} M_sun black-holes at L_{I, J} ~ 10^{44} erg s^{-1}, and by 10^{11} M_sun black-holes at L_{I, J} \~ 10^{46} erg s^{-1}. Future infrared observations from space(e.g.NGST) will probe cosmological evolution of black hole masses. For redshift z < 3, on the other hand, the observed optical/X-ray LFs can be fitted, if the initial QSO luminosity L_0 is below the Eddington luminosity. Interestingly, the best fitting values of l = L_0/L_{Edd} are different in B- and X-ray bands; l_B ~ 2.5 l_X. The reason for this discrepancy is briefly discussed.

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arXiv2000

Spectral Energy Distributions of Active Galactic Nuclei from an Accretion Disk with Advective Coronal Flow

To explain the broad-band spectral energy distributions (SED) of Seyfert nuclei and QSOs, we study the emission spectrum emerging from a vertical disk-corona structure composed of a two-temperature plasma by solving hydrostatic equilibrium and radiative transfer self-consistently. Our model can nicely reproduce the soft X-ray excess with alpha (L_frequency proportional to the frequency to the power of -alpha) of about 1.5 and the hard tail extending to about 50 keV with alpha about 0.5. The different spectral slopes (alpha about 1.5 below 2 keV and about 0.5 above) are the results of different emission mechanisms: unsaturated Comptonization in the former and a combination of Comptonization, bremsstrahlung, and reflection of the coronal radiation at the disk-corona boundary in the latter.

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arXiv1999

Temporal 1/f^αFluctuations from Fractal Magnetic Fields in Black Hole Accretion Flow

Rapid fluctuation with a frequency dependence of $1/f^α$ (with $α\simeq 1 - 2$) is characteristic of radiation from black-hole objects. Its origin remains poorly understood. We examine the three-dimensional magnetohydrodynamical (MHD) simulation data, finding that a magnetized accretion disk exhibits both $1/f^α$ fluctuation (with $α\simeq 2$) and a fractal magnetic structure (with the fractal dimension of $D \sim 1.9$). The fractal field configuration leads reconnection events with a variety of released energy and of duration, thereby producing $1/f^α$ fluctuations.

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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

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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