活 動 銀 河 中 心 核
Active Galactic Nuclei (AGNs)

活動銀河核の「輻射性なだれ」

ここ数年,HSTを含む観測の目覚ましい進展により,銀河内の活発な星形成過程と クェーサー/AGN現象に何らかの強い関連があるという証拠が急増してきた。昨今の HST観測では, 近傍のかなりのクェーサーが明るい母銀河の中心にあることが示され た。また, セイファート銀河の中心領域には, 爆発的星形成領域が多く見つかって きている。さらに最近, 遠方のクェーサーに見つかった大量の分子ガスとダストは, 母銀河である原始銀河がかなり短い時間で爆発的星形成を行い,これが何らかの形で クェーサー活動性と結び付いていることを示唆している。
我々は, クェーサー/AGNと爆発的星形成の強い繋がりを説明する物理的メカニズム として、星形成領域からの強力な輻射場による輻射摩擦によって, 中心核を取り囲 む回転ガス円盤から角運動量が効率よく抜き取られ, さながら「なだれ」的に中心 に落ち込むという「輻射性なだれ」モデルを提唱した。この「輻射性なだれ」は, 数pc-100pcの領域では, 通常のα粘性による降着よりも速い。また,銀河中心のバー 不安定による質量降着は, 数10pc程度までが限界であることがわかってきており, 「輻射性なだれ」は, バー不安定が効く数10pcまでと, α粘性が効く1pc以下の間を 繋ぐ第3の質量降着メカニズムと見ることができる。

References

• "Radiative Avalanche による AGN Fuelling"
  梅村雅之，福江純，嶺重慎， 理研研究会「X線でAGNに迫る」, 74-78 (1997) [ Proceedings (HTML) ]

• "Radiative Avalanche: Starburst Induced Fuelling to AGNs"
  M. Umemura, J. Fukue, and S. Mineshige, Astrophysical Journal Letters, 1997, 479, L97-L100

• "Radiative Avalanche Driven by Spherical Starbursts"
  J. Fukue, M. Umemura, and S. Mineshige, Publ. Astron. Soc. Japan, 1997, 49, 673-677

• Radiatively Driven Mass Accretion onto Galactic Nuclei by Circumnuclear Starbursts
  M. Umemura, J. Fukue, and S. Mineshige
  Mon. Not. Roy. Astron. Soc., in press (1998) [astro-ph/9806027, Source (393kb), PostScript (764kb) ]

巨大ブラックホール形成

マイクロレンズによるクェーサー構造の探究

• An X-Ray Microlensing Test of AU-Scale Accretion Disk Structure in Q2237+0305
  A. Yonehara, S. Mineshige, T. Manmoto, J. Fukue, M. Umemura, and E. L. Turner
  Astrophysical Journal Letters, 501, 41-44 (1998) [astro-ph/9804251]

• Microlens Diagnostics of Accretion Disks in Active Galactic Nuclei
  A. Yonehara, S. Mineshige, J. Fukue, M. Umemura, and E. L. Turner
  Astronomy & Astrophysics, submitted (1998) [astro-ph/9710190]

The innermost regions of quasars can be resolved by a gravitational-lens telescope on scales down to a few AU. This is because the radiation, especially X-rays from the innermost region can selectively be amplified during microlensing events. If detected, such X-ray variation will constrain the size of X-ray emitting region down to a few AU. Importantly, the maximum attainable resolution depends mainly on the observational sampling interval of the lens event, which can be much shorter than the crossing time. On the basis of this idea, we performed numerical simulations of microlensing of an optically-thick standard-type disk and an optically-thin, advection-dominated accretion flow (ADAF) by the so-called `caustics crossing' events. Calculated spectral variations and light curves show distinct behavior, depending on the photon energy. Optical-UV fluxes, which are likely to come from an optically thick part, exhibit gradual light changes over a few tens of days, since radiative cooling is balanced with viscous heating in the disk so that emissivity variation roughly reflects the shape of the potential well. Its variability timescale is consistent with that of the microlensing events observed in Q2237+0305. As for X-ray radiation which is produced in an optically thin part, in contrast, somewhat shorter intensity variation is expected at maximum light.
Currently, Q2237+0305 is being monitored in the optical range at Apache Point Observatory. Simultaneous multi-wavelength observations by HST and X-ray sattelites (e.g., ASCA, AXAF, XMM) are urgently required at the time of the microlens event to reveal a small scale structure of the central accretion disk around black hole.

活動銀河核における自己重力粘性円盤

• Self-Similar, Self-Gravitating Viscous Disk
  S. Mineshige and M. Umemura
  Astrophysical Journal Letters, 469, L49-L51 (1996)

• Self-Similar Collapse of Self-Gravitating Viscous Disk
  S. Mineshige and M. Umemura
  Astrophysical Journal, 480, 167-172 (1997) [astro-ph/9701189]

• Self-Similar Viscous Collapse of a Self-Gravitating, Polytropic Gas Disk
  S. Mineshige, K. Nakayama, and M. Umemura
  Publ. Astron. Soc. Japan, 49, 439-443 (1997)

We find a self-similar solution for the self-gravitational viscous disk. The disk structure is characterized by three parameters: viscosity parameter, alpha, the ratio of a disk thickness to a radius, and a mass-flow rate. The solution shows that a non-viscous, isothermal rotating disk modeled by Mestel can be extended to a viscous accretion disk. The solution is discussed in the context of formation of a quasar black hole.

活動銀河核の時間変動

• Optical Variability of Active Galactic Nuclei: Starbursts or Disk Instabilities ?
  T. Kawaguchi, S. Mineshige, M. Umemura, and E. L. Turner
  Astrophysical Journal, in press (1998) [astro-ph/9712006]

Aperiodic optical variability is a common property of Active Galactic Nuclei (AGNs), though its physical origin is still open to question. We compare light curves among the following two models and the observation of quasar 0957+561 in terms of structure function analysis, to make clear the origin of the optical -- ultraviolet variability in AGNs. 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 some instabilities in the accretion disk atmospheres around a supermassive black hole. We calculated fluctuating light curves and structure functions by simple Monte-Carlo simulations on the basis of the two models. Each resultant structure function possesses a power-law portion. The two models can be distinguished by the logarithmic slope, $\beta$. $\beta \sim$ 0.74--0.90 in the SB model and $\beta \sim$ 0.41--0.49 in the DI model, while the observed light curves exhibit $\beta \sim$ 0.35. Therefore, we conclude that the DI model is more favored over the SB model to explain the slopes of the observational structure function in the case of 0957+561.