- "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
We propose a novel mechanism for fuelling active galactic nuclei (AGNs),
i.e., a radiative avalanche, which is mass-accretion driven by
radiation drag exerted by stellar radiation from circumnuclear
starburst regions. If a surface layer of a rotating gas disk
is irradiated by intensive starlight, then it
could lose angular momentum via radiation drag, resulting in an avalanche
of the layer as an inevitable consequence.
Analyses show that
in an optically thick regime, the mass-accretion rate
via this radiative avalanche is in
dependent of not only the extinction coefficient of photon
absorbing matter but also the density distribution of the disk,
and the maximal mass-accretion rate
is simply described as dM/dt=O(L/c2)
with L being the bolometric luminosity of a starburst.
In a disk containing dust, the accretion timescale could be as short
as <108 yr. In the present context,
the intensity of radiative avalanche determines whether
a starburst galaxy is destined to posses an AGN or not.
The present model provides a solid physical mechanism
deduced from first principles to account for the
possible link between starburst activities and AGNs, which has been
suggested by a number of observations.
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)
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)
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.
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)
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.
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)
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.