Issue |
A&A
Volume 428, Number 2, December III 2004
|
|
---|---|---|
Page(s) | 353 - 363 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20041177 | |
Published online | 26 November 2004 |
Flare-induced fountains and buried flares in AGN
1
Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland e-mail: bcz@camk.edu.pl
2
associated to Observatoire de Paris, LUTH, 92195 Meudon, France
3
Observatoire de Paris, Section de Meudon, LUTH, Place Janssen, 92195 Meudon Cedex, France
Received:
28
April
2004
Accepted:
18
August
2004
We discuss the local physical changes at the surface of an AGN
accretion disk after the onset of a magnetic flare. The X-ray irradiation
by a flare creates a hot spot at the disk surface where the plasma both
heats up and expands in the vertical direction in order to regain the
hydrostatic equilibrium. Assuming
that the magnetic loop causing the flare is anchored deeply within the
disk interior, we derive analytical estimates for the vertical
dimension Hhot and the optical depth of the heated
atmosphere as a function of the position within the spot.
We perform computations for various values of the accretion rate
, the fraction fcor of radiation dissipated within the
disk corona, and the covering factor fcover of the disk surface
with flare-illuminated patches. It turns out that generally we
can distinguish three
characteristic radial zones within the disk showing a qualitatively
different behavior
of the heated material. In the innermost regions of the disk (inner zone)
the expansion of the disk material is restricted by strong gravitational
forces. Further out, the flare source, initially above the disk, soon
becomes embedded by the expanding disk atmosphere. At these intermediate
disk radii (middle zone) the material is optically thick thus
greatly modifying the
observed radiation by multiple Compton scattering. We show exemplary
spectra models obtained from Monte Carlo simulations illustrating the
trends. In the outermost
regions of the disk (outer zone) the expanding material is optically thin
and its influence
on the observed spectra is smaller but pressure gradients
in radial directions should cause the development of a fountain-like
dynamical structure
around the flare source. We discuss the observational consequences of our
results.
Key words: accretion, accretion disks / galaxies: active / X-rays: general / radiation mechanism: general
© ESO, 2004
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