Bibcode
Hönig, S. F.; Prieto, M. A.; Beckert, T.
Bibliographical reference
Astronomy and Astrophysics, Volume 485, Issue 1, 2008, pp.33-39
Advertised on:
7
2008
Journal
Citations
40
Refereed citations
37
Description
We investigate the idea that a sizable fraction of the
interferometrically unresolved infrared emission of the nucleus of NGC
1068 might originate from processes other than thermal dust emission
from the torus. We examine the contribution of free-free or synchrotron
emission to the central mid- and near-IR parsec-scale emitting region of
NGC 1068. Each mechanism is constrained using parsec scale radio data
available for NGC 1068 in the 109 {-}1011 Hz
regime, and compared with the highest-resolution interferometric data
available in the mid-infrared. It is shown that the unresolved emission
in the interferometric observation (⪉1 pc) is still dominated by
dust emission and not by contributions from synchrotron or free-free
emission. As previous studies suggest, the interferometric observations
appear to infer a clumpy structure of the dust distribution.
Extrapolation of the radio free-free or synchrotron emission to the IR
indicates that their contribution is <20% even for the unresolved
fraction of the interferometric flux. The slope of the available radio
data is consistent with a power law exponent α = 0.29 ±
0.07 that we interpret in terms of either free-free emission or
synchrotron radiation from quasi-monochromatic electrons. We apply
emission models for both mechanisms to obtain physical parameters.
Furthermore, we attempt to quantify the possible contribution of the
accretion disk to the near-infrared emission. It has been suggested,
that the unresolved K-band flux in VLTI/VINCI interferometric
observation at 46 m baseline (40% of the total K-band flux) might
originate in the accretion disk. By using an accretion disk spectrum
that has been adjusted to the luminosity and black hole mass of NGC
1068, we find that the expected accretion-disk flux in the K-band is
negligible. Moreover, the scenario of detecting the accretion disk
through holes in a clumpy torus is extremely unlikely. We conclude that
all current IR data of NGC 1068 trace the torus dust emission, favoring
a clumpy torus.
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