217 research outputs found
Verbose, Laconic or Just Right: A Simple Computational Model of Content Appropriateness under Length Constraints
Length constraints impose implicit requirements on the type of content that can be included in a text. Here we pro- pose the first model to computationally assess if a text deviates from these requirements. Specifically, our model predicts the appropriate length for texts based on content types present in a snippet of constant length. We consider a range of features to approximate content type, including syntactic phrasing, constituent compression probability, presence of named entities, sentence specificity and intersentence continuity. Weights for these features are learned using a corpus of summaries written by experts and on high quality journalistic writing. During test time, the difference between actual and predicted length allows us to quantify text verbosity. We use data from manual evaluation of summarization systems to assess the verbosity scores produced by our model. We show that the automatic verbosity scores are significantly negatively correlated with manual content quality scores given to the summaries
The origin of the infrared emission in radio galaxies. III. Analysis of 3CRR objects
We present Spitzer photometric data for a complete sample of 19 low redshift
(z<0.1) 3CRR radio galaxies as part of our efforts to understand the origin of
the prodigious mid- to far-infrared (MFIR) emission from radio-loud AGN. Our
results show a correlation between AGN power (indicated by [OIII] 5007 emission
line luminosity) and 24 micron luminosity. This result is consistent with the
24 micron thermal emission originating from warm dust heated directly by AGN
illumination. Applying the same correlation test for 70 micron luminosity
against [OIII] luminosity we find this relation to suffer from increased
scatter compared to that of 24 micron. In line with our results for the
higher-radio-frequency-selected 2Jy sample, we are able to show that much of
this increased scatter is due to heating by starbursts which boost the
far-infrared emission at 70 micron in a minority of objects (17-35%). Overall
this study supports previous work indicating AGN illumination as the dominant
heating mechanism for MFIR emitting dust in the majority of low to intermediate
redshift radio galaxies (0.03<z<0.7), with the advantage of strong statistical
evidence. However, we find evidence that the low redshift broad-line objects
(z<0.1) are distinct in terms of their positions on the MFIR vs. [OIII]
correlations.Comment: 31 pages, 3 figures, accepted for publication to Ap
Dust Emission from Active Galactic Nuclei
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central source, giving rise to Seyfert 1 line spectrum for
pole-on viewing and Seyfert 2 characteristics in edge-on sources. Although the
observed IR is in broad agreement with this scheme, the behavior of the 10
micron silicate feature and the width of the far-IR emission peak remained
serious problems in all previous modeling efforts. We show that these problems
find a natural explanation if the dust is contained in about 5-10 clouds along
radial rays through the torus. The spectral energy distributions (SED) of both
type 1 and type 2 sources are properly reproduced from different viewpoints of
the same object if the visual optical depth of each cloud is larger than about
60 and the clouds' mean free path increases roughly in proportion to radial
distance.Comment: 11 pages, submitted to ApJ Letter
Emission from Hot Dust in the Infrared Spectra of Gamma-ray Bright Blazars
A possible source of -ray photons observed from the jets of blazars
is inverse Compton scattering by relativistic electrons of infrared seed
photons from a hot, dusty torus in the nucleus. We use observations from the
Spitzer Space Telescope to search for signatures of such dust in the infrared
spectra of four -ray bright blazars, the quasars 4C 21.35, CTA102, and
PKS 1510089, and the BL Lacertae object ON231. The spectral energy
distribution (SED) of 4C 21.35 contains a prominent infrared excess indicative
of dust emission. After subtracting a non-thermal component with a power-law
spectrum, we fit a dust model to the residual SED. The model consists of a
blackbody with temperature K, plus a much weaker optically thin
component at K. The total luminosity of the thermal dust emission is
erg s. If the dust lies in an equatorial
torus, the density of IR photons from the torus is sufficient to explain the
-ray flux from 4C 21.35 as long as the scattering occurs within a few
parsecs of the central engine. We also report a tentative detection of dust in
the quasar CTA102, in which the luminosity of the infrared excess is erg s. However, in CTA102 the far-IR spectra are too
noisy to detect the m silicate feature. Upper limits to the luminosity
from thermal emission from dust in PKS 1510-089, and ON231, are,
, and erg s, respectively. These
upper limits do not rule out the possibility of inverse Compton up-scattering
of IR photons to -ray energies in these two sources. The estimated
covering factor of the hot dust in 4C 21.35, 22%, is similar to that of
non-blazar quasars; however, 4C 21.35 is deficient in cooler dust.Comment: 23 Pages, 5 Figures, 2 Tables, 1 Machine Readable Table. Accepted to
Ap
An original interferometric study of NGC 1068 with VISIR BURST mode images
We present 12.8 microns images of the core of NGC 1068 obtained with the
BURST mode of the VLT/VISIR. We trace structures under the diffraction limit of
one UT and we investigate the link between dust in the vicinity of the central
engine of NGC 1068, recently resolved by interferometry with MIDI, and more
extended structures. This step is mandatory for a multi-scale understanding of
the sources of mid-infrared emission in AGNs. A speckle processing of VISIR
BURST mode images was performed to extract very low spatial-frequency
visibilities, first considering the full field of VISIR BURST mode images and
then limiting it to the mask used for the acquisition of MIDI data. Extracted
visibilities are reproduced with a multi-component model. We identify two major
sources of emission: one compact < 85 mas, associated with the dusty torus, and
an elliptical one, (< 140) mas x 1187 mas at P.A.=-4 degrees from N to E. This
is consistent with previous deconvolution processes. The combination with MIDI
data reveals the close environment of the dusty torus to contribute to about 83
percent of the MIR flux seen by MIDI. This strong contribution has to be
considered in modeling long baseline interferometric data. It must be related
to the NS elongated component which is thought to originate from individually
unresolved dusty clouds and is located inside the ionization cone. Low
temperatures of the dusty torus are not challenged, emphasizing the scenarios
of clumpy torus.Comment: 10 pages, 7 figures, accepted for publication in A&
The power output of local obscured and unobscured AGN: crossing the absorption barrier with Swift/BAT and IRAS
The Swift/BAT 9-month catalogue of active galactic nuclei (AGN) provides an
unbiased census of local supermassive black hole accretion, and probes to all
but the highest levels of absorption in AGN. We explore a method for
characterising the bolometric output of both obscured and unobscured AGN by
combining the hard X-ray data from Swift/BAT (14-195keV) with the reprocessed
IR emission as seen with the IRAS all-sky surveys. This approach bypasses the
complex modifications to the SED introduced by absorption in the optical, UV
and 0.1-10 keV regimes and provides a long-term, average picture of the
bolometric output of these sources. We broadly follow the approach of Pozzi et
al. for calculating the bolometric luminosities by adding nuclear IR and hard
X-ray luminosities, and consider different approaches for removing non-nuclear
contamination in the large-aperture IRAS fluxes. Using mass estimates from the
M_BH-L_bulge relation, we present the Eddington ratios \lambda_Edd and 2-10 keV
bolometric corrections for a subsample of 63 AGN (35 obscured and 28
unobscured) from the Swift/BAT catalogue, and confirm previous indications of a
low Eddington ratio distribution for both samples. Importantly, we find a
tendency for low bolometric corrections (typically 10-30) for the obscured AGN
in the sample (with a possible rise from ~15 for \lambda_Edd<0.03 to ~32 above
this), providing a hitherto unseen window onto accretion processes in this
class of AGN. This finding is of key importance in calculating the expected
local black hole mass density from the X-ray background since it is composed of
emission from a significant population of such obscured AGN. Analogous studies
with high resolution IR data and a range of alternative models for the torus
emission will form useful future extensions to this work. (Abridged)Comment: 19 pages, 16 figures, 2 tables. Accepted for publication in MNRA
A Model for Type 2 Coronal Line Forest (CLiF) AGNs
We present a model for the classification of Coronal Line Forest Active Galactic Nuclei (CLiF AGNs). CLiF
AGNs are of special interest due to their remarkably large number of emission lines, especially forbidden highionization
lines (FHILs). Rose et al. suggest that their emission is dominated by reflection from the inner wall of
the obscuring region rather than direct emission from the accretion disk. This makes CLiF AGNs laboratories to
test AGN-torus models. Modeling an AGN as an accreting supermassive black hole surrounded by a cylinder of
dust and gas, we show a relationship between the viewing angle and the revealed area of the inner wall. From the
revealed area, we can determine the amount of FHIL emission at various angles. We calculate the strength of
[Fe VII]λ6087 emission for a number of intermediate angles (30°, 40°, and 50°) and compare the results with the
luminosity of the observed emission line from six known CLiF AGNs. We find that there is good agreement
between our model and the observational results. The model also enables us to determine the relationship between
the type 2:type 1 AGN fraction vs the ratio of torus height to radius, h/r
Infrared Spectral Energy Distributions of Seyfert Galaxies: Spitzer Space Telescope Observations of the 12 micron Sample of Active Galaxies
The mid-far-infrared spectral energy distributions (SEDs) of 83 active
galaxies, mostly Seyfert galaxies, selected from the extended 12 micron sample
are presented. The data were collected using all three instruments, IRAC, IRS,
and MIPS, aboard the Spitzer Space Telescope. The IRS data were obtained in
spectral mapping mode, and the photometric data from IRAC and IRS were
extracted from matched, 20 arcsec diameter circular apertures. The MIPS data
were obtained in SED mode, providing very low resolution spectroscopy (R ~ 20)
between ~ 55 and 90 microns in a larger, 20 by 30 arcsec synthetic aperture. We
further present the data from a spectral decomposition of the SEDs, including
equivalent widths and fluxes of key emission lines; silicate 10 and 18 micron
emission and absorption strengths; IRAC magnitudes; and mid-far infrared
spectral indices. Finally, we examine the SEDs averaged within optical
classifications of activity. We find that the infrared SEDs of Seyfert 1s and
Seyfert 2s with hidden broad line regions (HBLR, as revealed by
spectropolarimetry or other technique) are qualitatively similar, except that
Seyfert 1s show silicate emission and HBLR Seyfert 2s show silicate absorption.
The infrared SEDs of other classes with the 12 micron sample, including Seyfert
1.8-1.9, non-HBLR Seyfert 2 (not yet shown to hide a type 1 nucleus), LINER and
HII galaxies, appear to be dominated by star-formation, as evidenced by blue
IRAC colors, strong PAH emission, and strong far-infrared continuum emission,
measured relative to mid-infrared continuum emission.Comment: 78 pages, 13 figure
VLTI/VINCI observations of the nucleus of NGC 1068 using the adaptive optics system MACAO
We present the first near-infrared K-band long-baseline interferometric
measurement of the prototype Seyfert 2 galaxy NGC 1068 with resolution lambda/B
\~ 10 mas obtained with the Very Large Telescope Interferometer (VLTI) and the
two 8.2m Unit Telescopes UT2 and UT3. The adaptive optics system MACAO was
employed to deliver wavefront-corrected beams to the K-band commissioning
instrument VINCI. A squared visibility amplitude of 16.3 +/- 4.3 % was measured
for NGC 1068 at a sky-projected baseline length of 45.8 m and azimuth angle
44.9 deg. This value corresponds to a FWHM of the K-band intensity distribution
of 5.0 +/- 0.5 mas (0.4 +/- 0.04 pc) at the distance of NGC 1068) if it
consists of a single Gaussian component. Taking into account K-band speckle
interferometry observations (Wittkowski et al. 1998; Weinberger et al. 1999;
Weigelt et al. 2004), we favor a multi-component model for the intensity
distribution where a part of the flux originates from scales clearly smaller
than about 5 mas (<0.4 pc), and another part of the flux from larger scales.
The K-band emission from the small (< 5 mas) scales might arise from
substructure of the dusty nuclear torus, or directly from the central accretion
flow viewed through only moderate extinction.Comment: Accepted for publication in Astronomy and Astrophysics Letter
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