1,274 research outputs found
3C 216: A Powerful FRII Seyfert 1 Galaxy
3C 216 has a weak accretion flow luminosity, well below the Seyfert1/QSO
dividing line, weak broad emission lines (BELs) and powerful radio lobes. As a
consequence of the extreme properties of 3C 216, it is the most convincing
example known of an FR II radio source that is kinetically dominated: the jet
kinetic luminosity, , is larger than the total thermal luminosity (IR to
X-ray) of the accretion flow, . Using three independent estimators for
the central black hole mass, we find that the jet in 3C 216 is very
super-Eddington, , where is the long
term time averaged , calculated at 151 MHz. It is argued that 3C 216
satisfies the contemporaneous kinetically dominated condition, , either presently or in the past based on the rarity of
quasars. The existence of AGN is a strong constraint
on the theory of the central engine of FRII radio sources
Identifying Compact Symmetric Objects in the Southern Sky
We present results of multifrequency polarimetric VLBA observations of 20
compact radio sources. The observations represent the northern and southern
extensions of a large survey undertaken to identify Compact Symmetric Objects
(CSOs) Observed in the Northern Sky (COINS). CSOs are young radio galaxies
whose jet axes lie close to the plane of the sky, and whose appearance is
therefore not dominated by relativistic beaming effects. The small linear sizes
of CSOs make them valuable for studies of both the evolution of radio galaxies
and testing unified schemes for active galactic nuclei (AGN). In this paper we
report on observations made of 20 new CSO candidates discovered in the northern
and southern extremities of the VLBA Calibrator Survey. We identify 4 new CSOs,
and discard 12 core-jet sources. The remaining 4 sources remain candidates
pending further investigation. We present continuum images at 5 GHz and 15 GHz
and, where relevant, images of the polarized flux density and spectral index
distributions for the 8 new CSOs and CSO candidates.Comment: accepted to Ap
A Synoptic, Multiwavelength Analysis of a Large Quasar Sample
We present variability and multi-wavelength photometric information for the
933 known quasars in the QUEST Variability Survey. These quasars are grouped
into variable and non-variable populations based on measured variability
confidence levels. In a time-limited synoptic survey, we detect an
anti-correlation between redshift and the likelihood of variability. Our
comparison of variability likelihood to radio, IR, and X-ray data is consistent
with earlier quasar studies. Using already-known quasars as a template, we
introduce a light curve morphology algorithm that provides an efficient method
for discriminating variable quasars from periodic variable objects in the
absence of spectroscopic information. The establishment of statistically robust
trends and efficient, non-spectroscopic selection algorithms will aid in quasar
identification and categorization in upcoming massive synoptic surveys.
Finally, we report on three interesting variable quasars, including variability
confirmation of the BL Lac candidate PKS 1222+037.Comment: AJ, accepted for publication 15 Dec 200
Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN
Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we
study correlations between X-ray spectral features. The sample consists of 350
time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is
fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to
the central black hole that is reprocessed and absorbed by material around the
black hole. To test the robustness of our results, we performed Monte Carlo
simulations of the spectral sample. We find a complex relationship between the
iron line equivalent width (EW) and the underlying power law index (Gamma). The
data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2,
but finds a weak anti-correlation for steeper photon indices. We propose that
this relationship is driven by dilution of a disk spectrum (which includes the
narrow iron line) by a beamed jet component and, hence, could be used as a
diagnostic of jet-dominance. In addition, our sample shows a strong correlation
between the reflection fraction (R) and Gamma, but we find that it is likely
the result of modeling degeneracies. We also see the X-ray Baldwin effect (an
anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample
as a whole, but not for the individual galaxies and galaxy types.Comment: Accepted for publication in ApJ, 14 page
Radiation Pressure Supported Starburst Disks and AGN Fueling
We consider the structure of marginally Toomre-stable starburst disks under
the assumption that radiation pressure on dust grains provides the dominant
vertical support against gravity. This is particularly appropriate when the
disk is optically thick to its own IR radiation, as in the central regions of
ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for
star formation changes in the optically-thick limit, with the star formation
rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface
density and kappa is the mean opacity. We show that optically thick starburst
disks have a characteristic flux and dust effective temperature of F ~ 10^{13}
L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with
observations and find good agreement. We extend our model from many-hundred
parsec scales to sub-parsec scales and address the problem of fueling AGN. We
assume that angular momentum transport proceeds via global torques rather than
a local viscosity. We account for the radial depletion of gas due to star
formation and find a strong bifurcation between two classes of disk models: (1)
solutions with a starburst on large scales that consumes all of the gas with
little fueling of a central AGN and (2) models with an outer large-scale
starburst accompanied by a more compact starburst on 1-10 pc scales and a
bright central AGN. The luminosity of the latter models is in many cases
dominated by the AGN. We show that the vertical thickness of the starburst disk
on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration
in some Type 2 AGN. We also argue that the disk of young stars in the Galactic
Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes,
discussion tightened, references adde
AGN accretion disks as spatially resolved by polarimetry
A crucial difficulty in understanding the nature of the putative accretion
disk in AGNs is that some of its key intrinsic spectral signatures cannot be
observed directly. The strong emissions from the broad-line region (BLR) and
the obscuring torus, which are generally yet to be spatially resolved,
essentially 'bury' such signatures. Here we argue that we can actually isolate
the disk emission spectrum by using optical and near-infrared polarization of
quasars and uncover the important spectral signatures. In these quasars, the
polarization is considered to originate from electron scattering interior to
the BLR, so that the polarized flux shows the disk spectrum with all the
emissions from the BLR and torus eliminated. The polarized flux observations
have now revealed a Balmer edge feature in absorption and a blue near-infrared
spectral shape consistent with a specific and robust theoretical prediction.
These results critically verify the long-standing picture of an optically thick
and locally heated disk in AGNs.Comment: Proceedings for "The Universe under the Microscope" (AHAR 2008), held
in Bad Honnef (Germany) in April 2008, to be published in Journal of Physics:
Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart,
S. Pfalzner, and E. Ros (eds.
VLBI Polarimetry of 177 Sources from the Caltech-Jodrell Bank Flat-spectrum Survey
We present VLBA observations and a statistical analysis of 5 GHz VLBI
polarimetry data from 177 sources in the Caltech-Jodrell Bank flat-spectrum
(CJF) survey. The CJF survey, a complete, flux-density-limited sample of 293
extragalactic radio sources, gives us the unique opportunity to compare a broad
range of source properties for quasars, galaxies and BL Lacertae objects. We
focus primarily on jet properties, specifically the correlation between the jet
axis angle and the polarization angle in the core and jet. A strong correlation
is found for the electric vector polarization angle in the cores of quasars to
be perpendicular to the jet axis. Contrary to previous claims, no correlation
is found between the jet polarization angle and the jet axis in either quasars
or BL Lac objects. With this large, homogeneous sample we are also able to
investigate cosmological issues and AGN evolution.Comment: Accepted to the Astrophysical Journal: 37 pages, 14 figure
Obscuring Active Galactic Nuclei with Nuclear Starburst Disks
We assess the potential of nuclear starburst disks to obscure the
Seyfert-like AGN that dominate the hard X-ray background at z~1. Over 1200
starburst disk models, based on the theory developed by Thompson et al., are
calculated for five input parameters: the black hole mass, the radial size of
the starburst disk, the dust-to-gas ratio, the efficiency of angular momentum
transport in the disk, and the gas fraction at the outer disk radius. We find
that a large dust-to-gas ratio, a relatively small starburst disk, a
significant gas mass fraction, and efficient angular momentum transport are all
important to produce a starburst disk that can potentially obscure an AGN. The
typical maximum star-formation rate in the disks is ~10 solar masses per year.
Assuming no mass-loss due to outflows, the starburst disks feed gas onto the
black hole at rates sufficient to produce hard X-ray luminosities of
10^{43}-10^{44} erg s^{-1}. The starburst disks themselves should be detectable
at mid-infrared and radio wavelengths; at z=0.8, the predicted fluxes are ~1
mJy at 24microns and ~10-30 microJy at 1.4GHz. Thus, we predict a large
fraction of radio/X-ray matches in future deep radio surveys. The starburst
disks should be easily distinguished from AGN in future 100microns surveys by
Herschel with expected fluxes of ~5 mJy. Any AGN-obscuring starbursts will be
associated with hot dust, independent of AGN heating, resulting in observable
signatures for separating galactic and nuclear star-formation. Finally, because
of the competition between gas and star-formation, nuclear starbursts will be
associated with lower-luminosity AGN. Thus, this phenomenon is a natural
explanation for the observed decrease in the fraction of obscured AGN with
luminosity.Comment: 13 pages, 12 figures, 3 in color; accepted by Ap
Chandra observations of NGC4698: a Seyfert-2 with no absorption
We present Chandra ACIS-S observations of the enigmatic
Seyfert-2 galaxy NGC4698. This object together with several other bona-fide
Seyfert-2 galaxies show no absorption in the low spatial resolution ASCA data,
in contrast to the standard unification models. Our Chandra observations of
NGC4698 probe directly the nucleus allowing us to check whether nearby sources
contaminate the ASCA spectrum. Indeed, the Chandra observations show that the
ASCA spectrum is dominated by two nearby AGN. The X-ray flux of NGC4698 is
dominated by a nuclear source with luminosity L(0.3-8 keV) ~ 10^39, erg s-1
coincident with the radio nucleus. Its spectrum is well represented by a
power-law, ~ 2.2, obscured by a small column density of 5x10^20 cm-2 suggesting
that NGC4698 is an atypical Seyfert galaxy.
On the basis of its low luminosity we then interpret NGC4698 as a Seyfert
galaxy which lacks a broad-line region.Comment: 12 pages, to appear in Ap
Thermal Emission as a Test for Hidden Nuclei in Nearby Radio Galaxies
The clear sign of a hidden quasar inside a radio galaxy is the appearance of
quasar spectral features in its polarized (scattered) light. However that
observational test requires suitably placed scattering material to act as a
mirror, allowing us to see the nuclear light. A rather robust and more general
test for a hidden quasar is to look for the predicted high mid-IR luminosity
from the nuclear obscuring matter. The nuclear waste heat is detected and well
isolated in the nearest narrow line radio galaxy, Cen A. This confirms other
indications that Cen A does contain a modest quasar-like nucleus. However we
show here that M87 does not: at high spatial resolution, the mid-IR nucleus is
seen to be very weak, and consistent with simple synchrotron emission from the
base of the radio jet. This fairly robustly establishes that there are "real"
narrow line radio galaxies, without the putative accretion power, and with
essentially all the luminosity in kinetic form. Next we show the intriguing
mid-IR morphology of Cygnus A, reported previously by us and later discussed in
detail by Radomski et al. (2002). All of this mid-IR emission is consistent
with reprocessing by a hidden quasar, known to exist from spectropolarimetry by
Ogle et al. (1997) and other evidence.Comment: 21 pages, 5 figure
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