562 research outputs found
Radio Properties of Low Redshift Broad Line Active Galactic Nuclei
The question as to whether the distribution of radio-loudness in active
galactic nuclei (AGN) is actually bimodal has been discussed extensively in the
literature. Futhermore, there have been claims that radio-loudness depends on
black hole mass and Eddington ratio. We investigate these claims using the low
redshift broad line AGN sample of Greene & Ho (2007), which consists of 8434
objects at z < 0.35 from the Sloan Digital Sky Survey Fourth Data Release (SDSS
DR4). We obtained radio fluxes from the Very Large Array Faint Images of the
Radio Sky at Twenty-Centimeters (FIRST) survey for the SDSS AGN. Out of the
8434 SDSS AGN, 821 have corresponding observed radio fluxes in the FIRST
survey. We calculated the radio-loudness parameter (R) for all objects above
the FIRST detection limit (1 mJy), and an upper limit to R for the undetected
objects. Using these data, the question of radio bimodality is investigated for
different subsets of the total sample. We find no clear demarcation between the
radio-loud (RL, R > 10) and radio-quiet (RQ, R < 10) objects, but instead fill
in a more radio-intermediate population in a continuous fashion for all
subsamples. We find that 4.7% of the AGN in the flux-limited subsample are RL
based on core radio emission alone. We calculate the radio-loud fraction (RLF)
as both a function of black hole mass and Eddington ratio. The RLF decreases
(from 13% to 2%) as Eddington ratio increases over 2.5 order of magnitude. The
RLF is nearly constant (~5%) over 4 decades in black hole mass, except for an
increase at masses greater than 10^8 solar masses. We find for the FIRST
detected subsample that 367 of the RL AGN have black hole masses less than 10^8
solar masses, a large enough number to indicate that RL AGN are not a product
of only the most massive black holes in the local universe.Comment: 28 pages, 14 figures, accepted to A
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei
We present self-consistent models of the vertical structure and emergent
spectrum of AGN accretion disks. The central object is assumed to be a
supermassive Kerr black hole. We demonstrate that NLTE effects and the effects
of a self-consistent vertical structure of a disk play a very important role in
determining the emergent radiation, and therefore should be taken into account.
In particular, NLTE models exhibit a largely diminished H I Lyman discontinuity
when compared to LTE models, and the He II discontinuity appears strongly in
emission for NLTE models. Consequently, the number of ionizing photons in the
He II Lyman continuum predicted by NLTE disk models is by 1 - 2 orders of
magnitude higher than that following from the black-body approximation. This
prediction has important implications for ionization models of AGN broad line
regions, and for models of the intergalactic radiation field and the ionization
of helium in the intergalactic medium.Comment: 11 pages; 2 postscript figures; LaTeX, AASPP4 macro; to appear in the
Astrophysical Journal (Letters
Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei: I. Comparing the Photoionization and Reverberation Techniques
The masses and emission-line region sizes of Active Galactic Nuclei (AGNs)
can be measured by ``reverberation-mapping'' (measuring the lag of the
emission-line luminosity after changes in the continuum). We use tis technique
to calibrate similar size and mass estimates made by photoionization models of
the AGN line-emitting regions. We compile a sample of 19 AGNs with reliable
reverberation and spectroscopy data, twice the number available previously. The
data provide strong evidence that the BLR size and the emission-line width
measure directly the central mass. Two methods are used to estimate the
distance of the broad emission-line region (BLR) from the ionizing source: the
photoionization method (available for many AGNs but has large intrinsic
uncertainties), and the reverberation method (gives very reliable distances,
but available for only a few objects). The distance estimate is combined with
the velocity dispersion, derived from the broad Hb line profile, to estimate
the virial mass. Comparing the central masses calculated with the reverberation
method to those calculated using a photoionization model, we find a highly
significant, nearly linear correlation. This provides a calibration of the
photoionization method on the objects with presently available reverberation
data, which should enable mass estimates for all AGNs with measured Hb line
width. Comparing the BLR sizes given by the two methods also enables us to
estimate the ionizing EUV luminosity which is directly unobservable. We find it
to be typically ten times the visible (monochromatic luminosity at 5100A). The
inferred Eddington ratio of the individual objects in our sample are 0.001-0.03
(visible luminosity) and 0.01-0.3 (ionizing luminosity).Comment: 27 pages Latex, 8 figures. Accepted for publication in the
Astrophysical Journa
Determination of the X-ray reflection emissivity profile of 1H 0707-495
When considering the X-ray spectrum resulting from the reflection off the
surface of accretion discs of AGN, it is necessary to account for the variation
in reflected flux over the disc, i.e. the emissivity profile. This will depend
on factors including the location and geometry of the X-ray source and the disc
characteristics. We directly obtain the emissivity profile of the disc from the
observed spectrum by considering the reflection component as the sum of
contributions from successive radii in the disc and fitting to find the
relative weightings of these components in a relativistically-broadened
emission line. This method has successfully recovered known emissivity profiles
from synthetic spectra and is applied to XMM-Newton spectra of the Narrow Line
Seyfert 1 galaxy 1H 0707-495. The data imply a twice-broken power law form of
the emissivity law with a steep profile in the inner regions of the disc (index
7.8) and then a flat region between 5.6rg and 34.8rg before tending to a
constant index of 3.3 over the outer regions of the disc. The form of the
observed emissivity profile is consistent with theoretical predictions, thus
reinforcing the reflection interpretation.Comment: 9 pages, 10 figures. Accepted for publication in MNRA
The radio emission in radio-quiet quasars: the VLBA perspective
The origin of the radio emission in radio-quiet quasars (RQQ) is not
established yet. We present new VLBA observations at 1.6 and 4.9 GHz of ten RQQ
(nine detected), which together with published earlier observations of eight
RQQ (five detected), forms a representative sample of 18 RQQ drawn from the
Palomar-Green sample of low z (< 0.5) AGN. The spectral slope of the integrated
emission extends from very steep (alpha < -1.98) to strongly inverted (alpha =
+2.18), and the slopes of nine of the 14 objects are flat (alpha > -0.5). Most
objects have an unresolved flat-spectrum core, which coincides with the optical
Gaia position. The extended emission is generally steep-spectrum, has a low
brightness temperature (< 10^7 K), and is displaced from the optical core (the
Gaia position) by ~ 5-100 pc. The VLBA core flux is tightly correlated with the
X-ray flux, and follows a radio to X-ray luminosity relation of log L_R/L_X =
-6, for all objects with a black hole mass log M_BH/M_Sun < 8.5. The flatness
of the core emission implies a compact source size (< 0.1 pc), which likely
originates from the accretion disk corona. The mas-scale extended emission is
optically thin and of clumpy structure, and is likely produced by an outflow
from the center. Radio observations at higher frequencies can further test the
accretion disk coronal emission interpretation for the core emission in RQQ.Comment: Accepted for publication in MNRA
Infrared emission from interstellar dust cloud with two embedded sources: IRAS 19181+1349
Mid and far infrared maps of many Galactic star forming regions show multiple
peaks in close proximity, implying more than one embedded energy sources. With
the aim of understanding such interstellar clouds better, the present study
models the case of two embedded sources. A radiative transfer scheme has been
developed to deal with an uniform density dust cloud in a cylindrical geometry,
which includes isotropic scattering in addition to the emission and absorption
processes. This scheme has been applied to the Galactic star forming region
associated with IRAS 19181+1349, which shows observational evidence for two
embedded energy sources. Two independent modelling approaches have been
adopted, viz., to fit the observed spectral energy distribution (SED) best; or
to fit the various radial profiles best, as a function of wavelength. Both the
models imply remarkably similar physical parameters.Comment: 17 pages, 6 Figures, uses epsf.sty. To appear in Journal of
Astronophysics & Astronom
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