2,562 research outputs found
Variability and the X-ray/UV ratio of Active Galactic Nuclei
The observed relation between the X-ray radiation from AGNs, originating in
the corona, and the optical/UV radiation from the disk is usually described by
the anticorrelation between the UV to X-ray slope alpha_ox and the UV
luminosity. Many factors can affect this relation, including: enhanced X-ray
emission associated with the jets of radio-loud AGNs; X-ray absorption
associated with the UV Broad Absorption Line (BAL) outflows; other X-ray
absorption not associated with BALs; intrinsic X-ray weakness; UV and X-ray
variability, and non-simultaneity of UV and X-ray observations. The separation
of these effects provides information about the intrinsic alpha_ox-L_UV
relation and its dispersion, constraining models of disk-corona coupling. We
extract simultaneous data from the second XMM-Newton serendipitous source
catalogue and the XMM-Newton Optical Monitor Serendipitous UV Source Survey
Catalog, and derive the single-epoch alpha_ox indices. We use ensemble
structure functions to analyse multi-epoch data. We confirm the anticorrelation
of alpha_ox with L_UV, and do not find any evidence of a dependence of alpha_ox
on z. The dispersion in our simultaneous data (0.12) is not significantly
smaller than in previous non-simultaneous studies, suggesting that "artificial
alpha_ox variability" introduced by non-simultaneity is not the main cause of
dispersion. "Intrinsic alpha_ox variability", i.e., the true variability of the
X-ray to optical ratio, is instead important, and accounts for ~30% of the
total variance, or more. "Inter-source dispersion", due to intrinsic
differences in the average alpha_ox values from source to source, is also
important. The dispersion introduced by variability is mostly caused by the
long timescale variations, which are expected to be driven by the optical
variations.Comment: 16 pages, 10 figures, 1 table. Final version equal to the published
on
Mid Infrared Spectra of Radio Galaxies and Quasars
Spitzer Infrared Spectrograph (IRS) observations of 3C radio galaxies and
quasars shed new light on the nature of the central engines of AGN. Emission
from silicate dust obscuring the central engine can be used to estimate the
bolometric luminosity of an AGN. Emission lines from ions such as O IV and Ne V
give another indication of the presence or lack of a hidden source of far-UV
photons in the nucleus. Radio-loud AGN with relative-to-Eddington luminosity
ratios of L/L_Edd < 3E-3 do not appear to have broad optical emission lines,
though some do have strong silicate emission. Aromatic emission features from
star formation activity are common in low-luminosity radio galaxies. Strong
molecular hydrogen pure-rotational emission lines are also seen in some mid-IR
weak radio galaxies, caused by either merger shocks or jet shocks in the
interstellar medium.Comment: Conference proceedings to appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
The AGN Obscuring Torus -- End of the "Doughnut" Paradigm?
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central engine. The compact sizes (only a few pc) determined
in recent high-resolution observations require that the obscuring matter be
clumpy and located inside the region where the black-hole gravity dominates
over the galactic bulge. This location is in line with the scenario depicting
the torus as the region of the clumpy wind coming off the accretion disk in
which the clouds are dusty and optically thick. We study here the outflow
scenario within the framework of hydromagnetic disk winds, incorporating the
cloud properties determined from detailed modeling of the IR emission from
clumpy tori. We find that torus clouds were likely detected in recent water
maser observations of NGC 3079. In the wind scenario, the AGN main dynamic
channel for release of accreted mass seems to be switching at low luminosities
from torus outflow to radio jets. The torus disappears when the bolometric
luminosity decreases below about \E{42} erg/sec because the accretion onto the
central black hole can no longer sustain the required cloud outflow rate. This
disappearance seems to have been observed in both LINERs and radio galaxies.
With further luminosity decrease, suppression of cloud outflow spreads radially
inward from the disk's dusty, molecular region into its atomic, ionized zone,
resulting in disappearance of the broad emission line region at lower
luminosities, yet to be determined.Comment: ApJ Letters, to be publishe
New Results from a Near-Infrared Search for Hidden Broad-Line Regions in Ultraluminous Infrared Galaxies
This paper reports the latest results from a near-infrared search for hidden
broad-line regions (BLRs: FWHM >~ 2,000 km/s) in ultraluminous infrared
galaxies (ULIGs). The new sample contains thirty-nine ULIGs from the 1-Jy
sample selected for their lack of BLRs at optical wavelengths. The results from
this new study are combined with those from our previous optical and
near-infrared surveys to derive the fraction of all ULIGs with optical or
near-infrared signs of genuine AGN activity (either a BLR or [Si VI] emission).
Comparisons of the dereddened emission-line luminosities of the optical or
obscured BLRs detected in the ULIGs of the 1-Jy sample with those of optical
quasars indicate that the obscured AGN/quasar in ULIGs is the main source of
energy in at least 15 -- 25% of all ULIGs in the 1-Jy sample. This fraction is
30 -- 50% among ULIGs with L_ir > 10^{12.3} L_sun. These results are compatible
with those from recent mid-infrared spectroscopic surveys carried out with ISO.
(abridged)Comment: 40 pages including 10 figures and 3 tables (Table 3 should be printed
in landscape mode
Variability and the X-ray/UV ratio of active galactic nuclei. II. Analysis of a low-redshift Swift sample
Variability, both in X-ray and optical/UV, affects the well-known anti-correlation between the spectral index and the UV luminosity of active galactic nuclei, contributing part of the dispersion around the average correlation ("intra-source dispersion"), in addition to the differences among the time-average values from source to source ("inter-source dispersion"). We want to evaluate the intrinsic variations in individual objects, and their effect on the dispersion of the anti-correlation. We use simultaneous UV/X-ray data from Swift observations of a low-redshift sample, to derive the epoch-dependent indices. We correct for the host galaxy contribution by a spectral fit of the optical/UV data. We compute ensemble structure functions to analyse variability of multi-epoch data. We find a strong "intrinsic variability", which makes an important contribution ( of the total variance) to the dispersion of the anti-correlation ("intra-source dispersion"). The strong X-ray variability and weaker UV variability of this sample are comparable to other samples of low-z AGNs, and are neither due to the high fraction of strongly variable NLS1s, nor to dilution of the optical variability by the host galaxies. Dilution affects instead the slope of the anti-correlation, which steepens, once corrected, becoming similar to higher luminosity sources. The structure function of increases with the time lag up to 1 month. This indicates the important contribution of the intermediate-long timescale variations, possibly generated in the outer parts of the accretion disk
An Extraordinary Scattered Broad Emission Line in a Type 2 QSO
An infrared-selected, narrow-line QSO has been found to exhibit an
extraordinarily broad Halpha emission line in polarized light. Both the extreme
width (35,000 km/sec full-width at zero intensity) and 3,000 km/sec redshift of
the line centroid with respect to the systemic velocity suggest emission in a
deep gravitational potential. An extremely red polarized continuum and partial
scattering of the narrow lines at a position angle common to the broad-line
emission imply extensive obscuration, with few unimpeded lines of sight to the
nucleus.Comment: 4 pages, 1 figure, to appear in the Astrophysical Journal Letter
Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. I. The Nature of the Continuum Emission
We present the first long-slit spectra of the Seyfert 2 galaxy NGC 1068
obtained by the Space Telescope Imaging Spectrograph (STIS); the spectra cover
the wavelength range 1150 - 10,270 Angstroms at a spatial resolution of 0.05 -
0.1 arcsec and a spectral resolving power of 1000. In this first paper, we
concentrate on the far-UV to near-IR continuum emission from the continuum
``hot spot'' and surrounding regions extending out to +/- 6 arcsec (+/-432 pc)
at a position angle of 202 degrees In addition to the broad emission lines
detected by spectropolarimetry, the hot spot shows the ``little blue bump'' in
the 2000 - 4000 Ang. range, which is due to Fe II and Balmer continuum
emission. The continuum shape of the hot spot is indistinguishable from that of
NGC 4151 and other Seyfert 1 galaxies. Thus, the hot spot is reflected emission
from the hidden nucleus, due to electron scattering (as opposed to
wavelength-dependent dust scattering). The hot spot is ~0.3 arcsec in extent
and accounts for 20% of the scattered light in the inner 500 pc. We are able to
deconvolve the extended continuum emission in this region into two components:
electron-scattered light from the hidden nucleus (which dominates in the UV)
and stellar light (which dominates in the optical and near-IR). The scattered
light is heavily concentrated towards the hot spot, is stronger in the
northeast, and is enhanced in regions of strong narrow-line emission. The
stellar component is more extended, concentrated southwest of the hot spot,
dominated by an old (> 2 x 10 Gyr) stellar population, and includes a nuclear
stellar cluster which is ~200 pc in extent.Comment: 32 pages, Latex, includes 11 figures (postscript), to appear in the
Astrophysical Journa
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