1,213 research outputs found
Reverberation Mapping of High-Luminosity Quasars: First Results
Reverberation mapping of nearby active galactic nuclei has led to estimates
of broad-line-region (BLR) sizes and central-object masses for some 37 objects
to date. However, successful reverberation mapping has yet to be performed for
quasars of either high luminosity (above L_opt~10^{46} erg/s) or high redshift
(z>0.3). Over the past six years, we have carried out, at the Hobby-Eberly
Telescope, rest-frame-ultraviolet spectrophotometric monitoring of a sample of
six quasars at redshifts z=2.2--3.2, with luminosities of
L_opt~10^{46.4}--10^{47.6} erg/s, an order of magnitude greater than those of
previously mapped quasars. The six quasars, together with an additional five
having similar redshift and luminosity properties, were monitored
photometrically at the Wise Observatory during the past decade. All 11 quasars
monitored show significant continuum variations of order 10%--70%. This is
about a factor of two smaller variability than for lower luminosity quasars
monitored over the same rest-frame period. In the six objects which have been
spectrophotometrically monitored, significant variability is detected in the
CIV1550 broad emission line. In several cases the variations track the
continuum variations in the same quasar, with amplitudes comparable to, or even
greater than, those of the corresponding continua. In contrast, no significant
Ly\alpha variability is detected in any of the four objects in which it was
observed. Thus, UV lines may have different variability trends in
high-luminosity and low-luminosity AGNs. For one quasar, S5~0836+71 at z=2.172,
we measure a tentative delay of 595 days between CIV and UV-continuum
variations, corresponding to a rest-frame delay of 188 days and a central
black-hole mass of 2.6\times10^9 M_\odot.Comment: 11 pages, 6 figures, emulateapj, accepted for publication in The
Astrophysical Journa
XMM-Newton RGS observation of the warm absorber in Mrk 279
The Seyfert 1 galaxy Mrk 279 was observed by XMM-Newton in November 2005 in
three consecutive orbits, showing significant short-scale variability (average
soft band variation in flux ~20%). The source is known to host a two-component
warm absorber with distinct ionisation states from a previous Chandra
observation. We aim to study the warm absorber in Mrk 279 and investigate any
possible response to the short-term variations of the ionising flux, and to
assess whether it has varied on a long-term time scale with respect to the
Chandra observation. We find no significant changes in the warm absorber on
neither short time scales (~2 days) nor at longer time scales (two and a half
years), as the variations in the ionic column densities of the most relevant
elements are below the 90% confidence level. The variations could still be
present but are statistically undetected given the signal-to-noise ratio of the
data. Starting from reasonable standard assumptions we estimate the location of
the absorbing gas, which is likely to be associated with the putative dusty
torus rather than with the Broad Line Region if the outflowing gas is moving at
the escape velocity or larger.Comment: 10 pages, 9 figures, 6 tables. Accepted for publication in Astronomy
& Astrophysic
The Hard X-ray Spectral Slope as an Accretion-Rate Indicator in Radio-Quiet Active Galactic Nuclei
We present new XMM-Newton observations of two luminous and high
accretion-rate radio-quiet active galactic nuclei (AGNs) at z~2. Together with
archival X-ray and rest-frame optical spectra of three sources with similar
properties as well as 25 moderate-luminosity radio-quiet AGNs at z<0.5, we
investigate, for the first time, the dependence of the hard (>~2 keV) X-ray
power-law photon index on the broad H_beta emission-line width and on the
accretion rate across ~3 orders of magnitude in AGN luminosity. Provided the
accretion rates of the five luminous sources can be estimated by extrapolating
the well-known broad-line region size-luminosity relation to high luminosities,
we find that the photon indices of these sources, while consistent with those
expected from their accretion rates, are significantly higher than expected
from the widths of their H_beta lines. We argue that, within the limits of our
sample, the hard-X-ray photon index depends primarily on the accretion rate.Comment: 4 pages (emulateapj), 2 figures. Accepted for publication in ApJ
Letter
X-ray Characteristics of NGC 3516: A View through the Complex Absorber
We consider new Suzaku data for NGC 3516 taken during 2009, along with other
recent X-ray observations of the source. The cumulative characteristics of NGC
3516 cannot be explained without invoking changes in the line-of-sight
absorption. Contrary to many other well-studied Seyfert galaxies, NGC 3516 does
not show a positive lag of hard X-ray photons relative to soft photons over the
timescales sampled. In the context of reverberation models for the X-ray lags,
the lack of such a signal in NGC 3516 is consistent with flux variations being
dominated by absorption changes. The lack of any reverberation signal in such a
highly variable source disfavors intrinsic continuum variability in this case.
Instead, the colorless flux variations observed at high flux states for NGC
3516 are suggested to be a consequence of Compton-thick clumps of gas crossing
the line-of-sight.Comment: 12 pages, 6 figures, accepted for publication in The Astrophysical
Journa
The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541
We have analyzed the broad absorption line system of the bright (V=16.5)
high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along
with optical data from the MMT and the Steward Observatory 2.3m telescope.
These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,
corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object
therefore offers a rare opportunity to study broad absorption lines in the
rest-frame extreme UV. We find that the absorption system is dominated by very
high-ionization species, including O VI, NeVIII, and SiXII. We also identify
apparently saturated broad Lyman-series lines of order Ly-gamma and higher.
There is strong evidence for partial occultation of the QSO emission source,
particularly from the higher-order Lyman lines which indicate a covered
fraction less than 0.2. Overall, the data suggest a correlation between a
larger covered fraction and a higher state of ionization. We suggest that the
different covered fractions can be explained by either a special line of sight
through a disk-like geometry or by the existence of density fluctuations of a
factor >2 in the BAL gas. Our photoionization models of the system indicate a
large column density and high ionization state similar to that found in X-ray
``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap
Implications from the optical to UV flux ratio of FeII emission in quasars
We investigate FeII emission in Broad Line Region (BLR) of AGNs by analyzing
the FeII(UV), FeII(4570) and MgII emission lines in 884 quasars in the Sloan
Digital Sky Survey (SDSS) Quasar catalog in a redshift range of 0.727 < z <
0.804. FeII(4570)/FeII(UV) is used to infer the column density of FeII-emitting
clouds and explore the excitation mechanism of FeII emission lines. As
suggested before in various works, the classical photoionization models fail to
account for FeII(4570)/FeII(UV) by a factor of 10, which may suggest anisotropy
of UV FeII emission; otherwise, an alternative heating mechanism like shock is
working. The column density distribution derived from FeII(4570)/FeII(UV)
indicates that radiation pressure plays an important role in BLR gas dynamics.
We find a positive correlation between FeII(4570)/FeII(UV) and the Eddington
ratio. We also find that almost all FeII-emitting clouds are to be under
super-Eddington conditions unless ionizing photon fraction is much smaller than
that previously suggested. Finally we propose a physical interpretation of a
striking set of correlations between various emission-line properties, known as
``Eigenvector 1''.Comment: 10 pages, 10 figures, accepted for publication in MNRA
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium Disks
We have constructed a grid of non-LTE disk models for a wide range of black
hole mass and mass accretion rate, for several values of viscosity parameter
alpha, and for two extreme values of the black hole spin: the maximum-rotation
Kerr black hole, and the Schwarzschild (non-rotating) black hole. Our procedure
calculates self-consistently the vertical structure of all disk annuli together
with the radiation field, without any approximations imposed on the optical
thickness of the disk, and without any ad hoc approximations to the behavior of
the radiation intensity. The total spectrum of a disk is computed by summing
the spectra of the individual annuli, taking into account the general
relativistic transfer function. The grid covers nine values of the black hole
mass between M = 1/8 and 32 billion solar masses with a two-fold increase of
mass for each subsequent value; and eleven values of the mass accretion rate,
each a power of 2 times 1 solar mass/year. The highest value of the accretion
rate corresponds to 0.3 Eddington. We show the vertical structure of individual
annuli within the set of accretion disk models, along with their local emergent
flux, and discuss the internal physical self-consistency of the models. We then
present the full disk-integrated spectra, and discuss a number of
observationally interesting properties of the models, such as
optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region,
polarization, and number of ionizing photons. Our calculations are far from
definitive in terms of the input physics, but generally we find that our models
exhibit rather red optical/UV colors. Flux discontinuities in the region of the
hydrogen Lyman limit are only present in cool, low luminosity models, while
hotter models exhibit blueshifted changes in spectral slope.Comment: 20 pages, 31 figures, ApJ in press, spectral models are available for
downloading at http://www.physics.ucsb.edu/~blaes/habk
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