53 research outputs found
The Bull's-Eye Effect as a Probe of
We compare the statistical properties of structures normal and transverse to
the line of sight which appear in theoretical N-body simulations of structure
formation, and seem also to be present in observational data from redshift
surveys. We present a statistic which can quantify this effect in a
conceptually different way from standard analyses of distortions of the
power-spectrum or correlation function. From tests with --body experiments,
we argue that this statistic represents a new and potentially powerful
diagnostic of the cosmological density parameter, .Comment: Minor revisions; final version accepted for publication in ApJ
Letters. Latex, 16 pages, including 3 figures. Higher resolution versions of
figures, including supplementary figures not included in the manuscript, are
available at: ftp://kusmos.phsx.ukans.edu/preprints/melott/omeg
The Effect of Variability on the Estimation of Quasar Black Hole Masses
We investigate the time-dependent variations of ultraviolet (UV) black hole
mass estimates of quasars in the Sloan Digital Sky Survey (SDSS). From SDSS
spectra of 615 high-redshift (1.69 < z < 4.75) quasars with spectra from two
epochs, we estimate black hole masses, using a single-epoch technique which
employs an additional, automated night-sky-line removal, and relies on UV
continuum luminosity and CIV (1549A) emission line dispersion. Mass estimates
show variations between epochs at about the 30% level for the sample as a
whole. We determine that, for our full sample, measurement error in the line
dispersion likely plays a larger role than the inherent variability, in terms
of contributing to variations in mass estimates between epochs. However, we use
the variations in quasars with r-band spectral signal-to-noise ratio greater
than 15 to estimate that the contribution to these variations from inherent
variability is roughly 20%. We conclude that these differences in black hole
mass estimates between epochs indicate variability is not a large contributer
to the current factor of two scatter between mass estimates derived from low-
and high-ionization emission lines.Comment: 76 pages, 15 figures, 2 (long) tables; Accepted for publication in
ApJ (November 10, 2007
Spectral Variability of Quasars in the Sloan Digital Sky Survey. II: The C IV Line
We examine the variability of the high-ionizaton C IV line in a sample of 105
quasars observed at multiple epochs by the Sloan Digital Sky Survey. We find a
strong correlation between the change in the C IV line flux and the change in
the line width, but no correlations between the change in flux and changes in
line center and skewness. The relation between line flux change and line width
change is consistent with a model in which a broad line base varies with
greater amplitude than the line core. The objects studied here are more
luminous and at higher redshift than those normally studied for variability,
ranging in redshift from 1.65 to 4.00 and in absolute r-band magnitude from
roughly -24 to -28. Using moment analysis line-fitting techniques, we measure
line fluxes, centers, widths and skewnesses for the C IV line at two epochs for
each object. The well-known Baldwin Effect is seen for these objects, with a
slope beta = -0.22. The sample has a median intrinsic Baldwin Effect slope of
beta = -0.85; the C IV lines in these high-luminosity quasars appear to be less
responsive to continuum variations than those in lower luminosity AGN.
Additionally, we find no evidence for variability of the well known blueshift
of the C IV line with respect to the low-ionization Mg II line in the highest
flux objects, indicating that this blueshift might be useful as a measure of
orientation.Comment: 52 pages, 14 figures, accepted for publication in Ap
Cluster Winds Blow along Supercluster Axes
Within Abell galaxy clusters containing Wide-Angle Tailed radio sources, there is evidence of a ``prevailing wind'' which directs the WAT jets. We study the alignment of nine WAT jets and nearby clusters to test the idea that this wind may be a fossil of drainage along large-scale filaments. We also test this idea with a study of the alignment of WAT jets and filament axes. Statistical tests indicate no significant alignment of WAT jets towards nearest neighbour clusters, but a highly significant alignment with the long axis of the supercluster in which the cluster lies
On the variability of quasars: a link between Eddington ratio and optical variability?
Repeat scans by the Sloan Digital Sky Survey (SDSS) of a 278 square degree
stripe along the Celestial equator have yielded an average of over 10
observations each for nearly 8,000 spectroscopically confirmed quasars. Over
2500 of these quasars are in the redshift range such that the CIV emission line
is visible in the SDSS spectrum. Utilising the width of these CIV lines and the
luminosity of the nearby continuum, we estimate black hole masses for these
objects. In an effort to isolate the effects of black hole mass and luminosity
on the photometric variability of our dataset, we create several subsamples by
binning in these two physical parameters. By comparing the ensemble structure
functions of the quasars in these bins, we are able to reproduce the well-known
anticorrelation between luminosity and variability, now showing that this
anticorrelation is independent of the black hole mass. In addition, we find a
correlation between variability and the mass of the central black hole. By
combining these two relations, we identify the Eddington ratio as a possible
driver of quasar variability, most likely due to differences in accretion
efficiency.Comment: 13 pages, 5 figures, Accepted for publication in MNRA
Characteristic QSO Accretion Disk Temperatures from Spectroscopic Continuum Variability
Using Sloan Digital Sky Survey (SDSS) quasar spectra taken at multiple
epochs, we find that the composite flux density differences in the rest frame
wavelength range 1300-6000 AA can be fit by a standard thermal accretion disk
model where the accretion rate has changed from one epoch to the next (without
considering additional continuum emission components). The fit to the composite
residual has two free parameters: a normalizing constant and the average
characteristic temperature . In turn the characteristic temperature
is dependent on the ratio of the mass accretion rate to the square of the black
hole mass. We therefore conclude that most of the UV/optical variability may be
due to processes involving the disk, and thus that a significant fraction of
the UV/optical spectrum may come directly from the disk.Comment: 31 pages, 8 figure
Difference Imaging of Lensed Quasar Candidates in the SDSS Supernova Survey Region
Difference imaging provides a new way to discover gravitationally lensed
quasars because few non-lensed sources will show spatially extended, time
variable flux. We test the method on lens candidates in the Sloan Digital Sky
Survey (SDSS) Supernova Survey region from the SDSS Quasar Lens Search (SQLS)
and their surrounding fields. Starting from 20768 sources, including 49 SDSS
quasars and 36 candidate lenses/lensed images, we find that 21 sources
including 15 SDSS QSOs and 7 candidate lenses/lensed images are non-periodic
variable sources. We can measure the spatial structure of the variable flux for
18 of these sources and identify only one as a non-point source. This source
does not display the compelling spatial structure of the variable flux of known
lensed quasars, so we reject it as a lens candidate. None of the lens
candidates from the SQLS survive our cuts. Given our effective survey area of
order 0.71 square degrees, this indicates a false positive rate of order one
per square degree for themethod. The fraction of quasars not found to be
variable and the false positive rate should both fall if we analyze the full,
later data releases for the SDSS fields. While application of the method to the
SDSS is limited by the resolution, depth, and sampling of the survey, several
future surveys such as Pan-STARRS, LSST, and SNAP will avoid these limitations.Comment: Submitted to ApJ, 24 pages, 5 figure
Broad Absorption Line Variability in Repeat Quasar Observations from the Sloan Digital Sky Survey
We present a time-variability analysis of 29 broad absorption line quasars
(BALQSOs) observed in two epochs by the Sloan Digital Sky Survey (SDSS). These
spectra are selected from a larger sample of BALQSOs with multiple observations
by virtue of exhibiting a broad CIV 1549 absorption trough separated
from the rest frame of the associated emission peak by more than 3600 km
s. Detached troughs facilitate higher precision variability
measurements, since the measurement of the absorption in these objects is not
complicated by variation in the emission line flux. We have undertaken a
statistical analysis of these detached-trough BALQSO spectra to explore the
relationships between BAL features that are seen to vary and the dynamics of
emission from the quasar central engine. We have measured variability within
our sample, which includes three strongly variable BALs. We have also verified
that the statistical behavior of the overall sample agrees with current model
predictions and previous studies of BAL variability. Specifically, we observe
that the strongest BAL variability occurs among the smallest equivalent width
features and at velocities exceeding 12,000 km s, as predicted by recent
disk-wind modeling.Comment: 11 pages, 7 figures. Accepted for publication in Ap
Spectral Variability of Quasars in the Sloan Digital Sky Survey. I: Wavelength Dependence
Sloan Digital Sky Survey (SDSS) repeat spectroscopic observations have
resulted in multiple-epoch spectroscopy for roughly 2500 quasars observed more
than 50 days apart. From this sample, we identify 315 quasars that have varied
significantly between observations. We create an ensemble difference spectrum
(bright phase minus faint phase) covering rest-frame wavelengths from 1000 to
6000 Angstroms. This average difference spectrum is bluer than the average
single-epoch quasar spectrum; a power-law fit to the difference spectrum yields
a spectral index alpha_lambda = -2.00, compared to an index of alpha_lambda =
-1.35 for the single-epoch spectrum. The strongest emission lines vary only 30%
as much as the continuum. Due to the lack of variability of the lines, measured
photometric color is not always bluer in brighter phases, but depends on
redshift and the filters used. Lastly, the difference spectrum is bluer than
the ensemble quasar spectrum only for lambda_rest < 2500 Angstroms, indicating
that the variability cannot result from a simple scaling of the average quasar
spectrum.Comment: 47 pages, 14 figures, 3 tables, accepted for publication in Ap
The Ensemble Photometric Variability of ~25000 Quasars in the Sloan Digital Sky Survey
Using a sample of over 25000 spectroscopically confirmed quasars from the
Sloan Digital Sky Survey, we show how quasar variability in the rest frame
optical/UV regime depends upon rest frame time lag, luminosity, rest
wavelength, redshift, the presence of radio and X-ray emission, and the
presence of broad absorption line systems. The time dependence of variability
(the structure function) is well-fit by a single power law on timescales from
days to years. There is an anti-correlation of variability amplitude with rest
wavelength, and quasars are systematically bluer when brighter at all
redshifts. There is a strong anti-correlation of variability with quasar
luminosity. There is also a significant positive correlation of variability
amplitude with redshift, indicating evolution of the quasar population or the
variability mechanism. We parameterize all of these relationships. Quasars with
RASS X-ray detections are significantly more variable (at optical/UV
wavelengths) than those without, and radio loud quasars are marginally more
variable than their radio weak counterparts. We find no significant difference
in the variability of quasars with and without broad absorption line troughs.
Models involving multiple discrete events or gravitational microlensing are
unlikely by themselves to account for the data. So-called accretion disk
instability models are promising, but more quantitative predictions are needed.Comment: 41 pages, 21 figures, AASTeX, Accepted for publication in Ap
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