440 research outputs found
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
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
The Evolution of Quasar CIV and SiIV Broad Absorption Lines Over Multi-Year Time Scales
We investigate the variability of CIV 1549A broad absorption line (BAL)
troughs over rest-frame time scales of up to ~7 yr in 14 quasars at redshifts
z>2.1. For 9 sources at sufficiently high redshift, we also compare CIV and
SiIV 1400A absorption variation. We compare shorter- and longer-term
variability using spectra from up to four different epochs per source and find
complex patterns of variation in the sample overall. The scatter in the change
of absorption equivalent width (EW), Delta EW, increases with the time between
observations. BALs do not, in general, strengthen or weaken monotonically, and
variation observed over shorter (<months) time scales is not predictive of
multi-year variation. We find no evidence for asymmetry in the distribution of
Delta EW that would indicate that BALs form and decay on different time scales,
and we constrain the typical BAL lifetime to be >~30 yr. The BAL absorption for
one source, LBQS 0022+0150, has weakened and may now be classified as a
mini-BAL. Another source, 1235+1453, shows evidence of variable, blue continuum
emission that is relatively unabsorbed by the BAL outflow. CIV and SiIV BAL
shape changes are related in at least some sources. Given their high
velocities, BAL outflows apparently traverse large spatial regions and may
interact with parsec-scale structures such as an obscuring torus. Assuming BAL
outflows are launched from a rotating accretion disk, notable azimuthal
symmetry is required in the outflow to explain the relatively small changes
observed in velocity structure over times up to 7 yr
Probing Spectroscopic Variability of Galaxies & Narrow-Line Active Galactic Nuclei in the Sloan Digital Sky Survey
Under the unified model for active galactic nuclei (AGNs), narrow-line (Type
2) AGNs are, in fact, broad-line (Type 1) AGNs but each with a heavily obscured
accretion disk. We would therefore expect the optical continuum emission from
Type 2 AGN to be composed mainly of stellar light and non-variable on the
time-scales of months to years. In this work we probe the spectroscopic
variability of galaxies and narrow-line AGNs using the multi-epoch data in the
Sloan Digital Sky Survey (SDSS) Data Release 6. The sample contains 18,435
sources for which there exist pairs of spectroscopic observations (with a
maximum separation in time of ~700 days) covering a wavelength range of
3900-8900 angstrom. To obtain a reliable repeatability measurement between each
spectral pair, we consider a number of techniques for spectrophotometric
calibration resulting in an improved spectrophotometric calibration of a factor
of two. From these data we find no obvious continuum and emission-line
variability in the narrow-line AGNs on average -- the spectroscopic variability
of the continuum is 0.07+/-0.26 mag in the g band and, for the emission-line
ratios log10([NII]/Halpha) and log10([OIII]/Hbeta), the variability is
0.02+/-0.03 dex and 0.06+/-0.08 dex, respectively. From the continuum
variability measurement we set an upper limit on the ratio between the flux of
varying spectral component, presumably related to AGN activities, and that of
host galaxy to be ~30%. We provide the corresponding upper limits for other
spectral classes, including those from the BPT diagram, eClass galaxy
classification, stars and quasars.Comment: AJ accepte
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
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
Performance assessment of aero-assisted orbital transfer vehicles
Aero-assisted orbital transfer vehicles are analyzed. The aerodynamic characteristics over the flight profile and three- and six-degree-of-freedom performance analyses were determined. The important results, to date, are: (1) the aerodynamic preliminary analysis system, an interactive computer program, used to predict the aerodynamics (performance, stability, and control) for these vehicles; (2) the performance capability, e.g., maximum inclination change, maximum heating rate, and maximum sensed acceleration, can be determined using continuum aerodynamics only; (3) guidance schemes can be developed that allow for errors in atmospheric density prediction, mispredicted trim angle of attack, and off-nominal atmospheric interface conditions, even for vehicles with a low lift-to-drag ratio; and (4) multiple pass trajectories can be used to reduce the maximum heating rate
A Description of Quasar Variability Measured Using Repeated SDSS and POSS Imaging
We provide a quantitative description and statistical interpretation of the
optical continuum variability of quasars. The Sloan Digital Sky Survey (SDSS)
has obtained repeated imaging in five UV-to-IR photometric bands for 33,881
spectroscopically confirmed quasars. About 10,000 quasars have an average of 60
observations in each band obtained over a decade along Stripe 82 (S82), whereas
the remaining ~25,000 have 2-3 observations due to scan overlaps. The observed
time lags span the range from a day to almost 10 years, and constrain quasar
variability at rest-frame time lags of up to 4 years, and at rest-frame
wavelengths from 1000A to 6000A. We publicly release a user-friendly catalog of
quasars from the SDSS Data Release 7 that have been observed at least twice in
SDSS or once in both SDSS and the Palomar Observatory Sky Survey, and we use it
to analyze the ensemble properties of quasar variability. Based on a damped
random walk (DRW) model defined by a characteristic time scale and an
asymptotic variability amplitude that scale with the luminosity, black hole
mass, and rest wavelength for individual quasars calibrated in S82, we can
fully explain the ensemble variability statistics of the non-S82 quasars such
as the exponential distribution of large magnitude changes. All available data
are consistent with the DRW model as a viable description of the optical
continuum variability of quasars on time scales of ~5-2000 days in the rest
frame. We use these models to predict the incidence of quasar contamination in
transient surveys such as those from PTF and LSST.Comment: 33 pages, 19 figures, replaced with accepted version. Catalog is
available at http://www.astro.washington.edu/users/ivezic/macleod/qso_dr7
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