239 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
Ecosystem engineering creates a direct nutritional link between 600-m deep cold-water coral mounds and surface productivity
Cold-water corals (CWCs) form large mounds on the seafloor that are hotspots of biodiversity in the deep sea, but it remains enigmatic how CWCs can thrive in this food-limited environment. Here, we infer from model simulations that the interaction between tidal currents and CWC-formed mounds induces downwelling events of surface water that brings organic matter to 600-m deep CWCs. This positive feedback between CWC growth on carbonate mounds and enhanced food supply is essential for their sustenance in the deep sea and represents an example of ecosystem engineering of unparalleled magnitude. This ’topographically-enhanced carbon pump’ leaks organic matter that settles at greater depths. The ubiquitous presence of biogenic and geological topographies along ocean margins suggests that carbon sequestration through this pump is of global importance. These results indicate that enhanced stratification and lower surface productivity, both expected consequences of climate change, may negatively impact the energy balance of CWCs
Salient Factors in Predicting Student Success, Including Course Modality
Much discussion in higher education has focused upon predicting student learning, and how to identify students who may be at particular risk of failure. Little research has actually tackled that challenge, and research on the scholarship of teaching and learning (SoTL) in this areas is scarce; this study does so by measuring students across three semester of study in a variety of courses and course formats. Our results indicate that a set of characteristics predicting student success can be identified, and that course modality affects overall student success rate. Our results are discussed in terms of how they might inform faculty and administrators how best to identify at risk groups of students, and who other researchers might expand on these results to produce a more nuanced interactive understanding of the interplay among students, courses, course modalities, and other characteristics to identify ideal combinations of those factors leading to student success
Variability-selected quasars behind the Small Magellanic Cloud
We present followup spectroscopic observations of quasar candidates in the
Small Magellanic Cloud selected by Eyer from the OGLE database. Of twelve
observed objects identified as "QSO Candidate", five are confirmed quasars,
with the emission redshifts ranging from 0.28 to 2.16. Two of those quasars
were also recently identified independently in the MACHO database by Geha et
al. We discuss the prospects of using variability-based selection technique for
quasar searches behind other dense stellar fields. An additional criterion
utilizing the color-color diagram should reduce the number of stars in the
candidate lists.Comment: Revised version, AASTeX, 11 pages, 3 EPS figures, one table, accepted
14 Nov 2002 for publication in the Astronomical Journal, March 2003 issu
Ensemble Variability of Near-Infrared-Selected Active Galactic Nuclei
We present the properties of the ensemble variability for nearly 5000
near-infrared (NIR) AGNs selected from the catalog of Quasars and Active
Galactic Nuclei (13th Ed.) and the SDSS-DR7 quasar catalog. From 2MASS, DENIS,
and UKIDSS/LAS point source catalogs, we extract 2MASS-DENIS and 2MASS-UKIDSS
counterparts for cataloged AGNs by catalog cross-identification. We further
select variable AGNs based on an optimal criterion for selecting the variable
sources. The sample objects are divided into subsets according to whether NIR
light originates by optical or NIR emission in the rest frame; and we examine
the correlations of the ensemble variability with the rest-frame wavelength,
redshift, luminosity, and rest-frame time lag. In addition, we also examine the
correlations of variability amplitude with optical variability, radio
intensity, and radio-to-optical flux ratio. The rest-frame optical variability
of our samples shows known negative correlations with luminosity and positive
correlations with rest-frame time lag (i.e., the structure function, SF).
However, no well-known negative correlation exists between the rest wavelength
and optical variability. This inconsistency might be due to a biased sampling
of high-redshift AGNs. NIR variability in the rest frame is anticorrelated with
the rest wavelength, which is consistent with previous suggestions. However,
correlations of NIR variability with luminosity and rest-frame time lag are the
opposite of these correlations of the optical variability; that is, the NIR
variability is positively correlated with luminosity but negatively correlated
with the rest-frame time lag. Because these trends are qualitatively consistent
with the properties of radio-loud quasars reported by some previous studies,
most of our sample objects are probably radio-loud quasars. Finally, we also
discuss the negative correlations seen in the NIR SFs.Comment: 13 pages, 10 figures, Accepted for publication in Ap
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
Selecting Quasars by their Intrinsic Variability
We present a new and simple technique for selecting extensive, complete and
pure quasar samples, based on their intrinsic variability. We parametrize the
single-band variability by a power-law model for the light-curve structure
function, with amplitude A and power-law index gamma. We show that quasars can
be efficiently separated from other non-variable and variable sources by the
location of the individual sources in the A-gamma plane. We use ~60 epochs of
imaging data, taken over ~5 years, from the SDSS stripe 82 (S82) survey, where
extensive spectroscopy provides a reference sample of quasars, to demonstrate
the power of variability as a quasar classifier in multi-epoch surveys. For
UV-excess selected objects, variability performs just as well as the standard
SDSS color selection, identifying quasars with a completeness of 90% and a
purity of 95%. In the redshift range 2.5<z<3, where color selection is known to
be problematic, variability can select quasars with a completeness of 90% and a
purity of 96%. This is a factor of 5-10 times more pure than existing
color-selection of quasars in this redshift range. Selecting objects from a
broad griz color box without u-band information, variability selection in S82
can afford completeness and purity of 92%, despite a factor of 30 more
contaminants than quasars in the color-selected feeder sample. This confirms
that the fraction of quasars hidden in the 'stellar locus' of color-space is
small. To test variability selection in the context of Pan-STARRS 1 (PS1) we
created mock PS1 data by down-sampling the S82 data to just 6 epochs over 3
years. Even with this much sparser time sampling, variability is an
encouragingly efficient classifier. For instance, a 92% pure and 44% complete
quasar candidate sample is attainable from the above -selected catalog.Comment: 16 pages, 9 color figures and 5 tables - v3: Equations corrected and
text updated (see Erratum for details of corrections). Erratum:
http://adsabs.harvard.edu/abs/2010ApJ...721.1941S Original Paper:
http://adsabs.harvard.edu/abs/2010ApJ...714.1194
Time Domain Explorations With Digital Sky Surveys
One of the new frontiers of astronomical research is the exploration of time
variability on the sky at different wavelengths and flux levels. We have
carried out a pilot project using DPOSS data to study strong variables and
transients, and are now extending it to the new Palomar-QUEST synoptic sky
survey. We report on our early findings and outline the methodology to be
implemented in preparation for a real-time transient detection pipeline. In
addition to large numbers of known types of highly variable sources (e.g., SNe,
CVs, OVV QSOs, etc.), we expect to find numerous transients whose nature may be
established by a rapid follow-up. Whereas we will make all detected variables
publicly available through the web, we anticipate that email alerts would be
issued in the real time for a subset of events deemed to be the most
interesting. This real-time process entails many challenges, in an effort to
maintain a high completeness while keeping the contamination low. We will
utilize distributed Grid services developed by the GRIST project, and implement
a variety of advanced statistical and machine learning techniques.Comment: 5 pages, 2 postscript figures, uses adassconf.sty. To be published
in: "ADASS XIV (2004)", Eds. Patrick Shopbell, Matthew Britton and Rick
Ebert, ASP Conference Serie
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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