42 research outputs found
Societal Adaptation to Decadal Climate Variability in the United States
The search for evidence of decadal climatic variability (DCV) has a very long history. In the past decade, a research community has coalesced around a series of roughly biennial workshops that have emphasized description of past DCV events; their causes and their teleconnections responsible for droughts, floods, and warm and cold spells around the world; and recently, the predictability of DCV events. Researchers studying climate change put great emphasis on prospective impacts, but the DCV community has yet to do so. To begin rectifying this deficiency, a short but ambitious workshop was convened in Waikoloa, near Kona, Hawaii, from 26-28 April 2007. This workshop, sponsored by the Center for Research on the Changing Earth System (CRCES), NOAA, the U.S. Geological Survey, and the U.S. Army Corps of Engineers, brought together climatologists and sectoral specialists representing agriculture, water resources, economics, the insurance industry, and developing country interests
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
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 Fifth Data Release of the Sloan Digital Sky Survey
This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky
Survey (SDSS). DR5 includes all survey quality data taken through June 2005 and
represents the completion of the SDSS-I project (whose successor, SDSS-II will
continue through mid-2008). It includes five-band photometric data for 217
million objects selected over 8000 square degrees, and 1,048,960 spectra of
galaxies, quasars, and stars selected from 5713 square degrees of that imaging
data. These numbers represent a roughly 20% increment over those of the Fourth
Data Release; all the data from previous data releases are included in the
present release. In addition to "standard" SDSS observations, DR5 includes
repeat scans of the southern equatorial stripe, imaging scans across M31 and
the core of the Perseus cluster of galaxies, and the first spectroscopic data
from SEGUE, a survey to explore the kinematics and chemical evolution of the
Galaxy. The catalog database incorporates several new features, including
photometric redshifts of galaxies, tables of matched objects in overlap regions
of the imaging survey, and tools that allow precise computations of survey
geometry for statistical investigations.Comment: ApJ Supp, in press, October 2007. This paper describes DR5. The SDSS
Sixth Data Release (DR6) is now public, available from http://www.sdss.or
SDSS J124602.54+011318.8: A Highly Luminous Optical Transient at z=0.385
We report the discovery of a highly luminous optical transient (OT), SDSS
J124602.54+011318.8, associated with a galaxy at a redshift of 0.385. In this
paper we consider the possibility that the OT may be a GRB afterglow. Three
sets of images and two sets of spectra were obtained as part of the normal
operations of the Sloan Digital Sky Survey (SDSS). In the first two image sets,
observed two nights apart, the object appears as a point source at
. The third image set, observed about 410 days later, shows an
extended source which is more than 2.5 magnitudes fainter. The spectra were
observed about 400 and 670 days after the first two image sets, and both show
an apparently normal galaxy at a redshift of 0.385. Associating the OT with the
galaxy, the absolute magnitude was , which is over 4 magnitudes
brighter than the most luminous supernova ever measured. The spectral energy
distributions of the galaxy-subtracted OT derived from the first two image sets
are well-fit by single power-laws with indices of and -1.29
respectively, similar to most GRB afterglows. Based upon the luminosity of the
OT, non-detections in contemporaneous ROTSE-I images, and the change in
spectral slope, the OT, if an afterglow, was likely discovered early during a
``plateau'' or slowly-fading phase. The discovery of a GRB afterglow at this
stage of the SDSS is consistent with expectations, but only if the optical
emission is much less strongly beamed than the gamma-rays. We emphasize that
other explanations for the OT cannot be ruled out; a recent follow-up study by
[galyam02] provides strong evidence that this source is in fact an unusual AGN.Comment: Updated version to appear in Ap
The Seventh Data Release of the Sloan Digital Sky Survey
This paper describes the Seventh Data Release of the Sloan Digital Sky Survey
(SDSS), marking the completion of the original goals of the SDSS and the end of
the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most
of the roughly 2000 deg^2 increment over the previous data release lying in
regions of low Galactic latitude. The catalog contains five-band photometry for
357 million distinct objects. The survey also includes repeat photometry over
250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A
coaddition of these data goes roughly two magnitudes fainter than the main
survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2
in the Northern Galactic Cap, closing the gap that was present in previous data
releases. There are over 1.6 million spectra in total, including 930,000
galaxies, 120,000 quasars, and 460,000 stars. The data release includes
improved stellar photometry at low Galactic latitude. The astrometry has all
been recalibrated with the second version of the USNO CCD Astrograph Catalog
(UCAC-2), reducing the rms statistical errors at the bright end to 45
milli-arcseconds per coordinate. A systematic error in bright galaxy photometr
is less severe than previously reported for the majority of galaxies. Finally,
we describe a series of improvements to the spectroscopic reductions, including
better flat-fielding and improved wavelength calibration at the blue end,
better processing of objects with extremely strong narrow emission lines, and
an improved determination of stellar metallicities. (Abridged)Comment: 20 pages, 10 embedded figures. Accepted to ApJS after minor
correction
GRB Afterglows and Other Transients in the SDSS
The Sloan Digital Sky Survey (SDSS) will image one quarter of the sky centered on the northern galactic cap and produce a 3‐D map of galaxies and quasars found in the sample. An additional 225 deg2 southern survey will be imaged repeatedly on varying timescales. Here we discuss both archival searches in the SDSS catalog (such as SDSS J24602.54+011318.8) and active searches with the SDSS instruments (such as for GRB 010222) for GRB afterglows and other transient objects. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87288/2/349_1.pd
A Catalog of Broad Absorption Line Quasars in Sloan Digital Sky Survey Data Release 5
We present a catalog of 5039 broad absorption line (BAL) quasars (QSOs) in
the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) QSO catalog that have
absorption troughs covering a continuous velocity range >= 2000 km/s. We have
fit ultraviolet (UV) continua and line emission in each case, enabling us to
report common diagnostics of BAL strengths and velocities in the range -25,000
to 0 km/s for SiIV 1400, CIV 1549, AlIII 1857, and
MgII 2799. We calculate these diagnostics using the spectrum listed in
the DR5 QSO catalog, and also for spectra from additional SDSS observing epochs
when available. In cases where BAL QSOs have been observed with Chandra or
XMM-Newton, we report the X-ray monochromatic luminosities of these sources.
We confirm and extend previous findings that BAL QSOs are more strongly
reddened in the rest-frame UV than non-BAL QSOs and that BAL QSOs are
relatively X-ray weak compared to non-BAL QSOs. The observed BAL fraction is
dependent on the spectral signal-to-noise (S/N); for higher-S/N sources, we
find an observed BAL fraction of approximately 15%. BAL QSOs show a similar
Baldwin effect as for non-BAL QSOs, in that their CIV emission equivalent
widths decrease with increasing continuum luminosity. However, BAL QSOs have
weaker CIV emission in general than do non-BAL QSOs. Sources with higher UV
luminosities are more likely to have higher-velocity outflows, and the BAL
outflow velocity and UV absorption strength are correlated with relative X-ray
weakness. These results are in qualitative agreement with models that depend on
strong X-ray absorption to shield the outflow from over-ionization and enable
radiative acceleration....Comment: accepted to ApJ; 65 pages, 18 figures; adjustments made to abstract
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