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ABSTRACT: We report on a population of X-ray weak quasars with similar UV emission-line
properties to those of the remarkable quasar PHL 1811. All radio-quiet PHL 1811
analogs are notably X-ray weak by a mean factor of ~13, with hints of heavy
X-ray absorption. Correlations between the X-ray weakness and UV emission-line
properties suggest that PHL 1811 analogs may have extreme wind-dominated broad
emission-line regions (BELRs). We propose an AGN geometry that can potentially
unify the PHL 1811 analogs and the general population of weak-line quasars.
01/2012;
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Michael Berry,
Željko Ivezić,
Branimir Sesar,
Mario Jurić,
Edward F. Schlafly,
Jillian Bellovary,
Douglas Finkbeiner,
Dijana Vrbanec,
Timothy C. Beers,
Keira J. Brooks, [......],
Brian Yanny,
Steven R. Majewski,
Gillian R. Knapp,
James E. Gunn,
J. Allyn Smith,
Masataka Fukugita,
Steve Kent,
John Barentine,
Jurek Krzesinski,
Dan Long
[show abstract]
[hide abstract]
ABSTRACT: We use SDSS photometry of 73 million stars to simultaneously obtain best-fit
main-sequence stellar energy distribution (SED) and amount of dust extinction
along the line of sight towards each star. Using a subsample of 23 million
stars with 2MASS photometry, whose addition enables more robust results, we
show that SDSS photometry alone is sufficient to break degeneracies between
intrinsic stellar color and dust amount when the shape of extinction curve is
fixed. When using both SDSS and 2MASS photometry, the ratio of the total to
selective absorption, $R_V$, can be determined with an uncertainty of about 0.1
for most stars in high-extinction regions. These fits enable detailed studies
of the dust properties and its spatial distribution, and of the stellar spatial
distribution at low Galactic latitudes. Our results are in good agreement with
the extinction normalization given by the Schlegel et al. (1998, SFD) dust maps
at high northern Galactic latitudes, but indicate that the SFD extinction map
appears to be consistently overestimated by about 20% in the southern sky, in
agreement with Schlafly et al. (2010). The constraints on the shape of the dust
extinction curve across the SDSS and 2MASS bandpasses support the models by
Fitzpatrick (1999) and Cardelli et al. (1989). For the latter, we find an
$R_V=3.0\pm0.1$(random) $\pm0.1$(systematic) over most of the high-latitude
sky. At low Galactic latitudes (|b|<5), we demonstrate that the SFD map cannot
be reliably used to correct for extinction as most stars are embedded in dust,
rather than behind it. We introduce a method for efficient selection of
candidate red giant stars in the disk, dubbed "dusty parallax relation", which
utilizes a correlation between distance and the extinction along the line of
sight. We make these best-fit parameters, as well as all the input SDSS and
2MASS data, publicly available in a user-friendly format.
11/2011;
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ABSTRACT: We analyze the X-ray variability of 264 Sloan Digital Sky Survey
spectroscopic quasars using the Chandra public archive. This data set consists
of quasars with spectroscopic redshifts out to z~5 and covers rest-frame time
scales up to Delta t_sys 2000 d, with 3 or more X-ray observations available
for 82 quasars. It therefore samples longer time scales and higher luminosities
than previous large-scale analyses of AGN variablity. We find significant (>3
sigma) variation in ~30% of the quasars overall; the fraction of sources with
detected variability increases strongly with the number of available source
counts up to ~70% for sources with >1000 counts per epoch. Assuming the
distribution of fractional variation is Gaussian, its standard deviation is
~16% on >1 week time scales, which is not enough to explain the observed
scatter in quasar X-ray-to-optical flux ratios as due to variability alone. We
find no evidence in our sample that quasars are more variable at higher
redshifts (z > 2), as has been suggested in previous studies. Quasar X-ray
spectra vary similarly to some local Seyfert AGN in that they steepen as they
brighten, with evidence for a constant, hard spectral component that is more
prominent in fainter stages. We identify one highly-variable Narrow Line
Seyfert 1-type spectroscopic quasar in the Chandra Deep Field-North. We
constrain the rate of kilosecond-timescale flares in the quasar population
using ~8 months of total exposure and also constrain the distribution of
variation amplitudes between exposures; extreme changes (>100%) are quite rare,
while variation at the 25% level occurs in <25% of observations. [OIII] 5007A
emission may be stronger in sources with lower levels of X-ray variability; if
confirmed, this would represent an additional link between small-scale (corona)
and large-scale (narrow line region) AGN properties.
10/2011;
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[show abstract]
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ABSTRACT: We report the results from Chandra and XMM-Newton observations of a sample of 10 type 1 quasars selected to have unusual UV emission-line properties (weak and blueshifted high-ionization lines; strong UV Fe emission) similar to those of PHL 1811, a confirmed intrinsically X-ray weak quasar. These quasars were identified by the Sloan Digital Sky Survey at high redshift (z 2.2); eight are radio quiet while two are radio intermediate. All of the radio-quiet PHL 1811 analogs, without exception, are notably X-ray weak by a mean factor of 13. These sources lack broad absorption lines and have blue UV/optical continua, supporting the hypothesis that they are intrinsically X-ray weak like PHL 1811 itself. However, their average X-ray spectrum appears to be harder than those of typical quasars, which may indicate the presence of heavy intrinsic X-ray absorption. Our sample of radio-quiet PHL 1811 analogs supports a connection between an X-ray weak spectral energy distribution and PHL 1811-like UV emission lines; this connection provides an economical way to identify X-ray weak type 1 quasars. The fraction of radio-quiet PHL 1811 analogs in the radio-quiet quasar population is estimated to be 1.2%. We have investigated correlations between relative X-ray brightness and UV emission-line properties (e.g., C IV equivalent width and blueshift) for a sample combining our radio-quiet PHL 1811 analogs, PHL 1811 itself, and typical type 1 quasars. These correlation analyses suggest that PHL 1811 analogs may have extreme wind-dominated broad emission-line regions. Observationally, the radio-quiet PHL 1811 analogs appear to be a subset (30%) of radio-quiet weak-line quasars (WLQs). The existence of a subset of quasars in which high-ionization "shielding gas" covers most of the broad emission-line region (BELR), but little more than the BELR, could potentially unify the PHL 1811 analogs and WLQs. The two radio-intermediate PHL 1811 analogs are X-ray bright. X-ray spectral analyses and consideration of their multiwavelength properties suggest that one of them has jet-dominated X-ray emission, while the nature of the other remains unclear.
The Astrophysical Journal 06/2011; 736(1):28. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present measurements of reddening due to dust using the colors of stars in the Sloan Digital Sky Survey (SDSS). We measure the color of main-sequence turnoff stars by finding the "blue tip" of the stellar locus: the prominent blue edge in the distribution of stellar colors. The method is sensitive to color changes of order 18, 12, 7, and 8 mmag of reddening in the colors u – g, g – r, r – i, and i – z, respectively, in regions measuring 90' by 14'. We present maps of the blue tip colors in each of these bands over the entire SDSS footprint, including the new dusty southern Galactic cap data provided by the SDSS-III. The results disfavor the best-fit O'Donnell and Cardelli et al. reddening laws, but are described well by a Fitzpatrick reddening law with RV = 3.1. The Schlegel et al. (SFD) dust map is found to trace the dust well, but overestimates reddening by factors of 1.4, 1.0, 1.2, and 1.4 in u – g, g – r, r – i, and i – z largely due to the adopted reddening law. In select dusty regions of the sky, we find evidence for problems in the SFD temperature correction. A dust map normalization difference of 15% between the Galactic north and south sky may be due to these dust temperature errors.
The Astrophysical Journal 11/2010; 725(1):1175. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present measurements of reddening due to dust using the colors of stars in
the Sloan Digital Sky Survey (SDSS). We measure the color of main sequence
turn-off stars by finding the "blue tip" of the stellar locus: the prominent
blue edge in the distribution of stellar colors. The method is sensitive to
color changes of order 18, 12, 7, and 8 mmag of reddening in the colors u-g,
g-r, r-i, and i-z, respectively, in regions measuring 90' by 14'. We present
maps of the blue tip colors in each of these bands over the entire SDSS
footprint, including the new dusty southern Galactic cap data provided by the
SDSS-III. The results disfavor the best fit O'Donnell (1994) and Cardelli et
al. (1989) reddening laws, but are well described by a Fitzpatrick (1999)
reddening law with R_V = 3.1. The SFD dust map is found to trace the dust well,
but overestimates reddening by factors of 1.4, 1.0, 1.2, and 1.4 in u-g, g-r,
r-i, and i-z, largely due to the adopted reddening law. In select dusty regions
of the sky, we find evidence for problems in the SFD temperature correction. A
dust map normalization difference of 15% between the Galactic north and south
sky may be due to these dust temperature errors.
09/2010;
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ABSTRACT: We have compiled a sample of 14 of the optically brightest radio-quiet quasars ($m_{i}$~$\le$~17.5 and $z$~$\ge$~1.9) in the Sloan Digital Sky Survey Data Release 5 quasar catalog that have C IV mini-BALs present in their spectra. X-ray data for 12 of the objects were obtained via a Chandra snapshot survey using ACIS-S, while data for the other two quasars were obtained from archival XMM-Newton observations. Joint X-ray spectral analysis shows the mini-BAL quasars have a similar average power-law photon index ($\Gamma\approx1.9$) and level of intrinsic absorption ($N_H \lesssim 8\times 10^{21} \ {\rm cm}^{-2}$) as non-BMB (neither BAL nor mini-BAL) quasars. Mini-BAL quasars are more similar to non-BMB quasars than to BAL quasars in their distribution of relative X-ray brightness (assessed with $\Delta\alpha_{\rm ox}$). Relative colors indicate mild dust reddening in the optical spectra of mini-BAL quasars. Significant correlations between $\Delta\alpha_{\rm ox}$ and UV absorption properties are confirmed for a sample of 56 sources combining mini-BAL and BAL quasars with high signal-to-noise ratio rest-frame UV spectra, which generally supports models in which X-ray absorption is important in enabling driving of the UV absorption-line wind. We also propose alternative parametrizations of the UV absorption properties of mini-BAL and BAL quasars, which may better describe the broad absorption troughs in some respects. Comment: ApJ accepted; 21 pages, 11 figures, and 9 tables
09/2010;
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[show abstract]
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ABSTRACT: 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.
03/2010;
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[show abstract]
[hide abstract]
ABSTRACT: We investigate the variability of C IV λ1549 broad absorption line (BAL) troughs over rest-frame timescales of up to 7 yr in 14 quasars at redshifts z 2.1. For nine sources at sufficiently high redshift, we also compare the C IV and Si IV λ1400 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), ΔEW, increases with the time between observations. BALs do not, in general, strengthen or weaken monotonically, and variation observed over shorter (months) timescales is not predictive of multi-year variation. We find no evidence for asymmetry in the distribution of ΔEW that would indicate that BALs form and decay on different timescales, 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. C IV and Si IV 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.
The Astrophysical Journal 03/2010; 713(1):220. · 6.02 Impact Factor
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LSST Science Collaboration,
Paul A. Abell,
Julius Allison,
Scott F. Anderson,
John R. Andrew,
J. Roger P. Angel,
Lee Armus,
David Arnett,
S. J. Asztalos,
Tim S. Axelrod, [......],
Oliver Wiecha,
Benjamin F. Williams,
Beth Willman,
David Wittman,
Sidney C. Wolff,
W. Michael Wood-Vasey,
Przemek Wozniak,
Patrick Young,
Andrew Zentner,
Hu Zhan
[show abstract]
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ABSTRACT: A survey that can cover the sky in optical bands over wide fields to faint
magnitudes with a fast cadence will enable many of the exciting science
opportunities of the next decade. The Large Synoptic Survey Telescope (LSST)
will have an effective aperture of 6.7 meters and an imaging camera with field
of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over
20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with
fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a
total point-source depth of r~27.5. The LSST Science Book describes the basic
parameters of the LSST hardware, software, and observing plans. The book
discusses educational and outreach opportunities, then goes on to describe a
broad range of science that LSST will revolutionize: mapping the inner and
outer Solar System, stellar populations in the Milky Way and nearby galaxies,
the structure of the Milky Way disk and halo and other objects in the Local
Volume, transient and variable objects both at low and high redshift, and the
properties of normal and active galaxies at low and high redshift. It then
turns to far-field cosmological topics, exploring properties of supernovae to
z~1, strong and weak lensing, the large-scale distribution of galaxies and
baryon oscillations, and how these different probes may be combined to
constrain cosmological models and the physics of dark energy.
12/2009;
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ABSTRACT: We use UV/optical and X-ray observations of 272 radio-quiet Type 1 active galactic nuclei and quasars to investigate the C IV Baldwin Effect (BEff). The UV/optical spectra are drawn from the Hubble Space Telescope, International Ultraviolet Explorer and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra and XMM-Newton archives. We apply correlation and partial-correlation analyses to the equivalent widths (EWs), continuum monochromatic luminosities, and αox, which characterizes the relative X-ray to UV brightness. The EW of the C IV λ1549 emission line is correlated with both αox and luminosity. We find that by regressing l ν(2500 Å) with EW(C IV) and αox, we can obtain tighter correlations than by regressing l ν(2500 Å) with only EW(C IV). Both correlation and regression analyses imply that l ν(2500 Å) is not the only factor controlling the changes of EW(C IV); αox (or, equivalently, the soft X-ray emission) plays a fundamental role in the formation and variation of C IV. Variability contributes at least 60% of the scatter of the EW(C IV)-l ν(2500 Å) relation and at least 75% of the scatter of the of the EW(C IV)-αox relation. In our sample, narrow Fe Kα 6.4 keV emission lines are detected in 50 objects. Although narrow Fe Kα exhibits a BEff similar to that of C IV, its EW has almost no dependence on either αox or EW(C IV). This suggests that the majority of narrow Fe Kα emission is unlikely to be produced in the broad emission-line region. We do find suggestive correlations between the emission-line luminosities of C IV and Fe Kα, which could be potentially used to estimate the detectability of the Fe Kα line of quasars from rest-frame UV spectroscopic observations.
The Astrophysical Journal 08/2009; 702(1):767. · 6.02 Impact Factor
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Aleksandar M. Diamond-Stanic,
Xiaohui Fan,
W. N. Brandt,
Ohad Shemmer,
Michael A. Strauss,
Scott F. Anderson,
Christopher L. Carilli, Robert R. Gibson,
Linhua Jiang,
J. Serena Kim,
Gordon T. Richards,
Gary D. Schmidt,
Donald P. Schneider,
Yue Shen,
Paul S. Smith,
Marianne Vestergaard,
and Jason E. Young
[show abstract]
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ABSTRACT: We identify a sample of 74 high-redshift quasars (z > 3) with weak emission lines from the Fifth Data Release of the Sloan Digital Sky Survey and present infrared, optical, and radio observations of a subsample of four objects at z > 4. These weak emission-line quasars (WLQs) constitute a prominent tail of the Lyα + N v equivalent width distribution, and we compare them to quasars with more typical emission-line properties and to low-redshift active galactic nuclei with weak/absent emission lines, namely BL Lac objects. We find that WLQs exhibit hot (T ~ 1000 K) thermal dust emission and have rest-frame 0.1-5 μm spectral energy distributions that are quite similar to those of normal quasars. The variability, polarization, and radio properties of WLQs are also different from those of BL Lacs, making continuum boosting by a relativistic jet an unlikely physical interpretation. The most probable scenario for WLQs involves broad-line region properties that are physically distinct from those of normal quasars.
The Astrophysical Journal 06/2009; 699(1):782. · 6.02 Impact Factor
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Robert R. Gibson,
Linhua Jiang,
W. N. Brandt,
Patrick B. Hall,
Yue Shen,
Jianfeng Wu,
Scott F. Anderson,
Donald P. Schneider,
Daniel Vanden Berk,
S. C. Gallagher,
Xiaohui Fan,
and Donald G. York
[show abstract]
[hide abstract]
ABSTRACT: 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–1. We have fitted 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–1 for Si IV λ1400, C IV λ1549, Al III λ1857, and Mg II λ2799. 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 ratio (S/N); for higher S/N sources, we find an observed BAL fraction of 15%. BAL QSOs show a similar Baldwin effect as for non-BAL QSOs, in that their C IV emission equivalent widths decrease with increasing continuum luminosity. However, BAL QSOs have weaker C IV 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 overionization and enable radiative acceleration. In a scenario in which BAL trough shapes are primarily determined by outflow geometry, observed differences in Si IV and C IV trough shapes would suggest that some outflows have ion-dependent structure.
The Astrophysical Journal 02/2009; 692(1):758. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We examine the UV and X-ray properties of 256 radio-quiet SDSS quasars (QSOs)
observed in X-rays with Chandra and/or XMM-Newton in order to study the
relationship between QSOs with broad CIV absorption lines (BALs; width >2000
km/s) and those with CIV mini-BALs (here defined to have widths of 1000--2000
km/s). Our sample includes 42 BAL and 48 mini-BAL QSOs. The relative X-ray
brightness and hard spectral slopes of the mini-BAL population are, on average,
intermediate between those of BAL and non-BAL QSOs, as might be expected if
narrower and broader absorption line outflows are physically related. However,
a significant population of mini-BALs has outflow velocities higher than would
be expected for BAL QSOs of the same relative X-ray brightness. Consistenly
strong X-ray absorption is apparently not required to accelerate at least some
mini-BALs to high outflow velocities. Assuming the mini-BAL features are
correctly attributed to intrinsic CIV absorption, we suggest that their
observed properties may be explained if mini-BALs are "seeds" which can be
accelerated to form BALs when sufficient X-ray shielding is present.
We also examine several QSOs with broad CIV absorption that have been
recently reported to be unusually X-ray bright. Such cases are frequently
mini-BAL QSOs, which as a population are generally brighter in X-rays than BAL
QSOs. Pointed XMM-Newton observations also suggest that these sources (or
unresolved neighbors) may have been previously observed in a high flux state.
02/2009;
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[show abstract]
[hide abstract]
ABSTRACT: We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7 <= z <= 2.7 in order to characterize the relative UV and X-ray spectral properties of QSOs that do not have broad UV absorption lines (BALs). We constrain the fraction of X-ray weak, non-BAL QSOs and find that such objects are rare; for example, sources underluminous by a factor of 10 comprise $\la$2% of optically-selected SDSS QSOs. X-ray luminosities vary with respect to UV emission by a factor of $\la$2 over several years for most sources. UV continuum reddening and the presence of narrow-line absorbing systems are not strongly associated with X-ray weakness in our sample. X-ray brightness is significantly correlated with UV emission line properties, so that relatively X-ray weak, non-BAL QSOs generally have weaker, blueshifted CIV$\lambda$1549 emission and broader CIII]$\lambda$1909 lines. The CIV emission line strength depends on both UV and X-ray luminosity, suggesting that the physical mechanism driving the global Baldwin effect is also associated with X-ray emission. Comment: Accepted to ApJ
08/2008;
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[show abstract]
[hide abstract]
ABSTRACT: We use quantitative metrics to characterize the variation of CIV 1549 A broad absorption lines (BALs) over 3--6 (rest-frame) years in a sample of 13 quasars at 1.7 <= z <= 2.8 and compare the results to previous studies of BAL variability on shorter time scales. The strong BALs in our study change in complex ways over 3--6 yr. Variation occurs in discrete regions which are only a few thousand km/s wide, and the distribution of the change in absorption equivalent width broadens over time. We constrain the typical CIV BAL lifetime to be at least a few decades. While we do not find evidence to support a scenario in which the variation is primarily driven by photoionization on multi-year time scales, there is some indication that the variation is produced by changes in outflow geometry. We do not observe significant changes in the BAL onset velocity, indicating that the absorber is either far from the source or is being continually replenished and is azimuthally symmetric. It is not possible in a human lifetime to expand the time scales in our study by more than a factor of a few using optical spectroscopy. However, the strong variation we have observed in some BALs indicates that future studies of large numbers of BAL QSOs will be valuable to constrain BAL lifetimes and the physics of variation. Comment: To be published in ApJ
12/2007;
