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DRAFT VERSION NOVEMBER 12, 2004
Preprint typeset using IAThX style emulateapj v. 4/12/04
STRUCTURE FUNCTION ANALYSIS OF LONG-TERM QUASAR VARIABILITY
W. H. DE VRIES, R. H. BECKER
University of California, One Shields Ave, Davis, CA 95616 and
Lawrence Livermore National Laboratory, L-413, Livermore, CA 94550
R. L. WHITE, C. LOOMIS
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Draft version November 12, 2004
In our second paper on long-term quasar variability, we employ a much larger database of quasars
than in de Vries, Becker & White. This expanded sample, containing 35165 quasars from the Sloan
Digital Sky Survey Data Release 2, and 6413 additional quasars in the same area of the sky taken
from the 2dF QSO Redshift Survey, allows us to significantly improve on our earlier conclusions. As
before, all the historic quasar photometry has been calibrated onto the SDSS scale by using large
numbers of calibration stars around each quasar position. We find the following: (1) the outbursts
have an asymmetric light-curve profile, with a fast-rise, slow-decline shape; this argues against a
scenario in which micro-lensing events along the line-of-sight to the quasars are dominating the long-
term variations in quasars; (2) there is no turnover in the Structure Function of the quasars up to
time-scales of ~40 years, and the increase in variability with increasing time-lags is monotonic and
constant; and consequently, (3) there is not a single preferred characteristic outburst time-scale for
the quasars, but most likely a continuum of outburst time-scales, (4) the magnitude of the quasar
variability is a function of wavelength: variability increases toward the blue part of the spectrum, (5)
high-luminosity quasars vary less than low-luminosity quasars, consistent with a scenario in which
variations have limited absolute magnitude. Based on this, we conclude that quasar variability is
intrinsic to the Active Galactic Nucleus, is caused by chromatic outbursts / flares with a limited
luminosity range and varying time-scales, and which have an overall asymmetric light-curve shape.
Currently the model that has the most promise of fitting the observations is based on accretion disk
Subject headings: galaxies: active galaxies: statistics quasars: general
The cause of the long-term variability in quasars is still
a matter of debate. Unlike the short time-scale variations
(on the order of days), which are adequately described
in terms of relativistic beaming effects (e.g., Bregman et
al. 1990; Fan & Lin 2000; Vagnetti et al. 2003), the vari-
ations at much longer time-scales (years to decades) are
less understood. Current scenarios under consideration
are ranging from source intrinsic variations due to Active
Galactic Nucleus (AGN) accretion disk instabilities (e.g.,
Shakura & Sunyaev 1976; Rees 1984; Siemiginowska &
Elvis 1997; Kawaguchi et al. 1998; Starling et al. 2004),
and possible bursts of supernovae events close to the nu-
cleus (e.g., Terlevich et al. 1992; Cid Fernandes et al.
1996), to source extrinsic variations due to micro-lensing
events along the line-of-sight to the quasar (e.g., Hawkins
1993, 2002; Alexander 1995; Yonehara et al. 1999; Zack-
risson et al. 2003). See also the review article by Ulrich,
Maraschi & Urry (1997).
Determining which of the various proposed mecha-
nisms actually dominates quasar variability is best done
by studying it toward the longest possible time-baselines.
Depending on the mechanism, each has markedly differ-
ent variability "power" at the longer time-scales (e.g.,
Hawkins 2002). This means that if one would have a
quasar monitoring sample that is both large enough, and
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covers a large enough time-baseline, one could address
these issues adequately. Unfortunately, given the na-
ture of monitoring programs, this is not something that
can be started overnight. The longest quasar light-curve
monitoring programs are on the order of 20 years (e.g.,
Hawkins 1996), and will take a long time before they are
expanded significantly in time-baseline.
The way around this is by using historic photographic
plate material, in combination with a recent survey. Like
in our previous paper (de Vries, Becker & White 2003,
hereafter Paper I), we chose to use the Sloan Digital Sky
Survey (SDSS), Data Release 2 (DR2), in combination
with the historic Second Generation Guide Star Cata-
log (GSC2, McLean et al. 1998) and the Palomar Op-
tical Sky Survey (POSS, Reid et al. 1991). This allows
for photometric information on the quasars spanning up
to 50 years. The downside is that, unlike the monitor-
ing programs, we have typically a very sparse light-curve
sampling per quasar. However, since we will have a very
large number of them, the sampling across the complete
database will be very good. This obviously only works
if the variability of the quasars is due to a mechanism
common to all quasars. We proved the validity of this
concept in Paper I, and recently a similar approach has
been taken by Sesar et al. (2004).
1 The Guide Star Catalogue-II is a joint project of the Space
Telescope Science Institute and the Osservatorio Astronomico di
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de Vries, W; Becker, R; White, R & Loomis, C. STRUCTURE FUNCTION ANALYSIS OF LONG-TERM QUASAR VARIABILITY, article, November 15, 2004; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1410080/m1/3/?rotate=270: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.