Quasar Time Variability Data from Large Data Sets


Here we present a database of ~3.5 million photometric measurements for 80,000 spectroscopically confirmed quasars, to be used for time variability studies. The database consists of five different data sets: two with repeated SDSS imaging in five UV-to-IR photometric bands, one with photometric versus spectrophotometric SDSS measurements for three bands, and two with SDSS versus POSS imaging. The observed time lags span the range from 1 hour to 8 years for the SDSS data sets, and up to 50 years for the SDSS vs. POSS data sets.


The studies of quasars can be approximately classified as those based on flux vs. wavelength information (spectroscopic with high resolution, and lower-resolution spectral energy distribution studies extending from X-ray to radio range), studies with high spatial resolution, and studies of quasar time variability, both in continuum and emission lines. Such studies over the years have yielded detailed information on the physical environment of active galactic nuclei, leading to the current paradigm where a massive black hole is surrounded by an obscuring (dusty) torus from which it is accreting matter through a disk-like structure on the inner edge of the torus (e.g., Antonucci 1993, Urry & Padovani 1995, Krolik 1999). However, the time evolution is an important ingredient that is missing from the standard paradigm. We know galaxies and black holes evolve over time, but we do not know how. Fortunately, one of the defining characteristics of quasars is that their output is variable over time. It is this time dependence that allows us to probe the physics associated with regions very close to the black hole.

Recently, significant progress in the description of quasar variability has been made by employing Sloan Digital Sky Survey (SDSS) data; see this page for a list of relevant papers. The purpose of this catalog is to improve upon these studies by assembling and distributing a database of 3.5 million photometric measurements for 80,000 spectroscopically confirmed quasars detected by SDSS. This rich dataset will be used to develop a detailed and robust statistical description of the dependence of quasar continuum variability on time, wavelength, luminosity and other observables such as the properties of emission lines, and X-ray and radio emission.

Description of Data Sets

The main data sets that enable this program were obtained by the SDSS. In addition, long-term variability will be studied using photometric data obtained by the older photographic Palomar Observatory Sky Survey (POSS), as described below. For methods of accessing SDSS data products, and detailed product description, please see the SDSS EDR paper (Stoughton et al. 2002, Astronomical Journal, 123, 485).

For the purposes of this program, a quasar or QSO is any object listed in the SDSS catalog of spectroscopically confirmed quasars (Schneider et al. 2010). Its most recent fifth edition lists SDSS, 2MASS, ROSAT and FIRST data for 105,783 objects from 9380 deg2. For the spectroscopic target selection for quasars, see Richards et al. 2002.

The SDSS Photometric Variability Data
Although most of the sky was observed by SDSS only once, there are several different methods to measure variability using SDSS:

   Northern Survey: SDSS imaging data are obtained by drift scanning. Because of the scan overlaps, and because of the scan convergence near the survey poles, about 40% of the northern survey area (~4000 deg2) is surveyed at least twice. This method provides 2-epoch 5-band coverage for ~25,000 spectroscopically confirmed quasars.

   Southern Survey: About 290 square degrees of the southern survey area has already been observed ~60 times to search for variable objects and, by stacking the frames, to go deeper. This is the SDSS Stripe 82, which is 22h 24m < R.A. < 04h 08m and | Dec | < 1.27 deg. These multi-epoch data have time scales ranging from 3 hours to 8 years. This method provides well-sampled 5-band light curves for an unprecedented number of quasars.

   Spectrophotometry: As demonstrated by Vanden Berk et al. (2004), the spectrophotometric calibration of SDSS spectra is sufficiently accurate to synthesize magnitudes in the gri bands with errors as small as 0.05 mag. The time delay between imaging and spectroscopy ranges from a few days to a year. This method provides additional 2-epoch 3-band variability information for every single spectroscopically confirmed quasar (N=105,783), however we do not provide the spectrophotometry here.

The quasar sample sizes, time lags, and other relevant properties of these data are summarized in Table 1. Taking into account the number of epochs and bandpasses for each sample, the total number of all photometric measurements is 3.5 million.

The SDSS vs. POSS Variability Data
SDSS has obtained repeated scans in the southern survey area with time difference of up to 10 years. Longer time scales, up to 50 years, can be probed by comparing SDSS and POSS catalogs. Sesar et al. (2006) have addressed the problem of large systematic errors in POSS photometry by recalibrating several publicly available POSS catalogs (USNO-A2.0, USNO-B1.0, DPOSS and GSC2.2). A piecewise recalibration of the POSS data in 100 arcmin2 patches (one SDSS field) generally resulted in an improvement of photometric accuracy (rms) by nearly a factor of two, compared to the original data (POSS-I magnitudes can be improved to ~0.15 mag accuracy, and POSS-II magnitudes to ~0.10 mag accuracy). In addition to the smaller core width of the error distribution, the tails of the distribution become much steeper after the recalibration. These improvements are mostly due to the very dense grid of calibration stars provided by SDSS, which rectifies the intrinsic inhomogeneities of Schmidt plates. Sesar et al. (2006) have recalibrated the POSS-I and DPOSS data to DR2, and this catalog is available here. The main advantage of this data set is its very long time baseline. Sesar et al. (2006) also recalibrated the DPOSS data in 8,000 deg2 of sky from the SDSS Data Release 5. Here, we provide a catalog of all DR7 quasars with DPOSS observations (N=81,189), and provide the SDSS-DPOSS photometry in GRI bands which spans time lags up to 20 years.

Summary of the Available Variability Data:
Sample # of objects Photometric accuracy (mag) Bandpasses Observed time lag # epochs
Southern 9,258 0.02-0.04 ugriz 3 hr -- 9 yr 60
Northern 24,627 0.02-0.04 ugriz 1 hr -- 8 yr 2 or 3
SpecPhoto 80,000 0.05-0.08 gri 1 dy -- 1 yr 2
SDSS-POSS I 80,000 0.15-0.25 OE 45 -- 55 yr 2
SDSS-POSS II 80,000 0.10-0.15 GRI 5 -- 20 yr 2

The following links take you to pages that contain the data files as well as the file format and data assembly details:



We acknowledge support by NSF grant AST-0807500 to the University of Washington.


The reference entry for these catalogs is MacLeod et al., 2011, ApJ (submitted). In addition, we would greatly appreciate if you add the standard SDSS acknowledgement to your paper, found here.

Thank you very much,

   Chelsea MacLeod (1),
   Zeljko Ivezic (1),
   Branimir Sesar (2),
   Andrew C. Becker (1),
   and the SDSS Collaboration

(1) The University of Washington
(2) Caltech  

If you need more information, or have any comments or suggestions, please let us know!
Version 2 from Feb. 25, 2012 (revision history)

"Twinkle, twinkle quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns.
Twinkle, twinkle quasi-star
How I wonder what you are. "

George Gamow, "Quasar " 1964.
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