J/ApJ/728/26 QSO selection based on photometric variability (Macleod+, 2011)
Quasar selection based on photometric variability.
MacLeod C.L., Brooks K., Ivezic Z., Kochanek C.S., Gibson R., Meisner A.,
Kozlowski S., Sesar B., Becker A.C., de Vries W.H.
<Astrophys. J., 728, 26 (2011)>
=2011ApJ...728...26M 2011ApJ...728...26M
ADC_Keywords: QSOs ; Photometry, SDSS ; Models
Keywords: quasars: general
Abstract:
We develop a method for separating quasars from other variable point
sources using Sloan Digital Sky Survey (SDSS) Stripe 82 light-curve
data for ∼10000 variable objects. To statistically describe quasar
variability, we use a damped random walk (DRW) model parametrized by a
damping timescale, τ, and an asymptotic amplitude (structure
function), SF{infinite}. With the aid of an SDSS spectroscopically
confirmed quasar sample, we demonstrate that variability selection in
typical extragalactic fields with low stellar density can deliver
complete samples with reasonable purity (or efficiency, E). Compared
to a selection method based solely on the slope of the structure
function, the inclusion of the τ information boosts E from 60% to
75% while maintaining a highly complete sample (98%) even in the
absence of color information. For a completeness of C=90%, E is
boosted from 80% to 85%. Conversely, C improves from 90% to 97% while
maintaining E=80% when imposing a lower limit on τ. With the aid
of color selection, the purity can be further boosted to 96%, with
C=93%. Hence, selection methods based on variability will play an
important role in the selection of quasars with data provided by
upcoming large sky surveys, such as Pan-STARRS and the Large Synoptic
Survey Telescope (LSST). In summary, given an adequate survey cadence,
photometric variability provides an even better method than color
selection for separating quasars from stars.
Description:
To test our quasar selection method, we utilize the g-band light
curves from the SDSS Stripe 82 (S82), which covers the sky region
defined by 22:24<RA<04:08 and -1.27°<DEC<1.27° (an area of
∼290deg2). The light curves span about 10 years, and the
observations are clustered into yearly seasons about two to three
months long. There are on average more than 60 available epochs.
Because some observations were obtained in non-photometric conditions,
improved calibration techniques have been applied to the SDSS S82 data
by Ivezic et al. (2009AAS...21346003I) and Sesar et al.
(2007AJ....134.2236S 2007AJ....134.2236S), and we use their results. For these data,
photometric zero-point errors are 0.01-0.02mag.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 77 255 Targets for spectroscopic follow-up
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See also:
VII/260 : The SDSS-DR7 quasar catalog (Schneider+, 2010)
II/294 : The SDSS Photometric Catalog, Release 7 (Adelman-McCarthy+, 2009)
J/ApJS/199/3 : The quasars MMT-BOSS pilot survey (Ross+, 2012)
J/ApJS/194/45 : QSO properties from SDSS-DR7 (Shen+, 2011)
J/A+A/525/A37 : Variability of QSOs in SDSS Stripe 82 (Meusinger+, 2011)
J/AJ/137/3884 : Quasar candidates selected from the SDSS (Richards+, 2009)
J/ApJS/180/67 : Photometric selection of QSOs from SDSS. II. (Richards+, 2009)
J/ApJ/698/895 : Variations in QSOs optical flux (Kelly+, 2009)
J/A+A/497/81 : Variability-selected AGN in CDFS (Boutsia+, 2009)
J/AJ/133/1780 : GALEX/SDSS quasar catalog (Trammell+, 2007)
J/AJ/131/2788 : Redshifts in the SFQS survey (Jiang+, 2006)
J/AJ/131/2766 : Quasar luminosity function from SDSS-DR3 (Richards+, 2006)
J/ApJ/633/638 : 315 SDSS variable quasar sample (Wilhite+, 2005)
J/AJ/122/503 : SDSS photometry and redshift of QSOs (Anderson+, 2001)
J/AJ/121/31 : High-redshift quasars in SDSS (Fan+, 2001)
J/ApJS/123/377 : u*g*r*i*z* Photometry of stars, galaxies, QSOs (Newberg+ 1999)
http://www.sdss.org/ : SDSS home page
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 7 I7 --- ID [46516/7912248] Target identification number (1)
9- 18 F10.6 deg RAdeg Right Ascension in decimal degrees (J2000)
20- 28 F9.6 deg DEdeg Declination in decimal degrees (J2000)
30- 34 F5.2 mag imag [12.87/18.99] SDSS i band magnitude (2)
36- 40 F5.2 mag u-g SDSS (u-g) color index (2)
42- 46 F5.2 mag g-r SDSS (g-r) color index (2)
48- 52 F5.2 mag r-i SDSS (r-i) color index (2)
54- 58 F5.2 mag i-z SDSS (i-z) color index (2)
60- 66 F7.5 [d] logtau [2/7.5] Log of the DRW tau time scale (3)
68- 75 F8.5 mag SFinf [0.01/44.05] DRW long-term structure function
77 I1 --- Out [0/1] Outlier (3)
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Note (1): For unknown objects with τ≥100 days, ΔLnoise>2,
and i<19.
Note (2): All magnitudes and colors are corrected for Galactic extinction.
Note (3): DRW = damped random walk.
* For some objects, the time scale has saturated to τ=105days
due to insufficient light curve lengths.
* The outlier flag set to '1' means that the object fails the
τ≥100day and ΔLnoise>2 selection criteria after
rejecting the data point furthest from the median magnitude.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 03-Sep-2012