Exploring the Variable Sky with the Sloan Digital Sky Survey

Abstract

We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS Stripe 82, and contains 58 million photometric observations in the SDSS ugriz system for 1.4 million unresolved sources. In each photometric bandpass we compute various low-order lightcurve statistics and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g=20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands. The majority (2/3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as is the UV excess color selection (at high Galactic latitudes). We analyze the distribution of lightcurve skewness for quasars and find that is centered on zero. We find that about 1/4 of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of lightcurve skewness in the g-r vs. u-g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over six hundred RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled 2% accurate, multi-color lightcurves for ~2 million low-redshift quasars, and will discover at least 50 million variable stars.Comment: 41 pages, 14 figures, submitted to Astronomical Journa

Understanding extreme quasar optical variability with CRTS – I. Major AGN flares

“…Extreme quasars are, by definition, rare but the growing availability of large archives of astronomical time series, e.g. Sloan Digital Sky Survey (SDSS) Stripe 82 (Sesar et al 2007), LINEAR (Sesar et al 2011), Palomar Transient Factory (PTF; Rau et al 2009) and Pan-STARRS (Kaiser et al 2002), means that statistically valid samples of such objects can now be defined.…”

Oscillatory patterns in the light curves of five long-term monitored type 1 active galactic nuclei

“…Hence, optical variability studies give insight in geometry, physics and dynamics of AGN (Rees 1984;Torricelli-Campioni et al 2000;Hawkins 2002;Sesar et al 2007;Mushotzky et al 2011), but the used data must cover long time periods (Webb et al 1988). Because of this, AGN long-term monitoring programs are crucial for understanding physical aspects of AGN as well as for establishing cosmological constrains due to the wide distribution of AGN over different cosmological time-scales (Hönig 2014;Risaliti & Lusso 2017).…”

The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

“…A sample of 9254 variable quasars is obtained from cross-matching the spectroscopic confirmed SDSS DR7 quasars (Schneider et al 2010;Shen et al 2011) and a sample of 67507 variable sources in SDSS Stripe 82, which lies along the celestial equator in the Southern Galactic Hemisphere (22h 24m < α J2000 < 04h 08m, −1.27 • < δ J2000 < +1.27 • , ∼ 290 deg 2 ) and have repeated photometric observations (at least 4 per band, with a median of 10) measured in up to 10 years in the u ′ g ′ r ′ i ′ z ′ system (Fukugita et al 1996;MacLeod et al 2010;Ivezić et al 2007;Sesar et al 2007). Black hole masses, bolometric luminosities and Eddington ratios are obtained from the quasar catalog in Shen et al (2011), where the black hole masses of quasars were settled by fiducial virial mass estimates: Hβ (Vestergaard & Wilkes 2006) estimates for z < 0.7, [MgII] (Shen & Kelly 2010) estimates for 0.7 ≤ z < 1.9 and CIV] (Vestergaard & Wilkes 2006) estimates for z ≥ 1.9.…”

“…To measure the variability amplitude in each filter band for each quasar, we adopt the formalism similar to that used in Ai et al (2010) and Sesar et al (2007):…”

“…Otherwise, variability amplitudes are set to be 0. To get more reliable statistical results, in the following studies we only investigate quasar variability amplitudes in the SDSS g ′ , r ′ and i ′ band, as photometric errors in the u ′ and z ′ band are substantially larger (Sesar et al 2007). Among the 9148 quasars with measurable black hole mass (Shen & Kelly 2010), 8967 quasars have reliable variability indicators in the g ′ , r ′ and i ′ band, which means that variability in each band for a quasar is a non-zero value estimated from at least 10 data points, even after eliminating photometric outliers.…”