The QUEST-La Silla AGN Variability Survey

We present the characterization and initial results from the QUEST-La Silla active galactic nucleus (AGN) variability survey. This is an effort to obtain well-sampled optical light curves in extragalactic fields with unique multiwavelength observations. We present photometry obtained from 2010 to 2012 in the XMM-COSMOS field, which was observed over 150 nights using the QUEST camera on the European Southern Observatory (ESO) Schmidt telescope. The survey uses a broadband filter, the Q band, similar to the union of the g and the r filters, achieving an intrinsic photometric dispersion of 0.05 mag and a systematic error of 0.05 mag in the zero point. Since some detectors of the camera show significant nonlinearity, we use a linear correlation to fit the zero points as a function of the instrumental magnitudes, thus obtaining a good correction to the nonlinear behavior of these detectors. We obtain good photometry to an equivalent limiting magnitude of r∼ 20.5. The astrometry has an internal precision of ∼ 0\buildrel{\prime\prime}\over{.} 1 and an overall accuracy of 0\buildrel{\prime\prime}\over{.} 2 when compared to the Sloan Digital Sky Survey. Studying the optical variability of X-ray-detected sources in the XMM-COSMOS field, we find that the survey is ∼75%-80% complete to magnitudes r∼ 20, and ∼67% complete to a magnitude r∼ 21. Additionally, broad-line (BL) AGNs have larger variation amplitude than non-broad-line (NL) AGNs, with ∼80% of the BL AGNs classified as variable, while only ∼21% of the NL AGNs are classified as variable. We also find that ∼22% of objects classified as galaxies (GALs) are also variable. The determination and parameterization of the structure function ({{SF}}_{norm}(τ )=A{τ }^γ ) of the variable sources show that most BL AGNs are characterized by A\gt 0.1 and γ \gt 0.025. It is further shown that NL AGNs and GAL sources occupying the same parameter space in A and γ are very likely to correspond to obscured or low-luminosity AGNs. Our samples are, however, small, and we expect to revisit these results using larger samples with longer light curves obtained as part of our ongoing survey.