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Register a new userThe QUEST-La Silla AGN Variability Survey
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We present the characterization and initial results from the QUEST-La Silla AGN variability survey. This is an effort to obtain well sampled optical light curves in extragalactic fields with unique multi-wavelength 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 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 non-linearity, we use a linear correlation to fit the zero-points as a function of the instrumental magnitudes, thus obtaining a good correction to the non-linear behavior of these detectors. We obtain good photometry to an equivalent limiting magnitude of $rsim 20.5$. Studying the optical variability of X-ray detected sources in the XMM-COSMOS field, we find that the survey is $sim75-80$% complete to magnitudes $rsim20$, and $sim67$% complete to a magnitude $rsim21$. The determination and parameterization of the structure function (${SF}_{norm}(tau) = A tau^{gamma}$) of the variable sources shows that most BL AGN are characterized by $A > 0.1$ and $gamma > 0.025$. It is further shown that variable NL AGN and GAL sources occupying the same parameter space in $A$ and $gamma$ are very likely to correspond to obscured or low luminosity AGN. 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.
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We used data from the QUEST-La Silla Active Galactic Nuclei (AGN) variability survey to construct light curves for 208,583 sources over $sim 70$ deg$^2$, with a a limiting magnitude $r sim 21$. Each light curve has at least 40 epochs and a length of $geq 200$ days. We implemented a Random Forest algorithm to classify our objects as either AGN or non-AGN according to their variability features and optical colors, excluding morphology cuts. We tested three classifiers, one that only includes variability features (RF1), one that includes variability features and also $r-i$ and $i-z$ colors (RF2), and one that includes variability features and also $g-r$, $r-i$, and $i-z$ colors (RF3). We obtained a sample of high probability candidates (hp-AGN) for each classifier, with 5,941 candidates for RF1, 5,252 candidates for RF2, and 4,482 candidates for RF3. We divided each sample according to their $g-r$ colors, defining blue ($g-rleq 0.6$) and red sub-samples ($g-r>0.6$). We find that most of the candidates known from the literature belong to the blue sub-samples, which is not necessarily surprising given that, unlike for many literature studies, we do not cut our sample to point-like objects. This means that we can select AGN that have a significant contribution from redshifted starlight in their host galaxies. In order to test the efficiency of our technique we performed spectroscopic follow-up, confirming the AGN nature of 44 among 54 observed sources (81.5% of efficiency). From the campaign we concluded that RF2 provides the purest sample of AGN candidates.
We describe the La Silla-QUEST (LSQ) Variability Survey. LSQ is a dedicated wide-field synoptic survey in the Southern Hemisphere, focussing on the discovery and study of transients ranging from low redshift (z < 0.1) SN Ia, Tidal Disruption events, RR Lyr{ae} variables, CVs, Quasars, TNOs and others. The survey utilizes the 1.0-m Schmidt Telescope of the European Southern Observatory at La Silla, Chile, with the large-area QUEST camera, a mosaic of 112 CCDs with field of view of 9.6 square degrees. The LSQ Survey was commissioned in 2009, and is now regularly covering ~1000 square deg per night with a repeat cadence of hours to days. The data are currently processed on a daily basis. We present here a first look at the photometric capabilities of LSQ and we discuss some of the most interesting recent transient detections.
We present our statistical analysis of the connection between active galactic nuclei (AGN) variability and physical properties of the central supermassive black hole (SMBH). We constructed optical light curves using data from the QUEST-La Silla AGN variability survey. To model the variability, we used the structure function, among the excess variance and the amplitude from Damp Random Walk (DRW) modeling. For the measurement of SMBH physical properties, we used public spectra from the Sloan Digital Sky Survey (SDSS). Our analysis is based on an original sample of 2345 sources detected in both SDSS and QUEST-La Silla. For 1473 of these sources we could perform a proper measurement of the spectral and variability properties, and 1348 of these sources were classified as variable ($91.5%$). We found that the amplitude of the variability ($A$) depends solely on the rest frame emission wavelength and the Eddington ratio, where $A$ anti-correlates with both $lambda_{rest}$ and $L/L_{text{Edd}}$. This suggests that AGN variability does not evolve over cosmic time, and its amplitude is inversely related to the accretion rate. We found that the logarithmic gradient of the variability ($gamma$) does not correlate significantly with any SMBH physical parameter, since there is no statistically significant linear regression model with an absolute value of the slope higher than 0.1. Finally, we found that the general distribution of $gamma$ measured for our sample differs from the distribution of $gamma$ obtained for light curves simulated from a DRW process. For 20.6% of the variable sources in our sample, a DRW model is not appropriate to describe the variability, since $gamma$ differs considerably from the expected value of 0.5.
The LaSilla/QUEST Variability Survey (LSQ) and the Carnegie Supernova Project (CSP II) are collaborating to discover and obtain photometric light curves for a large sample of low redshift (z < 0.1) Type Ia supernovae. The supernovae are discovered in the LSQ survey using the 1 m ESO Schmidt telescope at the La Silla Observatory with the 10 square degree QUEST camera. The follow-up photometric observations are carried out using the 1 m Swope telescope and the 2.5 m du Pont telescopes at the Las Campanas Observatory. This paper describes the survey, discusses the methods of analyzing the data and presents the light curves for the first 31 Type Ia supernovae obtained in the survey. The SALT 2.4 supernova light curve fitter was used to analyze the photometric data, and the Hubble diagram for this first sample is presented. The measurement errors for these supernovae averaged 4%, and their intrinsic spread was 14%.
We systematically analyze X-ray variability of active galactic nuclei (AGNs) in the 7~Ms textit{Chandra} Deep Field-South survey. On the longest timescale ($approx~17$ years), we find only weak (if any) dependence of X-ray variability amplitudes on energy bands or obscuration. We use four different power spectral density (PSD) models to fit the anti-correlation between normalized excess variance ($sigma^2_{rm nxv}$) and luminosity, and obtain a best-fit power law index $beta=1.16^{+0.05}_{-0.05}$ for the low-frequency part of AGN PSD. We also divide the whole light curves into 4 epochs in order to inspect the dependence of $sigma^2_{rm nxv}$ on these timescales, finding an overall increasing trend. The analysis of these shorter light curves also infers a $beta$ of $sim 1.3$ that is consistent with the above-derived $beta$, which is larger than the frequently-assumed value of $beta=1$. We then investigate the evolution of $sigma^2_{rm nxv}$. No definitive conclusion is reached due to limited source statistics but, if present, the observed trend goes in the direction of decreasing AGN variability at fixed luminosity toward large redshifts. We also search for transient events and find 6 notable candidate events with our considered criteria. Two of them may be a new type of fast transient events, one of which is reported here for the first time. We therefore estimate a rate of fast outbursts $langledot{N}rangle = 1.0^{+1.1}_{-0.7}times 10^{-3}~rm galaxy^{-1}~yr^{-1}$ and a tidal disruption event~(TDE) rate $langledot{N}_{rm TDE}rangle=8.6^{+8.5}_{-4.9}times 10^{-5}~rm galaxy^{-1}~yr^{-1}$ assuming the other four long outbursts to be TDEs.
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