J/A+A/574/A112 COSMOS field variability-selected AGN nuclei (De Cicco+, 2015) ================================================================================ Variability-selected active galactic nuclei in the VST-SUDARE/VOICE survey of the COSMOS field. De Cicco D., Paolillo M., Covone G., Falocco S., Longo G., Grado A., Limatola L., Botticella M.T., Pignata G., Cappellaro E., Vaccari M., Trevese D., Vagnetti F., Salvato M., Radovich M., Brandt W.N., Capaccioli M., Napolitano N.R., Schipani P. =2015A&A...574A.112D ================================================================================ ADC_Keywords: Active gal. nuclei ; Redshifts ; Photometry Keywords: galaxies: active - X-rays: galaxies - quasars: general - supernovae: general - surveys Abstract: Active galaxies are characterized by variability at every wavelength, with timescales from hours to years depending on the observing window. Optical variability has proven to be an effective way of detecting AGNs in imaging surveys, lasting from weeks to years. In the present work we test the use of optical variability as a tool to identify active galactic nuclei in the VST multiepoch survey of the COSMOS field, originally tailored to detect supernova events. We make use of the multiwavelength data provided by other COSMOS surveys to discuss the reliability of the method and the nature of our AGN candidates. The selection on the basis of optical variability returns a sample of 83 AGN candidates; based on a number of diagnostics, we conclude that 67 of them are confirmed AGNs (81% purity), 12 are classified as supernovae, while the nature of the remaining 4 is unknown. For the subsample of AGNs with some spectroscopic classification, we find that Type 1 are prevalent (89%) compared to Type 2 AGNs (11%). Overall, our approach is able to retrieve on average 15% of all AGNs in the field identified by means of spectroscopic or X-ray classification, with a strong dependence on the source apparent magnitude (completeness ranging from 26% to 5%). In particular, the completeness for Type 1 AGNs is 25%, while it drops to 6% for Type 2 AGNs. The rest of the X-ray selected AGN population presents on average a larger rms variability than the bulk of non-variable sources, indicating that variability detection for at least some of these objects is prevented only by the photometric accuracy of the data. The low completeness is in part due to the short observing span: we show that increasing the temporal baseline results in larger samples as expected for sources with a red-noise power spectrum. Our results allow us to assess the usefulness of this AGN selection technique in view of future wide-field surveys. Description: The VLT Survey Telescope (VST) is located at Cerro Paranal Observatory; it is a joint venture between the European Southern Observatory (ESO) and the INAF-Osservatorio Astronomico di Capodimonte (OAC) in Napoli. The telescope is 2.65m in diameter and is equipped with the single focal plane detector OmegaCAM: a mosaic of 32 CCD detectors made up of 268 megapixels in total, corresponding to a 26cmx26cm area and a 1{deg}x1{deg} FoV, the resolution being 0.214"/pixel. The VST is dedicated to surveys in the wavelength range 0.3-1.0um. The survey provides data in the g, r, and i bands, with an observing frequency of approximately ten days for the g and i bands and three days for the r band, depending on the various observational constraints. We discuss here the analysis of 28 epochs in the r band, for which we have the best temporal sampling. The observations cover the period from December 2011 to May 2012. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table3.dat 57 83 List of the 83 optically variable sources in the secure sample (qualities 1 and 2) -------------------------------------------------------------------------------- Byte-by-byte Description of file: table3.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 2 I2 --- Seq [1/83] Sequential identification number 4- 5 I2 h RAh Right ascension (J2000) 7- 8 I2 min RAm Right ascension (J2000) 10- 13 F4.1 s RAs Right ascension (J2000) 15 A1 --- DE- Declination sign (J2000) 16- 17 I2 deg DEd Declination (J2000) 19- 20 I2 arcmin DEm Declination (J2000) 22- 25 F4.1 arcsec DEs Declination (J2000) 27- 31 F5.2 mag rmag [18.5/23] Average VST r(AB) magnitude 33- 36 F4.2 mag s_rmag rms of the light curve, <{sigma}^ltc^> (Eq.(2)) 38- 41 F4.2 --- S/G [0/1] SExtractor stellarity index (1=stellar) 43 I1 --- Qual [1/2] Quality label: 1=strong candidate, 2=likely variable candidate 45- 48 F4.1 --- signi [3/29] Significance (Eq.(3)) 50- 54 F5.3 --- z [0.3/3]?=- Spectroscopic redshift (1) 56- 57 I2 --- Class [-2/7] Source classification (2) -------------------------------------------------------------------------------- Note (1): Spectroscopic redshift by Civano et al. (2012ApJ...752...49C) or, when not available, by Brusa et al. (2010ApJ...716..348B, Cat. J/ApJ/716/348) Note (2): The classification index is the sum of different numbers corresponding to the following key: 0 = non-classified 1 = confirmed AGN through spectroscopy/SED 2 = confirmed AGN through X-ray/Optical diagram (X/O) 3 = confirmed AGN through X/O diagram (2) and spectroscopy/SED (1) 4 = confirmed AGN through color vs. color diagram (CCD) 6 = confirmed AGN through CCD (4) and X/O diagram (2) 7 = no counterpart in the other COSMOS catalogs inspected -1 = SN -2 = possible SN -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Patricia Vannier [CDS] 09-Mar-2015