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.
<Astron. Astrophys., 574, A112-112 (2015)>
=2015A&A...574A.112D 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°x1° 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 57 83 List of the 83 optically variable sources in
the secure sample (qualities 1 and 2)
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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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, <σltc> (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)
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Note (1): Spectroscopic redshift by Civano et al. (2012ApJ...752...49C 2012ApJ...752...49C) or,
when not available, by Brusa et al. (2010ApJ...716..348B 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
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 09-Mar-2015