Browsing by Author "De Cicco, D."

Now showing 1 - 8 of 8
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    A random forest-based selection of optically variable AGN in the VST-COSMOS field
    (2021) De Cicco, D.; Bauer, F. E.; Paolillo, M.; Cavuoti, S.; Sanchez-Saez, P.; Brandt, W. N.; Pignata, G.; Vaccari, M.; Radovich, M.
    Context. The survey of the COSMOS field by the VLT Survey Telescope is an appealing testing ground for variability studies of active galactic nuclei (AGN). With 54 r-band visits over 3.3 yr and a single-visit depth of 24.6 r-band mag, the dataset is also particularly interesting in the context of performance forecasting for the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST).Aims. This work is the fifth in a series dedicated to the development of an automated, robust, and efficient methodology to identify optically variable AGN, aimed at deploying it on future LSST data.Methods. We test the performance of a random forest (RF) algorithm in selecting optically variable AGN candidates, investigating how the use of different AGN labeled sets (LSs) and features sets affects this performance. We define a heterogeneous AGN LS and choose a set of variability features and optical and near-infrared colors based on what can be extracted from LSST data.Results. We find that an AGN LS that includes only Type I sources allows for the selection of a highly pure (91%) sample of AGN candidates, obtaining a completeness with respect to spectroscopically confirmed AGN of 69% (vs. 59% in our previous work). The addition of colors to variability features mildly improves the performance of the RF classifier, while colors alone prove less effective than variability in selecting AGN as they return contaminated samples of candidates and fail to identify most host-dominated AGN. We observe that a bright (r less than or similar to 21 mag) AGN LS is able to retrieve candidate samples not affected by the magnitude cut, which is of great importance as faint AGN LSs for LSST-related studies will be hard to find and likely imbalanced. We estimate a sky density of 6.2x10(6) AGN for the LSST main survey down to our current magnitude limit.
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    A structure function analysis of VST-COSMOS AGN
    (2022) De Cicco, D.; Bauer, F. E.; Paolillo, M.; Sanchez-Saez, P.; Brandt, W. N.; Vagnetti, F.; Pignata, G.; Radovich, M.; Vaccari, M.
    Context. We present our sixth work in a series dedicated to variability studies of active galactic nuclei (AGN), based on the survey of the COSMOS field by the VLT Survey Telescope (VST). Its 54 r-band visits over 3.3 yr and single-visit depth of 24.6 r-band mag make this dataset a valuable scaled-down version that can help forecast the performance of the Rubin Observatory Legacy Survey of Space and Time (LSST).
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    Alert Classification for the ALeRCE Broker System: The Light Curve Classifier
    (2021) Sánchez-Sáez, P.; Reyes, I.; Valenzuela, C.; Förster, F.; Eyheramendy, S.; Elorrieta, F.; Bauer, F. E.; Cabrera-Vives, G.; Estévez, P. A.; Catelan, Márcio; Pignata, G.; Huijse, P.; De Cicco, D.; Arévalo, P.; Carrasco-Davis, R.; Abril, J.; Kurtev, R.; Borissova, J.; Arredondo, J.; Castillo-Navarrete, E.; Rodríguez, D.; Ruz-Mieres, D.; Moya, A.; Sabatini-Gacitúa, L.; Sepúlveda-Cobo, C.; Camacho-Iñiguez, E.
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    Ensemble power spectral density of SDSS quasars in UV/optical bands
    (2024) Petrecca, V.; Papadakis, I. E.; Paolillo, M.; De Cicco, D.; Bauer, F. E.
    Context. Quasar variability has proven to be a powerful tool to constrain the properties of their inner engine and the accretion process onto supermassive black holes. Correlations between UV/optical variability and physical properties have been long studied with a plethora of different approaches and time-domain surveys, although the detailed picture is not yet clear. Aims. We analysed archival data from the SDSS Stripe-82 region to study how the quasar power spectral density (PSD) depends on the black hole mass, bolometric luminosity, accretion rate, redshift, and rest-frame wavelength. We developed a model-independent analysis framework that could be easily applied to upcoming large surveys such as the Legacy Survey of Space and Time (LSST). Methods. We used light curves of 8042 spectroscopically confirmed quasars, observed in at least six yearly seasons in five filters ugriz. We split the sample into bins of similar physical properties containing at least 50 sources, and we measured the ensemble PSD in each of them. Results. We find that a simple power law is a good fit to the power spectra in the frequency range explored. Variability does not depend on the redshift at a fixed wavelength. Instead, both PSD amplitude and slope depend on the black hole mass, accretion rate, and rest-frame wavelength. We provide scaling relations to model the observed variability as a function of the physical properties, and discuss the possibility of a universal PSD shape for all quasars, where frequencies scale with the black hole mass, while normalization and slope(s) are fixed (at any given wavelength and accretion rate).
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    Optically variable AGN in the three-year VST survey of the COSMOS field
    (2019) De Cicco, D.; Paolill, M.; Falocco, S.; Poulain, M.; Brandt, W. N.; Bauer, F. E.; Vagnetti, F.; Longo, G.; Grado, A.; Ragosta, F.; Botticella, M. T.; Pignata, G.; Vaccari, M.; Radovich, M.; Salvato, M.; Covone, G.; Napolitano, N. R.; Marchetti, L.; Schipani, P.
    Context. The analysis of the variability of active galactic nuclei (AGN) at different wavelengths and the study of possible correlations of different spectral windows are a current main field of inquiry. Optical variability has been largely used to identify AGN in multivisit surveys. The strength of a selection based on optical variability lies in the opportunity of analyzing data from surveys of large sky areas by ground-based telescopes. However, the effectiveness of optical variability selection with respect to other multiwavelength techniques has been poorly studied down to the depth that is expected from next-generation surveys.
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    The universal shape of the X-ray variability power spectrum of AGN up to z ∼ 3
    (2023) Paolillo, M.; Papadakis, I. E.; Brandt, W. N.; Bauer, F. E.; Lanzuisi, G.; Allevato, V.; Shemmer, O.; Zheng, X. C.; De Cicco, D.; Gilli, R.; Luo, B.; Thomas, M.; Tozzi, P.; Vito, F.; Xue, Y. Q.
    Aims. We study the ensemble X-ray variability properties of active galactic nuclei (AGN) over large ranges of timescale (20 ks <= T <= 14 yr), redshift (0 <= z less than or similar to 3), luminosity (10(40) erg s(-1) <= L-X <= 10(46) erg s(-1)), and black hole (BH) mass (10(6) <= M-circle dot <= 10(9)).
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    Variability and transient search in the SUDARE–VOICE field: a new method to extract the light curves
    (2020) Liu, D.; Deng, W.; Fan, Z.; Fu, L.; Covone, G.; Vaccari, M.; Radovich, M.; Capaccioli, M.; De Cicco, D.; Grado, A.; Marchetti, L.; Napolitano, N.; Paolillo, M.; Pignata, Giuliano; Ragost, F.
    The VLT Survey Telescope (VST) Optical Imaging of the CDFS and ES1 Fields Survey, in synergy with the SUDARE survey, is a deep optical ugri imaging of the CDFS and ES1 fields using the VST. The observations for the CDFS field comprise about 4.38 deg2 down to r ∼ 26 mag. The total on-sky time spans over 4 yr in this field, distributed over four adjacent sub-fields. In this paper, we use the multiepoch r-band imaging data to measure the variability of the detected objects and search for transients. We perform careful astrometric and photometric calibrations and point spread function modelling. A new method, referring to as differential running-average photometry, is proposed to measure the light curves of the detected objects. With the method, the difference of PSFs between different epochs can be reduced, and the background fluctuations are also suppressed. Detailed uncertainty analysis and detrending corrections on the light curves are performed. We visually inspect the light curves to select variable objects, and present some objects with interesting light curves. Further investigation of these objects in combination with multiband data will be presented in our forthcoming paper.
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    X-Ray Unveiling Events in a z ≈ 1.6 Active Galactic Nucleus in the 7 Ms Chandra Deep Field-South
    (2023) Yu, Li-Ming; Luo, Bin; Brandt, W. N.; Bauer, Franz E.; De Cicco, D.; Fabian, A.; Gilli, R.; Koekemoer, A.; Paolillo, M.; Schneider, D. P.; Shemmer, O.; Tozzi, P.; Trump, Jonathan R.; Vignali, C.; Vito, F.; Wang, J. -x.; Xue, Y. Q.
    We investigate the extreme X-ray variability of a z = 1.608 active galactic nucleus in the 7 Ms Chandra Deep Field-South (XID 403), which showed two significant X-ray brightening events. In the first event, XID 403 brightened by a factor of >2.5 in less than or similar to 6.1 rest-frame days in the observed-frame 0.5-5 keV band. The event lasted for approximate to 5.0-7.3 days, and then XID 403 dimmed by a factor of >6.0 in less than or similar to 6.1 days. After approximate to 1.1-2.5 yr in the rest frame (including long observational gaps), it brightened again, with the 0.5-5 keV flux increasing by a factor of >12.6. The second event lasted over 251 days, and the source remained bright until the end of the 7 Ms exposure. The spectrum is a steep power law (photon index Gamma = 2.8 +/- 0.3) without obscuration during the second outburst, and the rest-frame 2-10 keV luminosity reaches 1.5(-0.5)(+0.8) x 10(43) erg s(-1); there is no significant spectral evolution within this epoch. The infrared-to-UV spectral energy distribution of XID 403 is dominated by the host galaxy. There is no significant optical/UV variability and R-band (rest-frame approximate to 2500 angstrom) brightening contemporaneous with the X-ray brightening. The extreme X-ray variability is likely due to two X-ray unveiling events, where the line of sight to the corona is no longer shielded by high-density gas clumps in a small-scale dust-free absorber. XID 403 is probably a high-redshift analog of local narrow-line Seyfert 1 galaxies, and the X-ray absorber is a powerful accretion disk wind. On the other hand, we cannot exclude the possibility that XID 403 is an unusual candidate for tidal disruption events.