Browsing by Author "Gilli, Roberto"

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    A Large Population of Obscured AGN in Disguise as Low-luminosity AGN in Chandra Deep Field South
    (2020) Lambrides, Erini L.; Chiaberge, Marco; Heckman, Timothy; Gilli, Roberto; Vito, Fabio; Norman, Colin
    Population synthesis models of actively accreting supermassive black holes (or active galactic nuclei (AGN)) predict a large fraction that must grow behind dense, obscuring screens of gas and dust. Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In this paper, we use a sample of AGN derived from the deepest X-ray survey to date, the Chandra 7 Ms GOODS-South Survey, to investigate the nature of low-flux X-ray sources. We make full use of the extensive multiwavelength coverage of the GOODS-South field and cross-match our objects with wavelengths from the radio to the IR. We find that the low X-ray flux AGN in our sample have X-ray luminosities that indicate low-luminosity AGN classification, while their radio, IR, and optical counterparts indicate moderate to powerful AGN classification. We find that the predicted column densities are, on average, an order of magnitude higher than the calculated column densities via X-ray detections for X-ray-faint sources. We interpret our results as evidence of obscured AGN disguised as low-luminosity AGN via their X-ray luminosities. When we compare the estimation of the obscured AGN space density with and without these objects, we find a difference of 40% in the lowest X-ray luminosity regime probed by our sample.
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    A new, faint population of X-ray transients.
    (2017) Bauer, Franz Erik; Treister, Ezequiel; Schulze, Steve.; Schawinski, Kevin; Luo, B.; Alexander, D. M.; Brandt, W N.; Comastri, A.; Forster, Francisco; Gilli, Roberto
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    Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses
    (2019) Li, Junyao; Xue, Yongquan; Sun, Mouyuan; Liu, Teng; Vito, Fabio; Brandt, William N.; Hughes, Thomas M.; Yang, Guang; Tozzi, Paolo; Zhu, Shifu; Zheng, Xuechen; Luo, Bin; Chen, Chien-Ting; Vignali, Cristian; Gilli, Roberto; Shu, Xinwen
    We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (N-H > 10(23) cm(-2)). By fitting spectra with physical models, 436 (38%) sources with L-X > 10(42) erg s(-1) are confirmed to be highly obscured, including 102 Compton-thick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N-log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic N-H distribution and its evolution. The CT/highly obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (approximate to 7 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of N-H, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.
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    The XMM-SERVS Survey: XMM-Newton Point-source Catalogs for the W-CDF-S and ELAIS-S1 Fields
    (2021) Ni, Qingling; Brandt, W. N.; Chen, Chien-Ting; Luo, Bin; Nyland, Kristina; Yang, Guang; Zou, Fan; Aird, James; Alexander, David M.; Bauer, Franz Erik; Lacy, Mark; Lehmer, Bret D.; Mallick, Labani; Salvato, Mara; Schneider, Donald P.; Tozzi, Paolo; Traulsen, Iris; Vaccari, Mattia; Vignali, Cristian; Vito, Fabio; Xue, Yongquan; Banerji, Manda; Chow, Kate; Comastri, Andrea; Del Moro, Agnese; Gilli, Roberto; Mullaney, James; Paolillo, Maurizio; Schwope, Axel; Shemmer, Ohad; Sun, Mouyuan; Timlin III, John D.; Trump, Jonathan R.
    We present the X-ray point-source catalogs in two of the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) fields, W-CDF-S (4.6 deg(2)) and ELAIS-S1 (3.2 deg(2)), aiming to fill the gap between deep pencil-beam X-ray surveys and shallow X-ray surveys over large areas. The W-CDF-S and ELAIS-S1 regions were targeted with 2.3 and 1.0 Ms of XMM-Newton observations, respectively; 1.8 and 0.9 Ms exposures remain after flare filtering. The survey in W-CDF-S has a flux limit of 1.0 x 10(-14) erg cm(-2) s(-1) over 90% of its area in the 0.5-10 keV band; 4053 sources are detected in total. The survey in ELAIS-S1 has a flux limit of 1.3 x 10(-14) erg cm(-2) s(-1) over 90% of its area in the 0.5-10 keV band; 2630 sources are detected in total. Reliable optical-to-IR multiwavelength counterpart candidates are identified for approximate to 89% of the sources in W-CDF-S and approximate to 87% of the sources in ELAIS-S1. A total of 3129 sources in W-CDF-S and 1957 sources in ELAIS-S1 are classified as active galactic nuclei (AGNs). We also provide photometric redshifts for X-ray sources; approximate to 84% of the 3319/2001 sources in W-CDF-S/ELAIS-S1 with optical-to-near-IR forced photometry available have either spectroscopic redshifts or high-quality photometric redshifts. The completion of the XMM-Newton observations in the W-CDF-S and ELAIS-S1 fields marks the end of the XMM-SERVS survey data gathering. The approximate to 12,000 pointlike X-ray sources detected in the whole approximate to 13 deg(2) XMM-SERVS survey will benefit future large-sample AGN studies.