Browsing by Author "Civano, F."

Now showing 1 - 5 of 5
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    Discovery of the first heavily obscured QSO candidate at z > 6 in a close galaxy pair
    (2019) Vito, F.; Brandt, W. N.; Bauer, F. E.; Gilli, R.; Luo, B.; Zamorani, G.; Calura, F.; Comastri, A.; Mazzucchelli, C.; Mignoli, M.; Nanni, R.; Shemmer, O.; Vignali, C.; Brusa, M.; Cappelluti, N.; Civano, F.; Volonteri, M.
    While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting z > 6 quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first gigayear of the Universe from an observational point of view. We report the Chandra discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close (approximate to 5 kpc) galaxy pair at z = 6.515. One of the members is an optically classified type-1 QSO, PSO167-13. The companion galaxy was first detected as a [C II] emitter by Atacama large millimeter array (ALMA). An X-ray source is significantly (P = 0.9996) detected by Chandra in the 2-5 keV band, with < 1.14 net counts in the 0.5-2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of N-H > 2 x 10(24) cm(-2) and N-H > 6 x 10(23) cm(-2) at 68% and 90% confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at z > 6.
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    Inferring Compton-thick AGN candidates at z > 2 with Chandra using the > 8 keV rest-frame spectral curvature.
    (2017) Baronchelli, Linda; Ricci, Claudio; Treister, Ezequiel; Koss, M.; Schawinski, K.; Cardamone, C.; Civano, F.; Comastri, A.; Elvis, M.; Lanzuisi, G.; Marchesi, S.
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    NuSTAR J033202-2746.8: Direct Constraints on the Compton Reflection in a Heavily Obscured Quasar at z ≈ 2.
    (2014) Del Moro, A.; Bauer, Franz Erik; Treister, Ezequiel; Mullaney, J. R.; Alexander, D. M.; Comastri, A.; Stern, D.; Civano, F.; Ranalli, P.; Vignali, C.; Aird, J. A.
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    THE NuSTAR EXTRAGALACTIC SURVEY: A FIRST SENSITIVE LOOK AT THE HIGH-ENERGY COSMIC X-RAY BACKGROUND POPULATION
    (2013) Alexander, D. M.; Stern, D.; Del Moro, A.; Lansbury, G. B.; Assef, R. J.; Aird, J.; Ajello, M.; Ballantyne, D. R.; Bauer, F. E.; Boggs, S. E.; Brandt, W. N.; Christensen, F. E.; Civano, F.; Comastri, A.; Craig, W. W.; Elvis, M.; Grefenstette, B. W.; Hailey, C. J.; Harrison, F. A.; Hickox, R. C.; Luo, B.; Madsen, K. K.; Mullaney, J. R.; Perri, M.; Puccetti, S.; Saez, C.; Treister, E.; Urry, C. M.; Zhang, W. W.; Bridge, C. R.; Eisenhardt, P. R. M.; Gonzalez, A. H.; Miller, S. H.; Tsai, C. W.
    We report on the first 10 identifications of sources serendipitously detected by the Nuclear Spectroscopic Telescope Array (NuSTAR) to provide the first sensitive census of the cosmic X-ray background source population at greater than or similar to 10 keV. We find that these NuSTAR-detected sources are approximate to 100 times fainter than those previously detected at greater than or similar to 10 keV and have a broad range in redshift and luminosity (z = 0.020-2.923 and L10-40 keV approximate to 4 x 10(41)-5 x 10(45) erg s(-1)); the median redshift and luminosity are z approximate to 0.7 and L10-40 keV approximate to 3 x 10(44) erg s(-1), respectively. We characterize these sources on the basis of broad-band approximate to 0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared spectral energy distribution analyses. We find that the dominant source population is quasars with L10-40 keV > 10(44) erg s(-1), of which approximate to 50% are obscured with N-H greater than or similar to 10(22) cm(-2). However, none of the 10 NuSTAR sources are Compton thick (N-H greater than or similar to 10(24) cm(-2)) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L10-40 keV > 10(44) erg s(-1)) selected at greater than or similar to 10 keV of less than or similar to 33% over the redshift range z = 0.5-1.1. We jointly fitted the rest-frame approximate to 10-40 keV data for all of the non-beamed sources with L10-40 keV > 10(43) erg s(-1) to constrain the average strength of reflection; we find R < 1.4 for Gamma = 1.8, broadly consistent with that found for local active galactic nuclei (AGNs) observed at greater than or similar to 10 keV. We also constrain the host-galaxy masses and find a median stellar mass of approximate to 10(11) M-circle dot, a factor approximate to 5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass.
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    The X-ray properties of z > 6 quasars: no evident evolution of accretion physics in the first Gyr of the Universe
    (2019) Vito, F.; Brandt, W. N.; Bauer, F. E.; Calura, F.; Gilli, R.; Luo, B.; Shemmer, O.; Vignali, C.; Zamorani, G.; Brusa, M.; Civano, F.; Comastri, A.; Nanni, R.
    Context. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z approximate to 6. However, at z > 6 only approximate to 15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of the QSO population in the first Gyr of the Universe.