The XXL Survey

VI. The 1000 brightest X-ray point sources^⋆,⋆⋆

¹ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: sotiria.fotopoulou@unige.ch
² Argelander Institut für Astronomie, Universität Bonn, Auf dem Huegel 71, 53121 Bonn, Germany
³ IAASARS, National Observatory of Athens, 15236 Penteli, Greece
⁴ Lund Observatory, PO Box 43, 22100 Lund, Sweden
⁵ Service d’Astrophysique AIM, CEA Saclay, 91191 Gif-sur-Yvette, France
⁶ Aristotle University of Thessaloniki, Physics Department, 54124 Thessaloniki, Greece
⁷ Instituto Nacional de Astrofísica Óptica y Electrónica, AP 51 y 216, 72000 Puebla, Mexico
⁸ Université Aix Marseille, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) 13388 UMR 7326, Marseille, France
⁹ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00078 Monte Porzio Catone (Rome), Italy
¹⁰ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹¹ Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
¹² INAF, IASF Milano, via Bassini 15, 20133 Milano, Italy
¹³ Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
¹⁴ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
¹⁵ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
¹⁶ Main Astronomical Observatory, Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St., 03680 Kyiv, Ukraine
¹⁷ Australian Astronomical Observatory, PO Box 915, North Ryde NSW 1670, Australia
¹⁸ Department of Physics, University of Zagreb, Bijenicka cesta 32, 10000 Zagreb, Croatia
¹⁹ Astronomical Observatory, National Taras Schevchenko University of Kyiv, 3 Observatorna St., 04053 Kyiv, Ukraine
²⁰ Department of Astronomy and Space Sciences, Faculty of Science, Istanbul University, 34119 Istanbul, Turkey
²¹ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
²² Laboratoire d’Astrophysique de Marseille, Pôle de l’Étoile, Site de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
²³ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁴ H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK
²⁵ Astrophysics and Cosmology Research Unit, University of KwaZulu-Natal, 4041 Durban, South Africa
²⁶ Chalmers University of Technology Onsala Space Observatory, 439 92 Onsala, Sweden
²⁷ INAF−Osservatorio Astronomico di Brera, via Brera, 28, 20159 Milano, Italy
²⁸ Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁹ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
³⁰ National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, 1205 W. Clark St., Urbana, IL 61801, USA
³¹ Department of Astronomy, University of Illinois at Urbana-Champaign, W. Green Street, Urbana, IL 61801, USA
³² Department of Physics andAstronomy, Macquarie University, NSW 2109, Australia and Australian Astronomical Observatory PO Box 915, North Ryde NSW 1670, Australia
³³ INAF−Astronomical Observatory of Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
³⁴ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
³⁵ University of California, Davis, US
³⁶ Astrophysics Department Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
³⁷ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, Canada

Received: 18 September 2015
Accepted: 12 February 2016

Abstract

Context. X-ray extragalactic surveys are ideal laboratories for the study of the evolution and clustering of active galactic nuclei (AGN). Usually, a combination of deep and wide surveys is necessary to create a complete picture of the population. Deep X-ray surveys provide the faint population at high redshift, while wide surveys provide the rare bright sources. Nevertheless, very wide area surveys often lack the ancillary information available for modern deep surveys. The XXL survey spans two fields of a combined 50 deg² observed for more than 6Ms with XMM-Newton, occupying the parameter space that lies between deep surveys and very wide area surveys; at the same time it benefits from a wealth of ancillary data.

Aims. This paper marks the first release of the XXL point source catalogue including four optical photometry bands and redshift estimates. Our sample is selected in the 2 − 10 keV energy band with the goal of providing a sizable sample useful for AGN studies. The limiting flux is F_{2 − 10 keV} = 4.8 × 10^-14 erg s^-1 cm^-2.

Methods. We use both public and proprietary data sets to identify the counterparts of the X-ray point-like sources by means of a likelihood ratio test. We improve upon the photometric redshift determination for AGN by applying a Random Forest classification trained to identify for each object the optimal photometric redshift category (passive, star forming, starburst, AGN, quasi-stellar objects (QSO)). Additionally, we assign a probability to each source that indicates whether it might be a star or an outlier. We apply Bayesian analysis to model the X-ray spectra assuming a power-law model with the presence of an absorbing medium.

Results. We find that the average unabsorbed photon index is ⟨Γ⟩ = 1.85 ± 0.40 while the average hydrogen column density is log ⟨N_H⟩ = 21.07 ± 1.2 cm^-2. We find no trend of Γ or N_H with redshift and a fraction of 26% absorbed sources (log N_H> 22) consistent with the literature on bright sources (log L_x> 44). The counterpart identification rate reaches 96.7% for sources in the northern field, 97.7% for the southern field, and 97.2% in total. The photometric redshift accuracy is 0.095 for the full XMM-XXL with 28% catastrophic outliers estimated on a sample of 339 sources.

Conclusions. We show that the XXL-1000-AGN sample number counts extended the number counts of the COSMOS survey to higher fluxes and are fully consistent with the Euclidean expectation. We constrain the intrinsic luminosity function of AGN in the 2 − 10 keV energy band where the unabsorbed X-ray flux is estimated from the X-ray spectral fit up to z = 3. Finally, we demonstrate the presence of a supercluster size structure at redshift 0.14, identified by means of percolation analysis of the XXL-1000-AGN sample. The XXL survey, reaching a medium flux limit and covering a wide area, is a stepping stone between current deep fields and planned wide area surveys.