The XXL Survey

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 deg 2 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.

Super

“…The counterparts to the X-ray sources in the CDF-S and COSMOS are provided along with the optical-to-MIR multi-wavelength photometry by the original catalogs (Hsu et al 2014;Laigle et al 2016), where in both cases images were previously registered at the same reference and the photometry was PSF-homogenized. The counterparts to the SUPER targets in XMM-XXL and Stripe82X are known in the SDSS optical images and the corresponding associations to the X-ray sources are likewise given in the original catalogs (Fotopoulou et al 2016;LaMassa et al 2016;Ananna et al 2017). The remaining SDSS targets are WISE selected with follow-up in the X-ray band (Martocchia et al 2017).…”

Photometric redshifts for the next generation of deep radio continuum surveys – I. Template fitting

“…As illustrated in Sections 4.1-4.3, no single template set can perform well for all types of radio-detected galaxy. To obtain the best photometric redshift estimates for sources in future deep radio continuum surveys one would therefore ideally like to pre-classify every galaxy and fit it with the optimum method for that source type (as successfully implemented by Fotopoulou et al 2016). However, some of the key properties necessary for such a priori classification are potentially not going to be known at the time photometric redshifts are fitted.…”

The Chandra Cosmos-Legacy Survey: Source X-Ray Spectral Properties

“…A proper analysis of the different X-ray spectral parameters requires large datasets, with both good Xray statistics and complete multiwavelength characterization. Since developing these types of datasets is not trivial, even the dependences of the photon index and the intrinsinc absorption from other quantities (e.g., redshift, X-ray or bolometric luminosity) are still debated, as are the X-ray spectral properties of optically classified Type 1 and Type 2 AGN (see, e.g., Mateos et al 2005;Page et al 2005;Shemmer et al 2006;Young et al 2009;Sobolewska & Papadakis 2009;Lusso et al 2012;Lanzuisi et al 2013;Fotopoulou et al 2016).…”