[Paper Review] The XMM-Newton wide-field survey in the COSMOS field. The point-like X-ray source catalogue

N. Cappelluti, M. Brusa+25 more|ArXiv.org|Jan 15, 2009

Astrophysical Phenomena and ObservationsPhysics and Astronomy48 references169 citations

TL;DR

This paper presents the XMM-Newton wide-field X-ray survey in the COSMOS field, delivering a highly homogeneous catalogue of 1,887 point-like X-ray sources detected over 2.13 deg² with a limiting flux of ~1.7 × 10⁻¹⁵ erg cm⁻² s⁻¹ in the 0.5–2 keV band. The study derives precise logN-logS relations and X-ray colour-colour diagrams, confirming that ~20% of AGN-like sources show no optical signs of nuclear activity despite high X-ray luminosities, highlighting a population of hidden or Compton-thick AGN.

ABSTRACT

The COSMOS survey is a multiwavelength survey aimed to study the evolution of galaxies, AGN and the large scale structure. The XMM-COSMOS is a deep X-ray survey over the full 2 deg2 of the COSMOS area. It consists of 55 XMM-Newton pointings for a total exposure of ~1.5 Ms with an average vignetting corrected depth of 40 ks across the field of view and a sky coverage of 2.13 deg2. We present the catalogue of point-like X-ray sources detected with the EPIC CCD cameras, the logN-logS relations and the X-ray colour-colour diagrams. The analysis was performed in the 0.5-2 keV, 2-10 keV and 5-10 keV energy bands. The completeness of the catalogue as well as logN-logS have been calibrated using Monte Carlo simulations. The catalogs contains a total of 1887 unique sources detected in at least one band. The survey, that shows unprecedented homogeneity, has a flux limit of ~1.7x10-15 erg cm-2 s-1, ~9.3x10-15 erg cm-2 s-1 and ~1.3x10-14 erg cm-2 s-1 over 90% of the area (1.92 deg2) in the 0.5-2 keV, 2-10 keV and 5-10 keV energy band, respectively. Thanks to the rather homogeneous exposure over a large area, the derived logN-logS relations are very well determined over the flux range sampled by XMM-COSMOS. These relations have been compared with XRB synthesis models, which reproduce the observations with an agreement of ~10% in the 5-10 keV and 2-10 keV band, while in the 0.5-2 keV band the agreement is of the order of ~20%. The hard X-ray colors confirmed that the majority of the extragalactic sources, in a bright subsample, are actually Type I or Type II AGN. About 20% of the sources have X-ray luminosity typical of AGN (L_X >1042 erg/s) although they do not show any clear signature of nuclear activity in the optical spectrum.

Motivation & Objective

• To create a comprehensive, homogeneous X-ray point-source catalogue from the XMM-Newton survey in the COSMOS field, covering 2.13 deg² with deep exposure.
• To determine the logN-logS relations and source density maps with high statistical precision using Monte Carlo simulations for calibration.
• To characterize the X-ray properties of extragalactic sources using colour-colour diagrams and hardness ratios, identifying AGN types and obscured populations.
• To investigate the fraction of AGN with high X-ray luminosities but no optical signatures of nuclear activity.
• To validate the survey's consistency with X-ray background (XRB) population synthesis models across multiple energy bands.

Proposed method

• The survey used 55 XMM-Newton pointings totaling ~1.5 Ms, achieving an average exposure of ~40 ks across the field after vignetting correction.
• Source detection was performed using a maximum likelihood technique optimized for raster scan surveys, with a detection threshold of det_ml > 10.
• Monte Carlo simulations were used to calibrate source completeness, flux limits, and logN-logS relations across the 0.5–2 keV, 2–10 keV, and 5–10 keV energy bands.
• X-ray hardness ratios (HR1, HR2) and colour-colour diagrams were constructed to classify source spectral types, including Compton-thick candidates.
• Optical spectroscopy and multiwavelength data were used to cross-identify sources and classify AGN types, particularly in bright subsamples.
• The results were compared with XRB population synthesis models (Gilli et al. 2007; Treister & Urry 2006) to assess model consistency across energy bands.

Experimental results

Research questions

• RQ1 What is the distribution of X-ray source counts as a function of flux (logN-logS) in the 0.5–10 keV range across the COSMOS field?
• RQ2 How well do current X-ray background (XRB) population synthesis models reproduce the observed source counts in different energy bands?
• RQ3 What fraction of X-ray bright sources with L_X > 10⁴² erg/s show no optical signs of AGN activity?
• RQ4 Which X-ray colour-colour patterns indicate the presence of Compton-thick or heavily absorbed AGN?
• RQ5 To what extent does the survey’s homogeneous exposure minimize cosmic variance in source density measurements?

Key findings

• The XMM-COSMOS survey detected 1,887 unique X-ray point-like sources with det_ml > 10, distributed across 0.5–2 keV (1,621 sources), 2–10 keV (1,111 sources), and 5–10 keV (251 sources).
• The survey achieved a flux limit of ~1.7 × 10⁻¹⁵ erg cm⁻² s⁻¹ in the 0.5–2 keV band, ~9.3 × 10⁻¹⁵ erg cm⁻² s⁻¹ in the 2–10 keV band, and ~1.3 × 10⁻¹⁴ erg cm⁻² s⁻¹ in the 5–10 keV band over 90% of the 1.92 deg² area.
• The derived logN-logS relations show excellent agreement with XRB models within ~10% in the 2–10 keV and 5–10 keV bands, but a ~20% deviation in the 0.5–2 keV band.
• In the bright subsample with optical spectroscopy, 65% were classified as Type I AGN and ~15% as Type II AGN, with the remainder being star-forming or emission-line galaxies.
• Approximately 20% of sources with X-ray luminosities typical of AGN (L_X > 10⁴² erg/s) show no optical signatures of nuclear activity, indicating a population of hidden or Compton-thick AGN.
• Two sources (ID #2608 and #131) exhibit X-ray colours consistent with Compton-thick AGN with 3% and 1% leaking flux, respectively, in line with XRB model predictions.