High precision X-ray log N – log S distributions: implications for the obscured AGN population

Abstract

Context. Our knowledge of the properties of AGN, especially those of optical type-2 objects, is very incomplete. Because extragalactic source count distributions are dependent on the cosmological and statistical properties of AGN, they provide a direct method of investigating the underlying source populations. Aims. We aim to constrain the extragalactic source count distributions over a broad range of X-ray fluxes and in various energy bands to test whether the predictions from X-ray background synthesis models agree with the observational constraints provided by our measurements. Methods. We have used 1129 XMM-Newton observations at |b| > 20• covering a total sky area of 132.3 deg 2 to compile the largest complete samples of X-ray selected objects to date both in the 0.5−1 keV, 1−2 keV, 2−4.5 keV, 4.5−10 keV bands employed in standard XMM-Newton data processing and in the 0.5−2 keV and 2−10 keV energy bands more usually considered in source count studies. Our survey includes in excess of 30 000 sources and spans fluxes from ∼10 −15 to 10 −12 erg cm −2 s −1 below 2 keV and from ∼10 −14 to 10 −12 erg cm −2 s −1 above 2 keV where the bulk of the cosmic X-ray background energy density is produced. Results. The very large sample size we obtained means our results are not limited by cosmic variance or low counting statistics. A break in the source count distributions was detected in all energy bands except the 4.5−10 keV band. We find that an analytical model comprising 2 power-law components cannot adequately describe the curvature seen in the source count distributions. The shape of the log N(>S ) − log S is strongly dependent on the energy band with a general steepening apparent as we move to higher energies. This is due to the fact that non-AGN populations, comprised mainly of stars and clusters of galaxies, contribute up to 30% of the source population at energies <2 keV and at fluxes ≥10 −13 erg cm −2 s −1 , and these populations of objects have significantly flatter source count distributions than AGN. We find a substantial increase in the relative fraction of hard X-ray sources at higher energies, from ≥55% below 2 keV to ≥77% above 2 keV. However, the majority of sources detected above 4.5 keV still have significant flux below 2 keV. Comparison with predictions from the synthesis models suggest that the models might be overpredicting the number of faint absorbed AGN, which would call for fine adjustment of some model parameters such as the obscured to unobscured AGN ratio and/or the distribution of column densities at intermediate obscuration.

THE 31 DEG2 RELEASE OF THE STRIPE 82 X-RAY SURVEY: THE POINT SOURCE CATALOG

“…We then retain only objects where det_ml exceeds 15 (i.e., >5σ) in at least one energy band (see Loaring et al 2005;Mateos et al 2008, for a discussion of det_ml limits and their effects on Eddington bias in the derived Log N-Log S relation). We caution that care must be taken when determining the reliability of the reported fluxes, as the catalog includes the emldetect reported fluxes for every band where the source was detected (i.e.,  det ml _ 6).…”

“…The Chandra hard and full band fluxes from the source list were converted from the 2-7 keV and 0.5-7 keV ranges to 2-10 keV and 0.5-10 keV bands as described above. For reference, we also plot the Log N-Log S for a range of survey areas and depths: the deep, pencil-beam E-CDFS in the soft band (0.3 deg 2 ; Lehmer et al 2005) and the XMM-Newton survey of CDFS in the hard band (∼0.25 deg 2 ; Ranalli et al 2013); the moderate-area, moderate-depth Chandra COSMOSLegacy Survey (2.2 deg 2 ; Civano et al 2015;Marchesi et al 2015) in all three bands; and the wide-area 2XMMi Serendipitous Survey in the soft and hard bands (132 deg 2 ; Mateos et al 2008). The Stripe 82X Log N-Log S agrees with the reported trends from other surveys in the soft-band, the high-flux end in the hard and full bands, and with CDFS at the low-flux end (<´-2 10 14 erg s −1 ) in the hard band; discrepancies in these bands at lower fluxes (and between CDFS and COSMOS-Legacy and 2XMMi in the hard band at low fluxes) may be due to different methods for estimating survey sensitivity when generating area-flux curves and different assumed values for the power-law slope (Γ) when converting count rate to flux, and are not necessarily atypical when comparing number counts from different surveys.…”

“…Cumulative Log N-Log S relationship for the Stripe 82X survey (black circles) in the soft (top left), hard (top right) , and full (bottom) bands. For reference, we also plot the source number density for other surveys, spanning the gamut from deep, pencil-beam surveys (i.e., the 0.3 deg 2 ECDF-S and ∼0.25 deg 2 CDFS; Lehmer et al 2005;Ranalli et al 2013, respectively), to a moderate-area, moderate depth survey (the 2.2 deg 2 Chandra COSMOS-Legacy; Civano et al 2015;Marchesi et al 2015), and a wide-area survey (the 132 deg 2 2XMMi Serendipitous Survey; Mateos et al 2008). …”

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

“…To construct our sample of EXO50 sources in the bright flux regime we used one of the largest well defined and complete Xray source sample derived so far (discussed in Mateos et al 2008), based on the 2XMM source catalog 2 ).…”

“…Starting from a source list of 9431 sources (see Mateos et al 2008), these first selection criteria provide us with a list of about 600 X-ray emitting objects.…”