The XMM-NewtonWide Angle Survey (XWAS): the X-ray spectrum of type-1 AGN

Abstract

Aims. We discuss the broad band X-ray properties of one of the largest samples of X-ray selected type-1 AGN to date (487 objects in total), drawn from the XMM-Newton Wide Angle Survey (XWAS). The objects presented in this work cover 2−10 keV (rest-frame) luminosities from ∼10 42 −10 45 erg s −1 and are detected up to redshift ∼4. We constrain the overall properties of the broad band continuum, soft excess and X-ray absorption, along with their dependence on the X-ray luminosity and redshift. We discuss the implications for models of AGN emission. Methods. We fitted the observed 0.2−12 keV broad band spectra with various models to search for X-ray absorption and soft excess. The F-test was used with a significance threshold of 99% to statistically accept the detection of additional spectral components. Results. We constrained the mean spectral index of the broad band X-ray continuum to Γ = 1.96 ± 0.02 with intrinsic dispersion σ Γ = 0.27 +0.01 −0.02 . The continuum becomes harder at faint fluxes and at higher redshifts and hard (2−10 keV) luminosities. The dependence of Γ with flux is likely due to undetected absorption rather than to spectral variation. We found a strong dependence of the detection efficiency of objects on the spectral shape. We expect this effect to have an impact on the measured mean continuum shapes of sources at different redshifts and luminosities. We detected excess absorption in > ∼ 3% of our objects, with rest-frame column densities ∼a few ×10 22 cm −2 . The apparent mismatch between the optical classification and X-ray properties of these objects is a challenge for the standard orientation-based AGN unification model. We found that the fraction of objects with detected soft excess is ∼36%. Using a thermal model, we constrained the soft excess mean rest-frame temperature and intrinsic dispersion to kT ∼ 100 eV and σ kT ∼ 34 eV. The origin of the soft excess as thermal emission from the accretion disk or Compton scattered disk emission is ruled out on the basis of the temperatures detected and the lack of correlation of the soft excess temperature with the hard X-ray luminosity over more than 2 orders of magnitude in luminosity. Furthermore, the high luminosities of the soft excess rule out an origin in the host galaxy.

The X-ray spectral properties of the AGN population in theXMM-Newtonbright serendipitous survey

“…Because the errors on the photon index are not symmetric, we used the average value for the individual errors in each case. We obtain a mean value of Γ = 2.05 ± 0.03 with an intrinsic dispersion of σ = 0.26 ± 0.02 for the BSS and Γ = 1.98 ± 0.08 with an intrinsic dispersion of σ = 0.29 ± 0.05 for the HBSS, in agreement with previous works (Mateos et al 2010;Young et al 2009;Dadina 2008;Mainieri et al 2007;Page et al 2006;Mateos et al 2005a,b). Errors were extracted from the 1σ confidence contours.…”

“…We find an anti correlation between the power-law index and the X-ray luminosity (ρ = −0.21, probability = 8 × 10 −4 ) and marginally redshift (ρ = −0.10, probability = 0.09) with the Spearman rank correlation analysis. We also find a correlation with the 0.5−2 keV flux (ρ = 0.15, probability = 0.01), which could be caused by undetected intrinsic absorption as pointed out in Mateos et al (2010). In that work, the authors found a stronger correlation between the photon index and the source redshift, but in their case it mainly started above redshift 2 and our sample only contains 3 sources above that redshift.…”

“…An anticorrelation between the photon index and redshift and luminosity seems to be present, but it is within the intrinsic dispersion at each redshift or luminosity bin. A similar result is also found in Mateos et al (2010). These authors pointed out that the hardening of the spectra at higher luminosities and redshifts can be caused by a decrement in the detection efficiency for softer sources and an increment for harder sources, given that the sample is flux-limited.…”

“…In other cases, the resulting photon index is ∼1, much lower than the typical values for unabsorbed AGN 3 . In those cases, we fixed Γ to 1.9, a common value for unabsorbed AGN Mateos et al 2005a,b;Galbiati et al 2005;Tozzi et al 2006;Mateos et al 2010). If there was no intrinsic absorption detected, an upper limit, at 90% confidence level, is given.…”

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

“…The X-ray spectra are shown in Figure 2 (right panels). In the case of 2XMMJ100038.9+050955, the spectral quality does not allow us to constrain the power-law photon index and the intrinsic absorption at the same time, so we have fixed the power-law photon index Γ to 1.9, a common value for unabsorbed AGN (Mateos et al 2010, Corral et al 2011, Lanzuisi et al 2013). …”