X-ray spectral analysis of optically faint sources in the Chandra Deep Fields

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 01/2005; 358(2). DOI: 10.1111/j.1365-2966.2005.08821.x
Source: arXiv


We present the results of a detailed spectral analysis of optically faint hard X-ray sources in the Chandra deep fields selected on the basis of their high X-ray to optical flux ratio (X/O). The stacked spectra of high X/O sources in both Chandra deep fields, fitted with a single power-law model, are much harder than the spectrum of the X-ray background (XRB). The average slope is also insensitive to the 2-8 keV flux, being approximately constant around Gamma~1 over more than two decades, strongly indicating that high X/O sources represent the most obscured component of the XRB. For about half of the sample, a redshift estimate (in most of the cases a photometric redshift) is available from the literature. Individual fits of a few of the brightest objects and of stacked spectra in different redshift bins imply column densities in the range 10^{22-23.5} cm^{-2}. A trend of increasing absorption towards higher redshifts is suggested. Comment: 13 pages, 9 figures, 5 tables, accepted for pubblication in MNRAS

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    ABSTRACT: We investigate the presence of iron line emission among faint X-ray sources identified in the 1Ms Chandra Deep Field South and in the 2Ms Chandra Deep Field North. Individual source spectra are stacked in seven redshift bins over the range z=0.5-4. We find that iron line emission is an ubiquitous property of X-ray sources up to z~3. The measured line strengths are in good agreement with those expected by simple pre-Chandra estimates based on X-ray background synthesis models. The average rest frame equivalent width of the iron line does not show significant changes with redshift. Comment: 5 pages, 2 figures, ApJ Letters in press (include emulateapj.sty)
    Preview · Article · Jan 2005 · The Astrophysical Journal
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    ABSTRACT: We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope $\Gamma$ with an intrinsic redshifted absorption $N_{\rm H}$, a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both $\Gamma$ and $N_{\rm H}$ free. The weighted mean value for the slope of the power law is $\langle \Gamma \rangle \simeq 1.75 \pm 0.02$, and the distribution of best fit values shows an intrinsic dispersion of $\sigma_{\rm int} \simeq 0.30$. We do not find hints of a correlation between the spectral index $\Gamma$ and the intrinsic absorption column density $N_{\rm H}$.
We then investigate the absorption distribution for the whole sample, deriving the $N_{\rm H}$ values in faint sources by fixing $\Gamma = 1.8$. We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates.
By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic $N_{\rm H}$ distribution representative of the whole AGN population, $f(N_{\rm H}) {\rm d}N_{\rm H}$, from the observed one. $f(N_{\rm H})$ shows a lognormal shape, peaking around $\log(N_{\rm H})\simeq 23.1$ and with $\sigma \simeq 1.1$. Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN.
We find that the fraction of absorbed sources (with $N_{\rm H}>10^{22}$ cm$^{-2}$) in the sample is constant (at the level of about $75$%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical type I (type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample). 

    Full-text · Article · May 2006 · Astronomy and Astrophysics
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