XMM-Newton/Chandra Fe K line statistics and the role of ionized accretion discs

Thomas Boller (MPE)

The Fe K line profiles observed in the majority of active galactic nuclei are narrow and unresolved, in contrast to the expectation of relativistically blurred and broadened lines. To address this basic scientific aspect we consider the following. I. Individual XMM-Newton/Chandra spectra We have conducted a literature search of all AGN observed with XMM-Newton in the 2-10 keV band. Of about 60 objects which could potentially show a relativistic iron line, only 3 do so with relative confidence. The question arises: Why are the effects of relativistic motions in radio-quiet moderately luminous AGN not detectable in the XMM-Newton and Chandra spectra? II. Fe K line simulations We simulated Fe K_alpha emission under various condition to determine the likelihood of detecting the feature with a typical 40 ks XMM-Newton observation. First we considered a neutral accretion disc. Varying the observed flux from 10^{-11} down to 10^{-14}, and the equivalent width of the Fe K_alpha line from 400 down to 100 eV shows, that the relativistic line is clearly detected with XMM-Newton. We then considered if ionization plays a major role.We simulated accretion disc spectra for ionized discs and found that with relatively mildly ionization the effects of Compton broadening within the disc are already so strong that the line becomes undetectable for XMM-Newton. III. The first stacked XMM-Newton spectrum of type 1 AGN in the local universe We have stacked 36 moderately luminous PG quasars which do not show any evidence of relativistic lines. A broad and significant Fe K line is clearly visible in the ratio plot, supporting the results from the simulations that XMM-Newton lacks the sensitive to detect relativistic lines in ionized accretion discs.

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