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Current Research Highlight :: October 2007

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Summed hard X-ray spectrum of local AGN: a link to the cosmic X-ray background.

It is widely believed that the cosmic X-ray background (CXB) is the summed redshifted emission from all active galactic nuclei (AGN) in the visible Universe and hence a unique integral record of the history of growth of massive black holes. Scientists at the Max Planck Institute for Astrophysics have carried out yet another crucial test of this paradigm.

Fig. 1: Upper panel: Cumulative 3-300 keV spectrum of local AGN with hard X-ray luminosities (in the 17-60 keV or 3-20 keV energy band) ranging from 10⁴¹ to 10^43.5 erg/s, obtained by stacking individual AGN spectra measured by INTEGRAL and RXTE. The associated statistical and combined statistical and systematic uncertainties are shown by error bars and yellow-coloured regions, respectively. The solid line is the best-fit model of the data by a sum of absorbed and unabsorbed power-law spectra with a high-energy exponential cutoff. The dashed lines indicate the contributions of unobscured, obscured and Compton thick AGN. Lower panel: The same but for AGN with hard X-ray luminosities over 10^43.5 erg/s.

Fig. 2: Upper part of the plot: Solid line - the result of convolving the local cumulative AGN spectrum (sum of the two spectra shown in Fig. 1) with the redshift dependence of the AGN X-ray luminosity density inferred from deep extragalactic surveys conducted by Chandra and XMM-Newton. The vertically hatched region represents the associated statistical uncertainty and the horizontally hatched region the additional 20% systematic uncertainty in the overall normalization. The points with error bars represent the cosmic X-ray background spectrum measured by INTEGRAL during Earth occultation observations (Research Highlight October 2006). For comparison in the lower part of the plot the result of a similar calculation for the case of no cosmological evolution of AGN (i.e. the same total AGN luminosity per cubic Mpc at all redshifts as at z=0) is shown.

Over the recent years a wealth of observational data has provided further strong support to the picture in which the CXB is a superposition of AGN. In particular, the bulk of the CXB at energies below several keV has been directly resolved by X-ray telescopes into individual AGN. However, the peak of the CXB spectral energy distribution is located near 30 keV where 99% of the background emission remains unresolved and it was never directly demonstrated that the sum of the hard X-ray (between ten and several hundred keV) spectra of all AGN residing in a given volume of Universe is indeed compatible with the measured spectral energy distribution of the CXB (see Research Highlight October 2006 on a recent CXB measurement by INTEGRAL). Such crucial comparison has now been made by MPA researchers using all-sky hard X-ray surveys performed by the RXTE (Research Highlight February 2004) and INTEGRAL (Research Highlight October 2006 and Research Highlight May 2007) observatories.

The goal was to determine the cumulative spectrum of the local (at redshift z<0.1) AGN population in the broad energy band 3-300 keV. To this end a stacking analysis was performed of the spectra of 84 AGN detected by the PCA instrument aboard RXTE and of 68 AGN detected by the IBIS/ISGRI instrument aboard INTEGRAL, properly taking into account the space densities of AGN with different luminosities (using the well-known 1/Vmax technique).

The resulting summed spectrum (Figure 1) peaks at 50-80 keV and rolls over both below 20 keV (due to photoabsorption in obscured AGN which outnumber unobscured ones) and above 100-200 keV. This local cumulative AGN spectrum proves to be consistent, albeit within the significant statistical and systematic uncertainties, with the CXB spectral energy distribution in both shape and normalization (Figure 2) if the strong "cosmic downsizing" of AGN between redshifts z~1 and z=0, known from deep X-ray surveys with Chandra and XMM-Newton, has occured without significant changes in the average hard X-ray spectral shape of AGN.

The implication is that the popular concept of the CXB being a superposition of AGN is grossly correct. Improved measurements of the cumulative spectral distribution and cosmological evolution of AGN by current and future X-ray and hard X-ray astronomy missions will provide tighter constraints on the cosmic history of massive black hole growth and AGN unification schemes.

S. Sazonov, M. Revnivtsev, R. Krivonos, E. Churazov, R. Sunyaev

Reference:

S. Sazonov, R. Krivonos, M. Revnivtsev, E. Churazov, and R. Sunyaev, Cumulative hard X-ray spectrum of local AGN: a link to the cosmic X-ray background, submitted to Astronomy and Astrophysics, arXiv0708.3215

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