Measuring the cosmic hard X-ray background with INTEGRAL

An international team of astronomers, including researchers from the Max Planck Institute for Astrophysics, used the Earth as a giant screen to measure the combined X-ray flux of hundred thousands of active supermassive black holes at the centers of distant galaxies.

Fig. 1: 17-60 keV all-sky image obtained by INTEGRAL during four years of operations. The sources in this image were artificially blurred and the color map was stretched to make both strong and weak sources readily visible. The concentration of sources along the mid-plane of the image is due to neutron stars and stellar mass black holes in our Galaxy, while the majority of sources located far away from the Galactic plane are supermassive black holes in other galaxies. The Cosmic X-ray Background is composed of the emission of tens of millions of similar objects much further away from us. Superposed is an Earth image by ESA/EUMETSAT's Meteosat satellite. Blocking the emission from a distant source by the Earth disk allows astronomers to measure the intensity of the X-ray and gamma-ray background. An angular size of the Earth as seen from Integral during actual observations was smaller than shown in the image.

Fig. 2: Comparison of the CXB spectrum measured by INTEGRAL (blue points with error bars) with a model in which AGNs experience luminosity downsizing since z~1.5 till z=0, as suggested by deep extragalactic X-ray surveys, while the fractions of obscured and unobscured AGNs remain the same as measured locally by INTEGRAL. The two upper curves and the shaded regions around them reflect the current uncertainty in the AGN evolution at high redshifts (z>1-1.5). Also shown are the contributions to the CXB from AGNs with different degrees of obscuration.

Supermassive black holes millions and billions times heavier than our Sun are lurking in the centers of all galaxies. Many of these black holes are dormant now, but each had a period of rapid growth in its history when a vast amount of radiation was emitted. Such objects are observed as Seyfert galaxies in our vicinity and as powerful quasars in the distant Universe. All together the growing black holes are believed to be making a dominant contribution to the "cosmic X-ray background" (CXB) - hard radiation filling the space around us.

Understanding the nature of the CXB means that we know its flux and that we can account for all of it in terms of known populations of objects. At low energies (below few keV) deep surveys with large X-ray telescopes like XMM-Newton and Chandra already resolve 90% of the CXB, directly counting of order of 1000 sources per square degree of the sky. At energies higher than 10 or 20 keV such an exercise is far beyond the capabilities of existing instruments and other techniques have been employed by the INTEGRAL observatory to study the CXB above 20 keV.

First of all, along with an extensive study of the Galactic plane INTEGRAL used a fraction of its observing time to map the whole sky in the 17-60 keV band, identifying more than a hundred of active black holes (AGNs) in the nuclei of nearby galaxies. While these objects contribute at most few per cent of the CXB, the all sky survey by INTEGRAL provides un unbiased sample of local active black holes, including those surrounded by large masses of cold gas which absorbs soft X-ray radiation, making such objects invisible below few keV. Using this survey, the researchers have studied the distribution of nearby AGNs in hard X-ray luminosity and demonstrated that the fraction of obscured objects is high (~70%) in the faint end of the AGN luminosity function and low (~25%) in its bright end.

Secondly, aiming at measuring the total CXB flux, the INTEGRAL observatory spent several days looking directly at the Earth disk. At first glance it looks strange that in order to study emission from extragalactic sources one has to point the telescope at the Earth instead. In fact the Earth works as a giant shield, which blocks the emission of a distant source and causes a decrease in the flux seen by INTEGRAL. By measuring this decrease the combined emission from all distant sources, no matter how weak they are, can be measured. This is schematically shown in Fig. 1, where the Earth disk is superposed onto the all sky 20-50 keV map obtained by INTEGRAL. This task is not trivial since the Earth itself is a source of hard X-ray radiation, produced by cosmic rays interacting with the Earth atmosphere.

Now, knowing the total flux of the CXB and the detailed properties of the local population of active black holes, scientists are able to learn more on how such black holes have been evolving from early times (corresponding to redshifts z~1.5) till today (z=0, Fig. 2). It turns out that both the amplitude and spectral distribution of the CXB can be explained if the AGN population as a whole has experienced pure luminosity downsizing (from bright quasars to relatively weak Seyfert galaxies), as suggested by deep extragalactic X-ray surveys, while its other key properties such as the ratio of obscured to unobscured AGNs have not changed. This has important implications for theoretical models trying to unify different types of AGNs based on geometrical orientation and physical properties.

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


Churazov et al.:
"Integral observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth",
Astron. & Astrophys.; linkPfeilExtern.gifastro-ph/0608250

Sazonov et al.:
"Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations",
Astron. & Astrophys.; linkPfeilExtern.gifastro-ph/0608253

Churazov et al.:
"Earth X-ray albedo for CXB radiation in the 1-1000 keV band",
Astron. & Astrophys., in press; linkPfeilExtern.gifastro-ph/0608252