INTEGRAL Picture Of the Month
August 2008

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INTEGRAL chases obscured active galaxies in the deepest extragalactic hard X-ray survey

A large fraction of active galactic nuclei is thought to be hidden behind vast quantity of gas and dust that absorbs low-energy X-rays. Thanks to its hard X-ray imager IBIS, INTEGRAL is able to pierce this opaque layer and uncover the full population of active galactic nuclei.

A new study exploited this capability to perform the deepest extragalactic hard X-ray survey. This survey, centered around the quasar 3C 273 and the Coma cluster, uses 4 million seconds of exposure time and covers 12% of the sky. Thanks to the very large exposure time, the faintest sources are much fainter that those detected in previous extragalactic hard X-ray surveys. The figure on the left shows the effective exposure time map, with contours indicating the survey border and the regions exposed more than 10 000, 100 000, 300 000 and 500 000 seconds respectively. The positions of the 34 active galactic nuclei which have been detected in this survey are also marked with green dots.

The figure on the right shows the cumulative source counts as a function of minimum hard X-ray flux, separately for absorbed (blue) and non-absorbed (red) sources. The number of absorbed sources is about three times larger than that of non-absorbed sources, while the numbers for sources selected in the medium X-rays are approximately equal at these flux levels. This highlights the importance of pushing the sensitivity limit of hard X-ray surveys to its absolute minimum if we want to understand the population of active galaxies.


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