INTEGRAL has discovered hundreds of new sources in the hard X-ray range (> 20 keV). Most of these sources have an unknown nature, and multi-wavelength studies are necessary to try to classify them. This can be a challenging task since different diagnostics can lead to different classifications for the source type.
Occasionally, however, the sky is quite cooperative. The image above shows one example of a source discovered in hard X-rays by INTEGRAL: IGR J13042-1020. Multi-wavelength follow-up observations with the Swift satellite allowed the X-ray, UV, and optical counterparts to be identified. The image is a composite of the Swift/UVM2, UVW1, UVW2 observations. The small red circle and the large white circle represent the Swift/XRT soft X-ray and IBIS/ISGRI hard X-ray position, respectively. The UV/optical counterpart is the nucleus of the galaxy NGC 4939 which permits an easy identification of IGR J13042-1020 as an active galactic nucleus (AGN). The galaxy is located 140 million light-years away (at a redshift of z~0.01).
Examination of the X-ray spectra further enabled designation of the source as a probable Type 2 Seyfert AGN. These objects are powered by accretion of large amounts of material onto a central supermassive black hole. One of the characteristics of Type 2 AGN is the high absorption that can shield a significant amount of the soft X-rays, and hinders or prevents their discovery through scanning. By observing at higher energies that are unaffected by absorption, INTEGRAL has discovered dozens of such systems.
Photo credit: European Space Agency
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