IGR J22517+2218 was yet another unidentified object discovered by INTEGRAL/IBIS, but this time the quest for its identification turned out to be particularly rewarding. Follow up Swift/XRT observations identified its optical counterpart in MG3 J225155+2217, a quasar at z=3.668, the farthest object so far detected by INTEGRAL.
The image shows the detection of this new source by the ISGRI camera on IBIS in the 20-100 keV band while the zoom refers to a Swift/XRT observation covering the entire INTEGRAL error box: the brightest object detected in the 2-10 keV band is indeed the high redshift quasar. Superimposed on the image is the combined IBIS/XRT spectrum over the 0.4-100 keV band (or 2-500 keV in the source rest frame).
IGR J22517+2218 has a flat radio spectrum and is radio loud implying a blazar nature. Neverthless, the Source Spectral Energy Distribution is unusual compared to blazars of similar type: either it has the synchrotron peak in the X/gamma-ray band (i.e. much higher than generally observed) or the Compton peak in the MeV range (i.e. lower than typically measured). Other source peculiarities include a narrow line absorption system, spectral curvature below 1-2 keV and X/gamma-ray variability.
The rest frame luminosities are 2x1048 erg/s in hard X-rays (20-100 keV) and 5x1048 erg/s in the soft gamma-ray (100-500 KeV) interval, the highest seen by INTEGRAL in a blazar and 10 trillion times that of our Galaxy; thus IGR J22517+2218=MG3 J225155+2217 is a gamma-ray lighthouse shining from the edge of our Universe.
Credit: L. Bassani, IASF-Bologna, INAF. Details of this observations are described in Bassani et al. 2007, astro-ph(0709.3023), Ap.J.Lett. in press.
Download the picture [JPEG format: 167 Kb].
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