IGR J22517+2218=MG3 J225155+2217: A New Gamma-Ray Lighthouse in the Distant Universe

We report on the identification of a new soft gamma-ray source, namely, IGR J22517+2218, detected with INTEGRAL IBIS. The source, which has an observed 20-100 keV flux of ~4×10^-11 ergs cm^-2 s^-1, is spatially coincident with MG3 J225155+2217, a quasar at z=3.668. The Swift XRT 0.5-10 keV continuum is flat (Γ=1.5) with evidence for a spectral curvature below 1-2 keV either due to intrinsic absorption (N_H=3+/-2×10²² cm^-2) or to a change in slope (ΔΓ=0.5). X-ray observations indicate flux variability over a 6 day period that is further supported by a flux mismatch between Swift and INTEGRAL spectra. IGR J22517+2218 is radio-loud and has a flat radio spectrum; optically, it is a broad-line emitting quasar with the atypical property of hosting a narrow-line absorption system. The source spectral energy distribution is unusual compared to blazars of similar type: either it has the synchrotron peak in the X-ray/gamma-ray band (i.e., much higher than generally observed) or the Compton peak in the MeV range (i.e., lower than typically measured). IGR J22517+2218=MG3 J225155+2217 is the second most distant blazar detected above 20 keV and a gamma-ray lighthouse shining from the edge of our universe.