2008A&A...480..715M

We report on the high-redshift blazar identification of a new gamma-ray source, Swift J1656.3-3302, detected with the BAT imager onboard the Swift satellite and the IBIS instrument on the INTEGRAL satellite. Follow-up optical spectroscopy has allowed us to identify the counterpart as an R∼19mag source that shows broad Lyman-α, Si IV, He II, C IV, and C III] emission lines at redshift z=2.40±0.01. Spectral evolution is observed in X-rays when the INTEGRAL/IBIS data are compared to the Swift/BAT results, with the spectrum steepening when the source gets fainter. The 0.7-200keV X-ray continuum, observed with Swift/XRT and INTEGRAL/IBIS, shows the power law shape typical of radio loud (broad emission line) active galactic nuclei (with a photon index Γ∼1.6) and a hint of spectral curvature below ∼2keV, possibly due to intrinsic absorption (N_H∼7x10²²cm^–2) local to the source. Alternatively, a slope change (ΔΓ∼1) around 2.7keV can describe the X-ray spectrum equally well. At this redshift, the observed 20-100keV luminosity of the source is ∼10⁴⁸erg/s (assuming isotropic emission), making Swift J1656.3-3302 one of the most X-ray luminous blazars. This source is yet another example of a distant gamma-ray loud quasar discovered above 20keV. It is also the farthest object, among the previously unidentified INTEGRAL sources, whose nature has been determined a posteriori through optical spectroscopy.