High - redshift blazar identification for Swift J 1656 . 3 − 3302 ⋆

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 ∼ 19 mag 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 0.7–200 keV 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 ∼2 keV, possibly due to intrinsic absorption (NH ∼ 7×10 22 cm) local to the source. Alternatively, a slope change (∆Γ ∼ 1) around 2.7 keV can describe the X–ray spectrum equally well. At this redshift, the observed 20–100 keV luminosity of the source is ∼10 erg 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 20 keV. It is also the farthest object, among the previously unidentified INTEGRAL sources, whose nature has been determined a posteriori through optical spectroscopy.