Masetti, N., Mason, E., Landi, R., Giommi, P., Bassani, L., Malizia, A., Bird, A.J., Bazzano, A., Dean, A.J., Gehrels, N., Palazzi, E. and Ubertini, P. (2008) High-redshift blazar identification for Swift J1656.3-3302. Astronomy & Astrophysics, 480 (3), 715-721. (doi:10.1051/0004-6361:20078901).
Abstract
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 mag source that shows broad Lyman-, Si IV, He II, C IV, and C III] emission lines at redshift . 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 ) and a hint of spectral curvature below ~2 keV, possibly due to intrinsic absorption ( cm-2) local to the source. Alternatively, a slope change ( ) 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 ~1048 erg s-1 (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.
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