Volume 480, Number 3, March IV 2008
|Page(s)||715 - 721|
|Published online||25 January 2008|
High-redshift blazar identification for Swift J1656.3-3302*
INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: firstname.lastname@example.org
2 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
3 ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
4 School of Physics & Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
5 INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
6 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
Accepted: 14 January 2008
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-α, 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.
Key words: quasars: emission lines / quasars: individual: J1656.3-3302 / galaxies: high-redshift / galaxies: active / X-rays: galaxies / astrometry
Partly based on X-ray observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA, and on optical observations collected at ESO (La Silla, Chile) under programme 079.A–0171(A).
© ESO, 2008
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