Astronomy & Astrophysics

A&A 480, 715-721 (2008)
DOI: 10.1051/0004-6361:20078901

High-redshift blazar identification for Swift J1656.3-3302

N. Masetti¹, E. Mason², R. Landi¹, P. Giommi³, L. Bassani¹, A. Malizia¹, A. J. Bird⁴, A. Bazzano⁵, A. J. Dean⁴, N. Gehrels⁶, E. Palazzi¹, and P. Ubertini⁵

¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
e-mail: masetti@iasfbo.inaf.it
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

(Received 23 October 2007 / Accepted 14 January 2008)

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 $R\sim 19$ mag source that shows broad Lyman- $\alpha$ , Si IV, He II, C IV, and C III] emission lines at redshift $z = 2.40 \pm 0.01$ . 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 $\Gamma \sim 1.6$ ) and a hint of spectral curvature below ~2 keV, possibly due to intrinsic absorption ( $N_{\rm H} \sim 7\times 10 ^$ cm^-2) local to the source. Alternatively, a slope change ( $\Delta \Gamma \sim 1$ ) 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 ~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