Volume 533, September 2011
|Number of page(s)||11|
|Published online||30 August 2011|
Implications for the structure of the relativistic jet from multiwavelength observations of NGC 6251
SISSA/International School for Advance Studies, via Bonomea 265, 34136 Trieste, Italy
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
3 Istituto di Astrofisica e Fisica Cosmica - Bologna, INAF, via Gobetti 101, 40129 Bologna, Italy
4 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
5 Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
6 Department of Physics and Astronomy, Barnard College, Columbia University, New York NY 10027, USA
7 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
8 Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, 70126 Bari, Italy
9 INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
Received: 1 March 2011
Accepted: 15 June 2011
NGC 6251 is a luminous radio galaxy ≈104 Mpc away that was detected significantly with the Fermi Gamma-ray Space Telescope, and before that with EGRET (onboard the Compton Gamma-ray Observatory). Different observational constraints favor a nuclear origin for the γ-ray emission. Here we present a study of the spectral energy distribution of the core of NGC 6251, and give results of modeling in the one-zone synchrotron/SSC framework. The SSC model provides a good description of the radio to γ-ray emission but, as for other misaligned sources, predicts a lower Lorentz factor (Γ ~ 2.4) than typically found when modeling blazars. If the blazar unification scenario is correct, this seems to point to the presence of at least two emitting regions in these objects, one with a higher and one with a lower Lorentz factor. The solution of a structured jet, with a fast moving spine surrounded by a slow layer, is explored and the consequences of the two models for the jet energetics and evolution are discussed.
Key words: galaxies: jets / galaxies: active / gamma rays: galaxies / galaxies: individual: NGC 6251
© ESO, 2011
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