Fermi Large Area Telescope Detection of Extended Gamma-Ray Emission from the Radio Galaxy Fornax A

M. Ackermann , M. Ajello , L. Baldini , J. Ballet (1) , G. Barbiellini , D. Bastieri , R. Bellazzini , E. Bissaldi , R.D. Blandford , E.D. Bloom , R. Bonino , T.J. Brandt , J. Bregeon (2) , P. Bruel (3) , R. Buehler , S. Buson , G.A. Caliandro , R.A. Cameron , M. Caragiulo , P.A. Caraveo , E. Cavazzuti , C. Cecchi , E. Charles , A. Chekhtman , C.C. Cheung , G. Chiaro , S. Ciprini , J.M. Cohen , J. Cohen-Tanugi (2) , F. Costanza , S. Cutini , F. d'Ammando , D.S. Davis , A. de Angelis , F. de Palma , R. Desiante , S.W. Digel , N. Di Lalla , M. Di Mauro , L. Di Venere , C. Favuzzi , S.J. Fegan (3) , E.C. Ferrara , W.B. Focke , Y. Fukazawa , S. Funk , P. Fusco , F. Gargano , D. Gasparrini , M. Georganopoulos , N. Giglietto , F. Giordano , M. Giroletti , G. Godfrey , D. Green , I.A. Grenier (1) , S. Guiriec , E. Hays , J.W. Hewitt , A.B. Hill , T. Jogler , G. Jóhannesson , S. Kensei , M. Kuss , S. Larsson , L. Latronico , J. Li , L. Li , F. Longo , F. Loparco , P. Lubrano , J.D. Magill , S. Maldera , A. Manfreda , M. Mayer , M.N. Mazziotta , W. Mcconville , J.E. Mcenery , P.F. Michelson , W. Mitthumsiri , T. Mizuno , M.E. Monzani , A. Morselli , I.V. Moskalenko , S. Murgia , M. Negro , E. Nuss (2) , M. Ohno , T. Ohsugi , M. Orienti , E. Orlando , J.F. Ormes , D. Paneque , J.S. Perkins , M. Pesce-Rollins , F. Piron (2) , G. Pivato , T.A. Porter , S. Rainò , R. Rando , M. Razzano , A. Reimer , O. Reimer , J. Schmid (1) , C. Sgrò , D. Simone , E.J. Siskind , F. Spada , G. Spandre , P. Spinelli , Ł. Stawarz , H. Takahashi , J.B. Thayer , D.J. Thompson , D.F. Torres , G. Tosti , E. Troja , G. Vianello , K.S. Wood , M. Wood , S. Zimmer

1 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
2 UM2 - Université Montpellier 2 - Sciences et Techniques
3 LLR - Laboratoire Leprince-Ringuet

We report the Fermi Large Area Telescope detection of extended γ-ray emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8 data. After Centaurus A, this is now the second example of an extended γ-ray source attributed to a radio galaxy. Both an extended flat disk morphology and a morphology following the extended radio lobes were preferred over a point-source description, and the core contribution was constrained to be $\lt 14$% of the total γ-ray flux. A preferred alignment of the γ-ray elongation with the radio lobes was demonstrated by rotating the radio lobes template. We found no significant evidence for variability on ∼0.5 year timescales. Taken together, these results strongly suggest a lobe origin for the γ-rays. With the extended nature of the $\gt 100\,{\rm{MeV}}$ γ-ray emission established, we model the source broadband emission considering currently available total lobe radio and millimeter flux measurements, as well as X-ray detections attributed to inverse Compton (IC) emission off the cosmic microwave background (CMB). Unlike the Centaurus A case, we find that a leptonic model involving IC scattering of CMB and extragalactic background light (EBL) photons underpredicts the γ-ray fluxes by factors of about ∼2–3, depending on the EBL model adopted. An additional γ-ray spectral component is thus required, and could be due to hadronic emission arising from proton–proton collisions of cosmic rays with thermal plasma within the radio lobes.

Mots clés: galaxies: active; galaxies: individual; galaxies: jets; gamma rays: galaxies; radiation mechanisms: non-thermal

Domaines: Astrophysique [astro-ph]

M. Ackermann, M. Ajello, L. Baldini, J. Ballet, G. Barbiellini, et al.. Fermi Large Area Telescope Detection of Extended Gamma-Ray Emission from the Radio Galaxy Fornax A. Astrophys.J., 2016, 826 (1), pp.1. ⟨10.3847/0004-637X/826/1/1⟩. ⟨hal-01582675⟩