FERMI LARGE AREA TELESCOPE DETECTION OF EXTENDED GAMMA-RAY EMISSION FROM THE RADIO GALAXY FORNAX A
Authors: Ackermann M. , Ajello M. , Baldini L. , Ballet J. , Barbiellini G. , Bastieri D. , Bellazzini R. , Bissaldi E. , Blandford R. D. , Bloom E. D. , Bonino R. , Brandt T. J. , Bregeon J. , Bruel P. , Buehler R. , Buson S. , Caliandro G. A. , Cameron R. A. , Caragiulo M. , Caraveo P. A. , Cavazzuti E. , Cecchi C. , Charles E. , Chekhtman A. , Cheung C. C. , Chiaro G. , Ciprini S. , Cohen J. M. , Cohen-Tanugi J. , Costanza F. , Cutini S. , D’Ammando F. , Davis D. S. , Angelis A. de , Palma F. de , Desiante R. , Digel S. W. , Lalla N. Di , Mauro M. Di , Venere L. Di , Favuzzi C. , Fegan S. J. , Ferrara E. C. , Focke W. B. , Fukazawa Y. , Funk S. , Fusco P. , Gargano F. , Gasparrini D. , Georganopoulos M. , Giglietto N. , Giordano F. , Giroletti M. , Godfrey G. , Green D. , Grenier I. A. , Guiriec S. , Hays E. , Hewitt J. W. , Hill A. B. , Jogler T. , Jóhannesson G. , Kensei S. , Kuss M. , Larsson S. , Latronico L. , Li J. , Li L. , Longo F. , Loparco F. , Lubrano P. , Magill J. D. , Maldera S. , Manfreda A. , Mayer M. , Mazziotta M. N. , McConville W. , McEnery J. E. , Michelson P. F. , Mitthumsiri W. , Mizuno T. , Monzani M. E. , Morselli A. , Moskalenko I. V. , Murgia S. , Negro M. , Nuss E. , Ohno M. , Ohsugi T. , Orienti M. , Orlando E. , Ormes J. F. , Paneque D. , Perkins J. S. , Pesce-Rollins M. , Piron F. , Pivato G. , Porter T. A. , Rainò S. , Rando R. , Razzano M. , Reimer A. , Reimer O. , Schmid J. , Sgrò C. , Simone D. , Siskind E. J. , Spada F. , Spandre G. , Spinelli P. , Stawarz Ł. , Takahashi H. , Thayer J. B. , Thompson D. J. , Torres D. F. , Tosti G. , Troja E. , Vianello G. , Wood K. S. , Wood M. , Zimmer S. 
Published: July 2016

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

% 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

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.