Fermi Large Area Telescope detection of extended gamma-ray emission from the radio galaxy Fornax A
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Ackermann_2016_ApJ_826_1.pdf
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1.01 MB
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Date Issued
July 2016
Journal
Astrophysical journal
Publisher
American Astronomical Society
Citation
Ackermann, M., et al., "Fermi Large Area Telescope detection of extended gamma-ray emission from the radio galaxy Fornax A." Astrophysical journal 826 (July 2016): no. 1 doi 10.3847/0004-637X/826/1/1 ©2016 Author(s)
Version
Final published version
Abstract
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. ©2014
MIT Department
MIT Kavli Institute for Astrophysics and Space Research
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DOI of Published Version
https://doi.org/10.3847/0004-637X/826/1/1
