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Fermi LAT Discovery of Gamma-rays from the Giant Radio Lobes of Centaurus A

Fermi LAT Discovery of Gamma-rays from the Giant Radio Lobes of Centaurus A. C.C. Teddy Cheung (NRC/NRL) Lukasz Stawarz (ISAS/JAXA) Yasushi Fukazawa (Hiroshima) Jürgen Knödlseder (CESR) on behalf of the Fermi-LAT Collaboration. Gamma-ray Imaging of a Radio Galaxy.

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Fermi LAT Discovery of Gamma-rays from the Giant Radio Lobes of Centaurus A

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  1. Fermi LAT Discovery of Gamma-rays from the Giant Radio Lobes of Centaurus A C.C. Teddy Cheung (NRC/NRL) Lukasz Stawarz (ISAS/JAXA) Yasushi Fukazawa (Hiroshima) Jürgen Knödlseder (CESR) on behalf of the Fermi-LAT Collaboration

  2. Gamma-ray Imaging of a Radio Galaxy Over ½ of the total >100 MeV observed LAT flux in the lobes • Cen A • ATCA 1.4 GHz • Pre-launch expectation that lobes can produce γ-rays via inverse Compton process • Cen A is (uniquely) large enough to directly image with LAT -- derive physical conditions • What does this mean for LAT radio galaxies (& blazars) in general? • Feain et al. (2010)

  3. Cen A the Radio Galaxy • Nearest Radio-loud AGN • Radio source extent: L~10o • At D=3.7 Mpc, L=600 kpc • ☐

  4. Cen A the Radio Galaxy • Nearest Radio-loud AGN • Radio source extent: L~10o • At D=3.7 Mpc, L=600 kpc • ☐ • All-sky 408 MHz (Haslam et al. 1982)

  5. Not a Complete Surprise •  IC/CMB γ-ray lobes? Expect Lγ /LS ~ UCMB/UB ~ 1-10 •  LAT resolution = 0.8O at 1GeV so can image the radio structure • ? • ? • How far does high-energy spectrum extend ? • Cheung 2007 (1st GLAST symp)

  6. Expectations for Fermi LAT • Lobes detected by WMAP at 20-60 GHz • LAT • WMAP • IC/CMB • IC/EBL • LAT predictions for Cen A: • Hardcastle, et al. 2009 • Page et al. 2007

  7. Here’s what LAT sees in γ-rays • LAT • >200 MeV • WMAP • 22 GHz • PSF • Background (isotropic and diffuse) and field point sources subtracted • WMAP images provided by Nils Odegard (GSFC)

  8. Gamma-ray and Radio Counts Profiles • North Lobe • South Lobe • Regions 1 & 2 (north) and 4 & 5 (south) as defined in Hardcastle et al. 2009

  9. Relevant Photon Fields • CMB • EBL (various) • starlight • dust • Dominant photon fields are CMB and IR+optical extragalactic background light (EBL)

  10. Inverse Compton Emission • LAT • SAS-3 • First inverse Compton (IC) lobe measurements in γ-rays! • Good fits of LAT spectra with B ~ 1 μG in both lobes • Approximate energy equipartition, i.e., Ue/UB ~ 1 to 4 • IC/CMB X-rays in radio lobes: Ue/UB ~1-10

  11. Inverse Compton Emission: Close-up • Detected LAT emission dominated by IC/CMB component for the modeled electron energy spectra • EBL component dominating at highest-energies (>GeV)

  12. Gamma-ray probe of the EBL? • Different EBL models/compilations give (only) slight differences in predicted inverse Compton gamma-ray spectrum

  13. Inferred Physical Parameters • LAT detected γ-ray emission requires 0.1-1 TeV electrons in 100’s kpc-scale lobes that are probably accelerated in-situ • Non-thermal plasma / ambient gas in approximate pressure balance • Jet Calorimetry: • Etot = 1058 erg radiated in both lobes over 30 Myrs • Jet power ~7 x 1042 erg s-1 (~10-5Ledd) sub-Eddington, typical for low power radio galaxies • LAT γ-ray image requires a very extended source (L~10o in this case) – other examples?

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