Giant lobes of Centaurus A as seen in radio and γ-ray images obtained with the Fermi-LAT and Planck satellites

¹ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
e-mail: ryang@mpi-hd.mpg.de
² School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
³ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Building A28, School of Physics, The University of Sidney, NSW 2006, Sidney, Australia
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁵ MEPHI, Kashirskoe shosse 31, 115409 Moscow, Russia

Received: 7 June 2016
Accepted: 30 July 2016

Abstract

The γ-ray data of Fermi-LAT on the giant lobes of Centaurus A are analysed together with the high frequency radio data obtained with the Planck satellite. The large γ-ray photon statistics, accumulated during seven years of observations, and the recently updated Fermi-LAT collaboration software tools allow substantial extension of the detected γ-ray emission towards higher energy, up to 30 GeV, and lower energy, down to 60 MeV. Moreover, the new γ-ray data allow us to explore the spatial features of γ-ray emission of the lobes. For the north lobe, we confirm, with higher statistical significance, our earlier finding on the extension of γ-ray emission beyond the radio image. Moreover, the new analysis reveals significant spatial variation of γ-ray spectra from both lobes. On the other hand, the Planck observations at microwave frequencies contain important information on spectra of synchrotron emission in the cutoff region, and thus allow model-independent derivation of the strength of the magnetic field and the distribution of relativistic electrons based on the combined γ-ray and radio data. The interpretation of multiwavelength spectral energy distributions of the lobes within a pure leptonic model requires strong enhancement of the magnetic field at the edge of the south lobe. Alternatively, a more complex, leptonic-hadronic model of the γ-ray emission, postulating a non-negligible contribution of the π⁰-decay component at highest energies, can explain the γ-ray data with a rather homogeneous distribution of the magnetic field over the giant lobes.