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

Xiao-na Sun, Rui-zhi Yang, Benjamin Mckinley, Felix Aharonian

Published 2016-06-09Version 1

The {\gamma}-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 {\gamma}-ray photon statistics accumulated during seven years of observations, as well as the recently updated Fermi-LAT collaboration software tools allow substantial extension of the detected {\gamma}-ray emission towards both the higher, up to 30 GeV, and the lower, down to 60 GeV, energies. Moreover, the new {\gamma}-ray data allow us to explore the spatial features of {\gamma}-ray emission of the lobes. For the north lobe, we confirm with higher statistical significance, our earlier finding on extension of {\gamma}-ray emission beyond the radio image. Moreover, the new analysis reveals significant spatial variation of {\gamma}-ray spectra from both lobes. On the other hand, the Planck observations at microwave frequencies contain important information on spectrum 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 {\gamma}-ray and radio data. The interpretation of multiwavelength spectral energy distributions (SED) 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 {\gamma}-ray emission postulating a non-negligible contribution of the {\pi} 0 -decay component at highest energies, can explain the {\gamma}-ray data with a rather homogeneous distribution of the magnetic field over the giant lobes.