Parisa RoustazadehMarkus Boettcher45 min

Electron-Positron Pair Cascades in Blazars and Radio Galaxies Induced by VHE Gamma-Ray

The discovery of very high energy (VHE; E > 100 GeV) gamma-ray emission from intermediate- and low-frequency-peaked blazars suggests that gamma-gamma absorption and pair cascading might occur in these objects. I investigate the interaction of nuclear VHE gamma-rays with Ly-alpha line emission from the Broad Line Region or the thermal infrared radiation field from a circumnuclear dust torus through gamma-gamma absorption and pair production, and the subsequent Compton-supported pair cascades. I have developed a Monte-Carlo code to follow the spatial development of the cascade in full 3-dimensional geometry, and calculate the radiative output due to the cascade as a function of viewing angle with respect to the primary VHE gamma-ray beam (presumably the jet axis of the blazar). I demonstrate that this emission can explain the Fermi fluxes and spectra of radio galaxies Cen A and NGC 1275. It is demonstrated that the magnetic field from the AGN environment can not be determined from a fit of the cascade emission to the gamma-ray spectrum alone, and the degeneracy can only be lifted if the synchrotron emission from the cascades is observed as well. It is pointed out that the cascade synchrotron emission may produce spectral features reminiscent of the big blue bump observed in the spectral energy distributions of several blazars, and this idea is illustrated for 3C 279.