Is the High-Energy Emission from Centaurus A Compton-Scattered Jet Radiation?
NAVAL RESEARCH LAB WASHINGTON DC E O HULBURT CENTER FOR SPACE RESEARCH
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We consider whether the hard X-ray and soft gamma-ray emission from Centaurus A is beamed radiation from the active nucleus which is Compton-scattered into our line-of-sight. We derive the spectrum and degree of polarization of scattered radiation when incident beamed radiation is scattered from a cold kappa Tau mec2 electron cloud moving with bulk relativistic motion along the jet axis, and calculate results for an unpolarized, highly-beamed incident power-law photon source. The spectra of the scattered radiation exhibit a cut-off at gamma-ray energies due to electron recoil. The cut off energy depends on the observers viewing angle and the bulk Lorentz factor of the scattering medium. We fit the OSSE data from Centaurus A with this model and find that if the scatterers are not moving relativistically, then the angle the jet makes with respect to our line-of-sight is 61 degrees plus or minus 5 degrees. We predict a high degree of polarization of the scattered radiation below 300 keV. Future measurements with X-ray and gamma-ray polarimeters could be used to constrain or rule out such a scenario.