Is the High-Energy Emission from Centaurus A Compton-Scattered Jet Radiation?

Abstract

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 ($kT<<m_ec^2$) electron cloud moving with bulk relativistic motion along the jet axis, and calculate results for an unpolarized, highly-beamed incident power-law photon source. 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 $61^\circ\pm 5^\circ$. We predict a high degree of polarization of the scattered radiation below $\sim300$ keV. Future measurements with X-ray and gamma-ray polarimeters could be used to constrain or rule out such a scenario.