21 - Polarization of Compton X-rays from jets in AGN

Summary

We investigate the polarization of Compton scattered X-rays from relativistic jets in active galactic nuclei (AGN) using Monte-Carlo simulations. We consider three scenarios: scattering of photons from an accretion disk, scattering of cosmic microwave background (CMB) photons, and synchrotron self-Comptonization (SSC) within the jet. For Comptonization of thermal disk photons or CMB photons the maximum linear polarization attained is slightly over 20% at viewing angles close to 90°. The value decreases with the viewing inclination. For SSC, the maximum value may exceed 80%. The angle dependence is complicated, and it varies with the photon injection sites. Our study demonstrates that X-ray polarization, in addition to multi-wavelength spectra, can distinguish certain models for emission and particle acceleration in relativistic jets.

Introduction

Observations of extended jets in AGN by Chandra have revealed that the origins of their X-ray emission is less trivial than previous thought (see for X-ray jet surveys). The X-rays may arise from various processes. The polarization in the radio and optical bands suggests that the emission is generated by the synchrotron process. Thus, synchrotron and synchrotron self-Compton (SSC) emission are candidates for the X-ray continuum emission. However, the X-rays can also be generated from external Comptonization (EC) of disk black-body radiation or of the CMB. It has been suggested that X-ray polarization measurements are able to discriminate these competing emission mechanisms.