The High-Energy Emission Mechanisms in Quasar Jets

The emission mechanisms in extragalactic jets include both synchrotron and various inverse-Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR 1) and high-power (FR 2 and quasar) jets, because of the power-law nature of the spectra observed and high polarizations. However, at higher energies, the emission mechanism for high-power jets at kpc scales is hotly debated. Two mechanisms have been proposed: either inverse-Comptonization of CMB photons (IC-CMB), or synchrotron emission from a second, high-energy population of electrons. Recent observations with HST, Spitzer and Chandra have discovered two quasar jets, 3C 273 and PKS 1136+135, where in one or more knots a single spectral component is seen to connect the emission from optical through X-ray energies. This raises the possibility of using optical polarimetry as a diagnostic of the emission mechanism at X-ray as well as optical energies. Polarimetry is a useful diagnostic for synchrotron emission because the implied magnetic field vector of the emission in any band is aligned with the direction of the magnetic field in the emitting region, with the degree of polarization indicative of the magnetic field's ordering. We show that if the emission is due to IC-CMB, the emission should be unpolarized, because the seed photon population is unpolarized. We then discuss high-energy emission mechanisms for powerful jets in the light of the recent HST polarimetry observations reported by Niedzielski et al. We also discuss the constraints on both emission mechanism as well as other jet parameters (beaming, matter content) that can be added by using gamma-ray (Fermi LAT) observations.