Radio Band Observations of Blazar Variability

TL;DR

This study combines radio observations and simulations to understand blazar variability, revealing shocks in jets as key to outbursts and polarization changes, with implications for gamma-ray flaring and jet composition.

Contribution

It provides new radiative transfer simulations at arbitrary shock angles and first-time analysis of circular polarization spectral variability in blazars.

Findings

Shocks in jets cause flux and polarization outbursts.

Mode conversion explains circular polarization in blazars.

Jet composition is consistent with electron-proton plasma.

Abstract

The properties of blazar variability in the radio band are studied using the unique combination of temporal resolution from single dish monitoring and spatial resolution from VLBA imaging; such measurements, now available in all four Stokes parameters, together with theoretical simulations, identify the origin of radio band variability and probe the characteristics of the radio jet where the broadband blazar emission originates. Outbursts in total flux density and linear polarization in the optical-to-radio bands are attributed to shocks propagating within the jet spine, in part based on limited modeling invoking transverse shocks; new radiative transfer simulations allowing for shocks at arbitrary angle to the flow direction confirm this picture by reproducing the observed centimeter-band variations observed more generally, and are of current interest since these shocks may play a role in the gamma-ray flaring detected by Fermi. Recent UMRAO multifrequency Stokes V studies of bright blazars identify the spectral variability properties of circular polarization for the first time and demonstrate that polarity flips are relatively common. All-Stokes data are consistent with the production of circular polarization by linear-to-circular mode conversion in a region that is at least partially self-absorbed. Detailed analysis of single-epoch, multifrequency, all-Stokes VLBA observations of 3C 279 support this physical picture and are best explained by emission from an electron-proton plasma.