Constraining the Physical Conditions in the Jets of Gamma-Ray Flaring Blazars using Centimeter-Band Polarimetry and Radiative Transfer Simulations. I. Data and Models for 0420-014, OJ 287, and 1156+295

TL;DR

This study combines centimeter-band polarimetry and radiative transfer simulations to analyze jet conditions during gamma-ray flares in blazars, revealing shock properties, magnetic field structure, and jet orientation.

Contribution

It introduces a method to constrain jet and shock parameters during gamma-ray flares using combined observational data and simulations, highlighting the shock characteristics and magnetic field configurations.

Findings

Shocks in blazar jets are typically 3-4 in number during outbursts.

Shocks are predominantly forward-moving relative to the jet flow.

A significant ordered magnetic field component is necessary to explain spectral data.

Abstract

To investigate parsec-scale jet flow conditions during GeV gamma-ray flares detected by the Fermi Large Angle Telescope, we obtained centimeter-band total flux density and linear polarization monitoring observations from 2009.5 through 2012.5 with the 26-meter Michigan radio telescope for a sample of core-dominated blazars. We use these data to constrain radiative transfer simulations incorporating propagating shocks oriented at an arbitrary angle to the flow direction in order to set limits on the jet flow and shock parameters during flares temporally associated with gamma-ray flares in 0420-014, OJ 287, and 1156+295; these AGN exhibited the expected signature of shocks in the linear polarization data. Both the number of shocks comprising an individual radio outburst (3-4) and the range of the compression ratios of the individual shocks (0.5-0.8) are similar in all three sources; the shocks are found to be forward-moving with respect to the flow. While simulations incorporating transverse shocks provide good fits for 0420-014 and 1156+295, oblique shocks are required for modeling the OJ~287 outburst, and an unusually-low value of the low energy cutoff of the radiating particles' energy distribution is also identified. Our derived viewing angles and shock speeds are consistent with independent VLBA results. While a random component dominates the jet magnetic field, as evidenced by the low fractional linear polarization, to reproduce the observed spectral character requires that a significant fraction of the magnetic field energy is in an ordered axial component. Both straight and low pitch angle helical field lines are viable scenarios.