[Paper Review] Linear Polarization as a Probe of Gamma Ray Flaring Blazar Jets

M. F. Aller, H. D. Aller+1 more|ArXiv.org|Dec 16, 2009

Astrophysics and Cosmic Phenomena17 references3 citations

TL;DR

This study uses multifrequency centimeter-wave linear polarization and total flux density monitoring of gamma-ray-bright blazars to probe magnetic field changes in radio jets during gamma-ray flaring. It identifies shock-associated polarization swings and polarized flux outbursts as key signatures, with resolved flares in 3C 279 and 1502+106 indicating shock-driven jet activity linked to high-energy emission.

ABSTRACT

We describe and present initial results from a Fermi cycle 2 program designed to monitor the behavior of the centimeter-band linear polarization and total flux density emitted by gamma-ray-bright blazars during flaring. The goal of the program is to identify changes in the magnetic field structure in the radio jet associated with gamma-ray flaring and ultimately to test whether gamma-ray flaring is associated with the onset of shocks in the radio jet. Light curves illustrating radio band variability patterns are shown for sample program sources.

Motivation & Objective

• To identify changes in jet magnetic field structure during gamma-ray flaring using centimeter-band linear polarization monitoring.
• To test the hypothesis that gamma-ray flaring is triggered by shock formation in the radio jet, as suggested by earlier EGRET and Fermi data.
• To link radio band polarization variability with high-energy gamma-ray flares to constrain the location and nature of the emission site.
• To use multifrequency polarimetry and VLBA imaging to model shock dynamics and physical conditions in relativistic jets.
• To determine whether shock-induced magnetic field ordering and polarization swings precede or coincide with gamma-ray flares.

Proposed method

• Conducting daily to weekly monitoring of 30 gamma-ray-bright blazars at 14.5, 8.0, and 4.8 GHz using the University of Michigan 26-m radio telescope (UMRAO).
• Measuring total flux density, polarized flux, and electric vector position angle (EVPA) to track changes in magnetic field orientation and degree of order.
• Analyzing multifrequency light curves to detect shock signatures such as EVPA swings and polarized flux outbursts.
• Combining single-dish UMRAO data with VLBA imaging from MOJAVE and BU 43 GHz programs to resolve jet structure and constrain shock geometry.
• Applying shock-in-jet modeling using transfer codes that allow arbitrary shock orientations, informed by polarization and flux data.
• Using spectral indices and frequency-dependent variability to infer emission region location, distinguishing between self-absorbed (4.8 GHz) and core (14.5 GHz) regions.

Experimental results

Research questions

• RQ1 Do gamma-ray flares in blazars coincide with measurable changes in the linear polarization and EVPA of the radio jet?
• RQ2 Is the observed polarization swing and increase in fractional polarization during flares consistent with a shock-induced compression of the magnetic field?
• RQ3 Can shock-in-jet models constrained by multifrequency polarization and flux data reproduce the observed variability patterns in gamma-ray flaring sources?
• RQ4 Are the physical conditions in the jet—such as shock strength, Doppler factor, and particle energy spectrum—consistent with gamma-ray emission models?
• RQ5 Do sources with high gamma-ray activity but low radio variability exhibit frequency-dependent time delays or masking due to multiple components?

Key findings

• Resolved radio band flares with EVPA swings and polarized flux outbursts were detected in 1502+106, 3C 279, 0727-115, 0805-077, and 1510-089, indicating shock activity.
• In 1502+106, a systematic EVPA swing and polarized flux outburst began in February 2009, tracking at 14.5 and 8.0 GHz but diverging at 4.8 GHz, suggesting different emission regions.
• The 4.8 GHz data show a self-absorbed spectrum, indicating emission from a region further out in the jet than the higher-frequency core emission.
• In 3C 279, polarization variations in 2009 April–May were consistent with a transverse shock model, similar to mid-1980s events successfully modeled previously.
• Some gamma-ray flaring sources, such as NRAO 190, showed no large-amplitude radio variability, suggesting possible frequency-dependent delays or component masking.
• Long-term monitoring of NRAO 530 shows low flux levels in both radio and gamma-ray bands since Fermi launch, supporting a broadband emission scenario with correlated activity.