Gamma-ray light curve and VLBI polarization connection in Mrk 421

TL;DR

This study combines gamma-ray and high-resolution radio observations of Mrk 421 to explore the connection between gamma-ray activity and radio polarization, revealing simultaneous peaks and polarization rotations linked to gamma-ray variability.

Contribution

It provides the most comprehensive temporal and spatial radio data for a TeV blazar and establishes new constraints on jet physics and gamma-ray emission regions.

Findings

Radio and gamma-ray peaks are simultaneous, indicating a linked emission region.

Polarization angle rotation correlates with gamma-ray activity, suggesting magnetic field changes.

Constraints on jet Doppler factor and magnetic field configuration are derived.

Abstract

We present Fermi-LAT and multi-frequency, multi-epoch VLBA data for the TeV blazar Mrk421. We collected the data during a long and intensive multi-frequency campaign in 2011. We study the gamma-ray light curve, the photon index evolution and their connection to the radio data on sub-parsec scales, including total intensity, polarized flux density, polarization angle, spectral index, and rotation measure both in the core and the jet region. The VLBA data were obtained at 15 and 24 GHz for 12 epochs and at 43 GHz for 23 epochs, thus providing the best temporal and spatial coverage in the radio band ever achieved for a TeV blazar. We provide significant constraints on the jet Doppler factor, the presence of proper motion, the magnetic field configuration, and an intriguing connection between variability in the radio data and the gamma-ray light curve: the total intensity and polarized core emission reach a peak simultaneously to the main gamma-ray peak, followed by a rotation of the polarization angle at low frequency. Opacity-related, long wavelength polarization swings are also detected later in the year, possibly related to secondary peaks in the gamma-ray light curve, setting constraints on the physics of the gamma-ray zone.