# The magnetic field structure in CTA 102 from high resolution mm-VLBI observations during the flaring state in 2016-2017

## TL;DR

This study reveals the magnetic field structure in CTA 102's jet during a flaring state using high-resolution mm-VLBI polarimetric observations, showing evidence of large-scale helical magnetic fields and their role in jet activity.

## Contribution

First high-resolution mm-VLBI polarimetric analysis of CTA 102 during a flaring period, identifying magnetic field configurations and their connection to jet dynamics and high-energy flares.

## Key findings

- Detection of a Faraday rotation gradient indicating helical magnetic fields.
- Observation of magnetic field changes during superluminal component crossing.
- Correlation between jet interactions and multi-wavelength flares.

## Abstract

Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to shed light on the crucial physical processes giving rise to the jet formation, as well as to its extraordinary radiation output up to gamma-ray energies. We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm-VLBI polarimetric observations, reaching an unprecedented resolution (~50 microarcsec). We also investigate the variability and physical process occurring in the source during the observing period which coincides with a very active state of the source till high-energies. The Faraday rotation analysis between 3 and 7mm shows a gradient in rotation measure with a maximum value of ~6X10^4 rad/m^2 and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7mm EVPAs orientation is different when the component is exiting the core or crossing a stationary feature at ~0.1 mas. The interaction between the superluminal component and a recollimation shock at ~0.1 mas could have triggered the multi-wavelengths flares. The variability Doppler factor associated with such interaction is large enough to explain the high energy emission, as we infer from the analysis of gamma-ray and X-ray data, and it is in agreement with the Doppler factor obtained to explain the extraordinary optical flare by Raiteri et al.(2017).