Magnetic field strength constraints on $\gamma$-ray flaring regions in the flat spectrum radio quasar PKS 1222+216

TL;DR

This study analyzes multi-wavelength data of PKS 1222+216 to constrain magnetic fields and locate gamma-ray emission regions, revealing interactions between jet components and non-equipartition magnetic conditions.

Contribution

It provides new estimates of magnetic field strengths and emission region locations in a blazar, using long-term multi-wavelength observations and component interaction analysis.

Findings

Gamma-ray flare coincides with jet component interaction.

Magnetic field scales as B∝r^{-0.3}, indicating non-equipartition.

Gamma-ray emission region is about 9.2 pc from the central engine.

Abstract

We present a multi-wavelength study of the Flat Spectrum Radio Quasar PKS 1222+216, analyzing its long-term variability of radio data obtained in 2013-2020 from the iMOGABA, MOJAVE, and VLBA-BU-BLAZAR programs, along with $\gamma$-ray data from Fermi-LAT. We found that the radio flux densities at 15, 22, 43, and 86 GHz declined exponentially by 37%-56% over a year following a $\gamma$-ray flare in November 2014. We estimated jet physical parameters through Gaussian model fitting of VLBA 43 GHz data, identifying 10 jet components. The cooling timescales of the jet emission regions, i.e., newly ejected components C9, C10, and C11, range from 43 to 222 days, with the estimated jet viewing angles of approximately 8 degrees and magnetic field strengths of 77-134 mG in the jet emission regions. Additionally, by determining the magnetic field strength at different frequencies, we found that the magnetic field scales as $B\propto r^{-0.3\pm0.04}$, indicating a non-equipartition condition ($k_\text{r}\gtrsim 1$) or a slow decline in magnetic field strength profile ($m<1$). By analyzing component ejection times, we discovered that the $\gamma$-ray flare in 2014 coincided with the interaction between the moving component C9 and the stationary feature A1. We estimated that the $\gamma$-ray emission region is located at $9.2\pm1.0$ pc from the central engine, beyond the BLR and dusty torus, suggesting that the seed photons for inverse Compton scattering originate from the jet itself, external CMB radiation, or a surrounding sheath. Our results favor a scenario where $\gamma$-ray emission originates further downstream from the central engine through interactions between moving and stationary components. Additionally, our study presents an alternative method for estimating magnetic field strengths in AGNs undergoing long-term synchrotron cooling based on the associated timescale.