Origin and Evolution of the Multi-band Variability in the Flat Spectrum Radio Source 4C 38.41

TL;DR

This study investigates the multi-band variability of the flat spectrum radio quasar 4C 38.41, linking radio and gamma-ray flares with jet activity and magnetic field properties, supporting the shock-in-jet model.

Contribution

It provides multi-frequency VLBI observations correlating gamma-ray flares with jet component ejections and analyzes magnetic field evolution during flares.

Findings

Gamma-ray flares coincide with ejection of new VLBI components.

Magnetic fields remain stable around 0.1 mG during flares.

The source is particle dominated, with magnetic fields much lower than equipartition values.

Abstract

The flat spectrum radio quasar 4C 38.41 showed a significant increase of its radio flux density during the period 2012 March - 2015 August which correlates with gamma-ray flaring activity. Multi-frequency simultaneous VLBI observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional monitoring observations at various bands across the electromagnetic spectrum. The epochs of the maxima for the two largest gamma-ray flares coincide with the ejection of two respective new VLBI components and the evolution of the physical properties seem to be in agreement with the shock-in-jet model. Derived synchrotron self absorption magnetic fields, of the order of 0.1 mG, do not seem to dramatically change during the flares, and are much smaller, by a factor 10,000, than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle dominated emitting region.