Exploring the Variability of the Flat-spectrum Radio Source 1633+382. II. Physical Properties

The flat-spectrum radio quasar 1633+382 (4C 38.41) showed a significant increase of its radio flux density during the period 2012 March-2015 August, which correlates with γ-ray flaring activity. Multi-frequency simultaneous very long baseline interferometry (VLBI) observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional radio monitoring observations with the OVRO 40 m telescope, the Boston University VLBI program, and the Submillimeter Array. The epochs of the maxima for the two largest γ-ray flares coincide with the ejection of two respective new VLBI components. Analysis of the spectral energy distribution indicates a higher turnover frequency after the flaring events. The evolution of the flare in the turnover frequency-turnover flux density plane probes the adiabatic losses in agreement with the shock-in-jet model. The derived synchrotron self-absorption magnetic fields, of the order of 0.1 mG, do not seem to change dramatically during the flares, and are much weaker, by a factor 10⁴, than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle-dominated emitting region.