Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies

TL;DR

This study analyzes the multiwavelength variability of blazar 4C 38.41 during its 2011 outburst, revealing contributions from thermal disc emission, jet polarization, and jet orientation changes affecting observed flux and polarization.

Contribution

It provides a comprehensive multiwavelength analysis of 4C 38.41's variability, highlighting the role of jet orientation and inhomogeneous jet structure in flux and polarization changes.

Findings

Optical-UV emission includes QSO-like thermal component and polarized jet emission.

No clear correlation between optical and radio light curves, but optical and gamma-ray fluxes are correlated with fading over time.

Variability can be explained by changes in jet orientation and Doppler boosting, with estimated Doppler factors between 7 and 21.

Abstract

The quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum. We present the results of low-energy multifrequency monitoring by the GASP project of the WEBT consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. In the optical-UV band, several results indicate that there is a contribution from a QSO-like emission component, in addition to both variable and polarised jet emission. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R(QSO) ~ 17.85 - 18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and \gamma-ray flux apparently fades in time, likely because of an increasing optical to \gamma-ray flux ratio. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor \delta. Under the hypothesis that in the period 2008-2011 all the \gamma-ray and optical variability on a one-week timescale were due to changes in \delta, this would range between ~ 7 and ~ 21. If the variability were caused by changes in the viewing angle \theta\ only, then \theta\ would go from ~ 2.6 degr to ~ 5 degr. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.