Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT campaign and its extension

TL;DR

The 2012 outburst of blazar CTA 102 was extensively observed across multiple wavelengths, revealing co-spatial optical and gamma-ray emission, with flux relationships and polarization changes explained by jet dynamics and Doppler effects.

Contribution

This study provides a comprehensive multi-wavelength analysis of CTA 102's 2012 outburst, highlighting the co-spatial emission regions and the impact of Doppler factor variations on observed flux relationships.

Findings

Optical and gamma-ray emissions are co-spatial with a ~1 hour lag.

Gamma-ray and optical fluxes follow SSC mechanism with different relations during outburst and post-outburst.

Spectral hardening correlates with increased Doppler shift and jet orientation changes.

Abstract

After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and gamma-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray emission regions. The relation between the gamma-ray and optical fluxes is consistent with the SSC mechanism, with a quadratic dependence of the SSC gamma-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the gamma-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in gamma-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.