The radio delay of the exceptional 3C 454.3 outburst. Follow-up WEBT observations in 2005-2006

TL;DR

This study analyzes multi-frequency radio and optical observations of blazar 3C 454.3 during its 2005-2006 outburst, revealing complex correlations and delays that inform jet structure and variability mechanisms.

Contribution

It provides detailed multi-frequency monitoring and interprets the variability using a combined intrinsic and geometric model of jet behavior.

Findings

Radio outburst lasted over a year with delayed peaks at lower frequencies.

The major radio peak is linked to a minor optical flare, not the initial optical outburst.

Variability explained by jet disturbances and curved-jet geometry effects.

Abstract

In spring 2005 the blazar 3C 454.3 was observed in an unprecedented bright state from the near-IR to the hard X-ray frequencies. A mm outburst peaked in June-July 2005, and it was followed by a flux increase at high radio frequencies. In this paper we report on multifrequency monitoring by the WEBT aimed at following the further evolution of the outburst in detail. In particular, we investigate the expected correlation and time delays between the optical and radio emissions in order to derive information on the variability mechanisms and jet structure. A comparison among the light curves at different frequencies is performed by means of visual inspection and discrete correlation function, and the results are interpreted with a simple model taking into account Doppler factor variations of geometric origin. The high-frequency radio light curves show a huge outburst starting during the dimming phase of the optical one and lasting more than 1 year. The first phase is characterized by a slow flux increase, while in early 2006 a major flare is observed. The lower-frequency radio light curves show a progressively delayed and fainter event, which disappears below 8 GHz. We suggest that the radio major peak is not physically connected with the spring 2005 optical one, but it is actually correlated with a minor optical flare observed in October-November 2005. This interpretation involves both an intrinsic and a geometric mechanism. The former is represented by disturbances travelling down the emitting jet, the latter being due to the curved-jet motion, with the consequent differential changes of viewing angles of the different emitting regions.