The long-term optical spectral variability of BL Lacertae

TL;DR

This study analyzes long-term optical spectral variability of BL Lacertae, revealing a bluer-when-brighter trend, spectral evolution during low-amplitude events, and spectral variations leading flux changes, explained by jet Doppler factor variations.

Contribution

It provides detailed analysis of spectral variability and introduces a model linking spectral changes to jet viewing angle variations.

Findings

Spectral slope correlates with flux following a power-law.

Spectral variations lead flux variations by about 4 days.

No significant delays between B and R-band variations.

Abstract

We present the results from a study of the long-term optical spectral variations of BL Lacertae, using the long and well-sampled B and R-band light curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time intervals of 1 day. The relation between spectral slope and flux (the spectrum gets bluer as the source flux increases) is well described by a power-law model, although there is significant scatter around the best-fitting model line. To some extent, this is due to the spectral evolution of the source (along well-defined loop-like structures) during low-amplitude events, which are superimposed on the major optical flares, and evolve on time scales of a few days. The "bluer-when-brighter" mild chromatism of the long-term variations of the source can be explained if the flux increases/decreases faster in the B than in the R band. The B and R-band variations are well correlated, with no significant, measurable delays larger than a few days. On the other hand, we find that the spectral variations lead those in the flux light curves by ~ 4 days. Our results can be explained in terms of Doppler factor variations due to changes in the viewing angle of a curved and inhomogeneous emitting jet.