Long-term optical and radio variability of BL Lacertae

TL;DR

This study analyzes over four decades of optical and radio data of BL Lacertae, revealing periodicities, correlations, and spectral trends, and proposes a jet-based model to explain its long-term variability.

Contribution

It provides the first comprehensive long-term multi-band light curves and identifies periodicities and spectral behaviors, proposing a new jet precession and shock model for variability.

Findings

Detected ~1.26-year optical periodicity.

Identified ~7.50-year radio periodicity.

Observed bluer-when-brighter spectral trend.

Abstract

Well-sampled optical and radio light curves of BL Lacertae in B, V, R, I bands and 4.8, 8.0, 14.5 GHz from 1968 to 2014 were presented in this paper. A possible $1.26 \pm 0.05$ yr period in optical bands and a $7.50 \pm 0.15$ yr period in radio bands were detected based on discrete correlation function, structure function as well as Jurkevich method. Correlations among different bands were also analyzed and no reliable time delay was found between optical bands. Very weak correlations were detected between V band and radio bands. However, in radio bands the variation at low frequency lagged that at high frequency obviously. The spectrum of BL Lacertae turned mildly bluer when the object turned brighter, and stronger bluer-when-brighter trends were found for short flares. A scenario including a precessing helical jet and periodic shocks was put forward to interpret the variation characteristics of BL Lacertae.