The correlated optical and radio variability of BL Lacertae^*

WEBT data analysis 1994–2005

¹ 
INAF, Osservatorio Astronomico di Torino, Italy e-mail: villata@oato.inaf.it 

² Astronomical Institute, St.-Petersburg State University, Russia 

³ Pulkovo Observatory, Russia 

⁴ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia 

⁵ Abastumani Astrophysical Observatory, Georgia 

⁶ Department of Astronomy, University of Michigan, MI, USA 

⁷ Max-Planck-Institut für Radioastronomie, Germany 

⁸ Armenzano Astronomical Observatory, Italy 

⁹ Institute of Theoretical Physics and Astrophysics, Masaryk University, Czech Republic 

¹⁰ Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, Uzbekistan 

¹¹ Institute for Astrophysical Research, Boston University, MA, USA 

¹² Astro Space Center of Lebedev Physical Institute, Russia 

¹³ Metsähovi Radio Observatory, Helsinki University of Technology TKK, Finland 

¹⁴ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland 

¹⁵ Dipartimento di Fisica, Università di Perugia, Italy 

¹⁶ Boltwood Observatory, ON, Canada 

¹⁷ INAF, Osservatorio Astrofisico di Catania, Italy 

¹⁸ Institut de Ciències de l'Espai (IEEC-CSIC), Barcelona, Spain 

¹⁹ Université de Bordeaux, Observatoire Aquitain des Sciences de l'Univers, France 

²⁰ CNRS, Laboratoire d'Astrophysique de Bordeaux – UMR5804, France 

²¹ Agrupació Astronòmica de Sabadell, Spain 

²² INAF, Osservatorio Astronomico di Roma, Italy 

²³ INAF, Osservatorio Astronomico di Collurania Teramo, Italy 

²⁴ INAF, Istituto di Radioastronomia, Sezione di Noto, Italy 

²⁵ Department of Physics and Astronomy, Georgia State University, GA, USA 

²⁶ Department of Physics and Astronomy, University of Southampton, UK 

²⁷ Nordic Optical Telescope, Roque de los Muchachos Astronomical Observatory, TF, Spain 

²⁸ Department of Physics, University of Colorado Denver, CO, USA 

²⁹ National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, China 

³⁰ Physics Department, Yunnan University, China

Received: 13 March 2009
Accepted: 7 April 2009

Abstract

Context. Since 1997, BL Lacertae has undergone a phase of high optical activity, with the occurrence of several prominent outbursts. Starting from 1999, the Whole Earth Blazar Telescope (WEBT) consortium has organized various multifrequency campaigns on this blazar, collecting tens of thousands of data points. One of the main issues in the study of this huge dataset has been the search for correlations between the optical and radio flux variations, and for possible periodicities in the light curves. The analysis of the data assembled during the first four campaigns (comprising also archival data to cover the period 1968–2003) revealed a fair optical-radio correlation in 1994–2003, with a delay of the hard radio events of ~100 days. Moreover, various statistical methods suggested the existence of a radio periodicity of ~8 years.

Aims. In 2004 the WEBT started a new campaign to extend the dataset to the most recent observing seasons, in order to possibly confirm and better understand the previous results.

Methods. In this campaign we have collected and assembled about 11 000 new optical observations from twenty telescopes, plus near-IR and radio data at various frequencies. Here, we perform a correlation analysis on the long-term R-band and radio light curves.

Results. In general, we confirm the ~100-day delay of the hard radio events with respect to the optical ones, even if longer (~200–300 days) time lags are also found in particular periods. The radio quasi-periodicity is confirmed too, but the “period” seems to progressively lengthen from 7.4 to 9.3 years in the last three cycles. The optical and radio behaviour in the last forty years suggests a scenario where geometric effects play a major role. In particular, the alternation of enhanced and suppressed optical activity (accompanied by hard and soft radio events, respectively) can be explained in terms of an emitting plasma flowing along a rotating helical path in a curved jet.