The optical-gamma correlation in BL Lacertae

¹ INAF, Osservatorio Astrofisico di Torino
² Dip. di Fisica, Università degli Studi di Perugia, Perugia
³ INAF, Istituto di Radioastronomia, Bologna
⁴ Astron. Inst., St.-Petersburg State Univ., Russia
⁵ Pulkovo Observatory, St.-Petersburg, Russia
⁶ Isaac Newton Institute of Chile, St.-Petersburg Branch
⁷ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
⁸ Maidanak Observatory of the Ulugh Beg Astronomical Institute, Uzbekistan
⁹ Steward Observatory, University of Arizona, Tucson, AZ, USA
¹⁰ Instituto de Astrofisica de Canarias (IAC), La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofisica, Universidad de La Laguna, La Laguna, Tenerife, Spain

Published online: 9 December 2013

Abstract

We present multifrequency light curves of BL Lacertae from February 2008 to October 2012. Lowenergy data (optical and millimetre) were acquired in the framework of a GASP-WEBT project. High-energy data (ultraviolet, X-ray, and γ-ray) come from observations of the Swift, RXTE, and Fermi satellites. After a period of moderate activity, in May 2011 the source suddenly started to flare at γ and optical-UV frequencies. Activity at millimetre wavelengths and X rays began 3-4 months later. This behaviour offered a good opportunity to study the correlation among flux variability in different bands, in particular between the best-sampled optical and γ-ray light curves. However, even in this fortuitous case, we can only define a general correlation with likely no time lag, but with a lag uncertainty of ±1 day. Indeed, the data reveal a complex relationship between the γ and optical fluxes, which cannot be unveiled because of the small gaps in the sampling of this extremely variable source.