Optical and radio variability of BL Lacertae^⋆

¹ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
e-mail: haritma@shao.ac.cn
² Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, 263002 Nainital, India
³ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shosse Blvd., 1784 Sofia, Bulgaria
⁴ Department of Physics, The College of New Jersey, PO Box 7718, Ewing, NJ 08628-0718, USA
⁵ Radio Astronomy Laboratory of Crimean Astrophysical Observatory, Crimea
⁶ Taras Shevchenko National University of Kyiv, 01601 Kiev, Ukraine
⁷ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁸ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁹ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
¹⁰ ZAH, Landessternwarte Heidelberg, Königsthul 12, 69117 Heidelberg, Germany
¹¹ Engelhardt Astronomical Observatory, Kazan Federal University, 420 000 Tatarstan, Russia
¹² Aalto University Metsähovi Radio Observatory, 00076 Aalto, Finland
¹³ Aalto University Department of Radio Science and Engineering, 00076 Aalto, Finland

Received: 15 May 2015
Accepted: 3 August 2015

Abstract

Context. We extensively observed the prototype blazar, BL Lacertae, in optical and radio bands during an active phase in the period 2010–2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multi-frequency data to learn about the mechanisms producing variability.

Aims. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multi-frequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands.

Methods. We performed optical observations in B, V, R, and I-bands from seven telescopes in Bulgaria, Georgia, Greece, and India and obtained radio data at 36.8, 22.2, 14.5, 8, and 4.8 GHz frequencies from three telescopes in Crimea, Finland, and USA.

Results. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical bands of ~250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical “mini-flares” on short timescales. Variations on longer term timescales are mildly chromatic with the superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks.

Conclusions. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source.