Optical and radio behaviour of the BL Lacertae object 0716+714 ^*

¹ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy e-mail: raiteri@to.astro.it
² Osservatorio Astronomico, Università di Perugia, via B. Bonfigli, 06126 Perugia, Italy e-mail: Gino.Tosti@fisica.unipg.it
³ Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, 00185 Roma, Italy e-mail: roberto.nesci@uniroma1.it
⁴ Dept. of Astronomy, Dennison Bldg., U. Michigan, Ann Arbor, MI 48109, USA e-mail: margo@astro.lsa.umich.edu
⁵ Metsähovi Radio Observatory, 02540 Kylmälä, Finland e-mail: hte@alpha.hut.fi
⁶ Abastumani Observatory, 383762 Abastumani, Georgia e-mail: okur@kheta.ge
⁷ Astrophysikalisches Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Landessternwarte Heidelberg-Königstuhl, Königstuhl 12, 69117 Heidelberg, Germany
⁹ Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, 33 Astronomical Str., Tashkent 700052, Uzbekistan e-mail: mansur@astrin.uzsci.net
¹⁰ Isaac Newton Institute of Chile, Uzbekistan Branch
¹¹ Physics Department, University of Crete, 710 03 Heraklion, Crete, Greece e-mail: jhep@physics.uoc.gr
¹² IESL, Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: p459kri@mpifr-bonn.mpg.de
¹⁴ IRAM, Avd. Div. Pastora 7NC, 18012 Granada, Spain e-mail: ungerechts@iram.es
¹⁵ Vallinfreda Astronomical Station, Vallinfreda (RM), Italy e-mail: f.montagni@libero.it
¹⁶ Dipartimento di Fisica Generale, Università di Torino, via Pietro Giuria 1, 10125 Torino, Italy e-mail: ostorero@to.astro.it

Corresponding author: C. M. Raiteri, raiteri@to.astro.it

Received: 16 January 2003
Accepted: 14 February 2003

Abstract

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data, which all together constitute the widest optical and radio database available on this object, are presented here for the first time. Four major optical outbursts were observed at the beginning of 1995, in late 1997, at the end of 2000, and in fall 2001. In particular, an exceptional brightening of 2.3 mag in 9 days was detected in the R band just before the BeppoSAX pointing of October 30, 2000. A big radio outburst lasted from early 1998 to the end of 1999. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5–6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness; a clear spectral steepening trend was observed during at least one long-lasting dimming phase. Moreover, the optical spectrum became steeper after , the change occurring in the decaying phase of the late-1997 outburst. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week–month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one. On the contrary, the process responsible for the big radio outbursts does not seem to affect the optical emission.