Optical and radio variability of the BL Lacertae object AO 0235+16: A possible 5-6 year periodicity

¹ Osservatorio Astronomico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
² Dept. of Astronomy, Dennison Bldg., U. Michigan, Ann Arbor, MI 48109, USA
³ Landessternwarte Königstuhl, W-6900 Heidelberg 1, Germany
⁴ Abastumani Astrophysical Observatory, Georgia
⁵ Astrophysikalisches Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Dipartimento di Fisica, Università La Sapienza, Roma, P.le A. Moro 2, 00185 Roma, Italy
⁷ Tuorla Observatory, 21500 Piikkiö, Finland
⁸ Dipartimento di Fisica Generale, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy
⁹ Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁰ Cattedra di Astrofisica, Università di Perugia, Via B. Bonfigli, 06126 Perugia, Italy
¹¹ Foggy Bottom Observatory, Colgate University, Hamilton, NY, USA
¹² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Mount Stromlo and Siding Spring Observatories, Canberra, Australia
¹⁴ Beijing Astronomical Observatory, Beijing, China
¹⁵ Teide Observatory, Tenerife, Spain
¹⁶ Dept. of Chemistry, Physics, & Astronomy, Francis Marion University, PO Box 100547, Florence, SC 29501-0547, USA
¹⁷ Vainu Bappu Observatory, Indian Institute of Astrophysics, Dept. of Science & Technology, Kavalur, India
¹⁸ Dept. of Physics and Astronomy, University of Victoria, British Columbia, Canada
¹⁹ Dept. of Physics, University of Colorado at Denver, PO Box 173364, Denver, CO 80217-3364, USA

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

Received: 19 June 2001
Accepted: 7 August 2001

Abstract

The BL Lacertae object AO 0235+16 is well known for its extreme optical and radio variability. New optical and radio data have been collected in the last four years by a wide international collaboration, which confirm the intense activity of this source: on the long term, overall variations of in the R band and up to a factor 18 in the radio fluxes were detected, while short-term variability up to in a few hours and in one day was observed in the optical band. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) first-light campaign organized in November 1997, involving a dozen optical observatories. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection and Discrete Correlation Function (DCF) analysis. On the long term, the autocorrelation function of the optical data shows a double-peaked maximum at 4100-4200 days (11.2-11.5 years), while a double-peaked maximum at 3900-4200 days (10.7-11.5 years) is visible in the radio autocorrelation functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio correlation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts repeat quasi-regularly with a periodicity of ~5.7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspection and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, but in other cases they lead the radio events. Moreover, a deep inspection of the radio light curves suggests that in at least two occasions (the 1992-1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.