Optical variability of the BL Lacertae object GC 0109+224

Multiband behaviour and time scales from a 7-years monitoring campaign

¹ Physics Department, University of Perugia, via A. Pascoli, 06123 Perugia, Italy
² Astronomical Observatory, University of Perugia, via B. Bonfigli, 06126 Perugia, Italy
³ INAF, Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese, Torino, Italy
⁴ Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, Astronomicheskaya 33, Tashkent 700052, Uzbekistan

Corresponding author: S. Ciprini, stefano.ciprini@pg.infn.it

Received: 28 October 2002
Accepted: 6 January 2003

Abstract

We present the most continuous data base of optical observations ever published on the BL Lacertae object GC 0109+224, collected mainly by the robotic telescope of the Perugia University Observatory in the period November 1994-February 2002. These observations have been complemented by data from the Torino Observatory, collected in the period July 1995-January 1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid optical variations and six major outbursts were observed at the beginning and end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the beginning of 2002. Fast and large-amplitude drops characterized its flux behaviour. The R_c magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy), with a mean value of 14.9 (3.38 mJy). In the periods where we collected multi-filter observations, we analyzed colour and spectral indexes, and the variability patterns during some flares. The long-term behaviour seems approximatively achromatic, but during some isolated outbursts we found evidence of the typical loop-like hysteresis behaviour, suggesting that rapid optical variability is dominated by non-thermal cooling of a single emitting particle population. We performed also a statistical analysis of the data, through the discrete correlation function (DCF), the structure function (SF), and the Lomb-Scargle periodogram, to identify characteristic times scales, from days to months, in the light curves, and to quantify the mode of variability. We also include the reconstruction of the historical light curve and a photometric calibration of comparison stars, to favour further extensive optical monitoring of this interesting blazar.