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Astron. Astrophys. Suppl. Ser. 130, 109-115

$BVR_{\rm c}I_{\rm c}$ monitoring of ON 231 during the great outburst in 1994-1997[*]

G. Tosti1, M. Fiorucci1, M. Luciani1, N. Rizzi1 - M. Villata2, C.M. Raiteri2, G. De Francesco2, L. Lanteri2, M. Chiaberge 2, A. Peila2, M. Cavallone2, G. Sobrito2 - M. Maesano3, E. Massaro3, F. Montagni3, R. Nesci3 - G. Ghisellini4 - L.O. Takalo5 - A. Sillanpää5 - S. Katajainen5 - P. Heinämäki5 - K. Nilsson5 - T. Pursimo5

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1 - Cattedra di Astrofisica, Università di Perugia, Via A. Pascoli, I-06100 Perugia, Italy
2 - Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, Italy
3 - Istituto Astronomico, Università di Roma ``La Sapienza", via G.M. Lancisi 29, I-00161 Roma, Italy
4 - Osservatorio Astronomico di Brera, via Bianchi, 46, I-22055 Merate, Italy
5 - Tuorla Observatory, Tuorla, FIN-21500 Piikkiö, Finland

Received August 5; accepted October 27, 1997


We present the most continuous data base of optical multiband data ever published on the BL Lacertae object ON 231 (W Com). The data have been collected during an intensive and coordinated $BVR_{\rm c}I_{\rm c}$ monitoring campaign carried out in the period from March 1994 to March 1997. During our campaign, the source brightness was at the highest level ever observed. The light curve shows a complex structure, characterized by the presence of three major outbursts having the observed maxima in March 1995, February 1996, and January 1997, when ON 231 reached its historical maximum ($B \simeq 14.2$). Variability on time scales from a few hours up to a month have frequently been observed and the light curve seems to be the superposition of many flares with different amplitudes and time scales. The broad-band optical spectral energy distribution is characterized by a spectral slope which correlates with the flux level. In particular, the higher is the flux the flatter is the spectrum.

Key words: BL Lacertae objects: general; individual -- ON231

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